Growing High-Yielding Sweet Potatoes

Healthy sweet potato plants. A few insect bites won’t hurt!
Photo Kathryn Simmons

Growing High-Yielding Sweet Potatoes

In May I gave information on planting sweet potatoes. Hopefully that went well for you, and by now you have a large patch of healthy green vines. Let’s keep it that way! Here I will tell you about what the growing plants need, and the pests, disease and afflictions to avoid.

Read more

I have a Sweet Potato slideshow. I have lot of other posts on sweet potatoes (mostly about propagating our own slips, or about harvesting). Click to see the links in last month’s post

The ATTRA publication Sweetpotato: Organic Production is a good introduction.

Oklahoma State University Extension Sweet Potato Production is a clear concise publication (although it’s not organic).

Sweet potato development

This paragraph was included in Planting Sweet Potatoes, and I’m repeating it here, as a good reality check on what you can expect.

Regardless of how early in the season you plant them out, they will not make flowers earlier, or start making tubers sooner. Both flower and tuber initiation are triggered by day length. Each variety has its own internal clock. Most varieties take 90–110 days from planting out to reach a good size, if the weather is warm enough.

The first month or so after transplanting is the root development stage. Roots can go 8’ (2.4 m) deep in 40 days. Don’t be alarmed at the lack of above-ground action. The second month or so is the vine growth stage. The roots begin to store starch and sugar close to the stem base. During the last month of growth for that variety (3rd or 4th month), the potatoes develop. Make sure you dig them up before the soil temperature gets down to 55˚F (13˚C) – the week of the average first fall frost is about right.

Growing sweet potatoes – Three Ws: water, warmth, weed-free
  1. Water
  • The critical time to maintain sufficient moisture is after transplanting for at least the first 20-40 days while roots are developing. By now, most growers will be beyond this most-important watering period. But if it’s less than 40 days since you planted them out, keep the soil moist. Use your fingers to test the soil for dampness.
  • Once they are established, sweet potato plants are fairly drought-tolerant. But if you want high yields, they’ll need water once a week, either from the sky, or provided by you. Note that if you are using plastic mulch, rain won’t go through it, so I hope you installed drip irrigation below the plastic. If not, lay it on top, right beside the plants. Aim to provide an inch (2.5 cm) of water per week.
  1. Warmth
  • Sweet potatoes need warmth! Heat determines success; the number of days from planting does not.
  • Provided the weather is warm enough, most varieties take 90–110 days from planting out to reach a good size.
  • You can gain warmth in a cold climate, by planting inside a hoophouse or low tunnel covered in clear plastic. Ventilate in hot weather.
  • Growing Degree Days (heat units) are a tool for measuring accumulated heat, but you don’t need to calculate GDDs to get a good crop!
  • Early varieties take 1200 GDDs to grow a good crop.
  • To calculate GDDs, take the day’s high temperature (max) and the day’s low temperature (min) and add them together. Divide by 2 and subtract the base temperature of 55F. (Apologies to the rest of the world – I only know this method using Fahrenheit, but I’m sure you can find out how to do the calculations in Celsius). There are phone apps that will do the calculation for you.
  • Example: For a daytime max of 90F, and a night-time min of 70F, you get 25 GDDs – just about perfect for sweet potatoes. 90+70=160. 160/2=80. 80-55=25. At 25 GDDs a day, you theoretically only need about 48 days to get a crop. There are some other limits to daily plant growth – the likely minimum for a decent crop is about 76 days.
  • In a plastic tunnel, you can get 20 GDDs a day or more, rather than the 5 you might get outdoors.

    Water, warmth and no weeds – all that growing sweet potatoes need.
    Photo Nina Gentle
  1. Weed-free
  • Cultivate to remove weeds until the vines cover the ground, after which very little weeding will be needed.
  • If you have plastic mulch, walk through pulling weeds, and drop them on the plastic to cook. If you are growing on bare soil, hoe while the weeds are small, and pull if the weeds and the vines get ahead of you.
  • Weeding is generally not onerous because the sweet potato vines cover the ground within 6 weeks of planting and smother any newly emerging weeds.
Our motion sensor sprinkler and the outer layer of our fence around the sweet potato patch at the end of May.
Photo Pam Dawling
Solar electric fence controller for our sweet potato patch.
Photo Wren Vile
Pest mammals
  • Deer eat sweet potato plants at all stages, including digging out the roots in the fall. Dogs, fences and guns are the three most effective methods of deer control. The plants can be covered with row cover or plastic net for the growing season. Motion-sensor sprayers work well if maintained.
  • Rabbits eat the foliage. Plant the slips on black plastic to hold back weeds, then put wire hoops over the rows and cover with row cover for 3–4 weeks while the plants are young. Even after the plants are large rabbits can cause substantial losses.
  • Groundhogs dig and eat the roots. They can be trapped with baits of fruit. What’s for dinner?
  • Pocket Gophers search out sweet potatoes to eat. Their mounds may be hidden under the foliage and the plants may survive as they only eat the larger roots, leaving no crop.
  • Voles move in from grassy areas to live under the mulch and feed as fast as the roots form. They eat the roots from the top down leaving the outer shell in the soil where they have feasted. Cats are the best control.
  • Rats love the roots. Cats or dogs are the best methods of control.
  • Field Mice build nests under black plastic and eat the roots emerging from the ground.
Why not eat some sweet potato leaves as summer greens?
Photo Nina Gentle
Human “pests” of sweet potatoes

 You can eat sweet potato leaves yourself and it takes several meals to reduce yields of the tubers. Some researchers working in Vietnam, discovered that harvesting 25%, 50%, 75% or 100% of the vines every 15, 20, or 30 days (ignoring the information about the season of the year and the varieties) gave the sort of results you might expect. Harvesting tops every 20 days gave highest yields of greens. Harvesting 50% of the greens each time gave highest total yields of greens. Harvesting not more than 25% or 50% of the greens each time gave the highest eventual tuber yields, after 120 days. Researchers in Tanzania came up with the clear information that harvesting three times at one month intervals gave the highest greens production, but the tuber yield was affected tremendously. Harvesting tops twice in a growing period proved the best in leaf production as well as root yields. So, clip 25-50% of the tops of each plant up to twice in one summer, and you’ll still get a good yield of roots.

Also see the University of Arkansas Extension Nutritional and Medicinal Qualities of Sweetpotato Tops and Leaves. This publication explains how to plant slips 2” (5 cm) apart, specifically for greens (vine tips) and harvest 6 times between the end of April and the end of October.

 Insect pests of sweet potatoes

See North Carolina State University, Pests of Sweetpotato  for photos, drawings and details.

Although there are many insect pests that feed on sweet potato vines and leaves, most do very little damage, and hunting them down is not justified.

Healthy sweet potato patch, with some deer nibbling and weeds..
Photo Wren Vile
Pests that feed on foliage
  • Sweet potato flea beetles – Tiny black/bronze oval beetles (1.6 mm long), with reddish-yellow legs, and ridged wing covers; make small shot-holes in leaves or grooves in the upper surface of the leaves. Damaged areas turn brown and die. See below about larvae.
  • Sweet potato weevil adults and larvae do feed on the foliage, but mostly go for the roots (see below).
  • Caterpillars of three kinds:
    • Southern armyworms – Gray-black larvae up to 36 mm long with green or pink tints; pale longitudinal stripes and pairs of triangular spots along the back; pale yellow heads with bright red-brown marks. They feed on leaves and tips of vines, and congregate around the bases of plants during the middle of the day.
    • Sweet potato hornworms – First instar: white with a black horn; later instars (up to 90 mm long): green or brown with black diagonal lines down each side and a black horn, with a green or brown head with black stripes. They defoliate plants and often hide under leaves near the bases of plants.
    • Yellow-striped armyworms – Pale gray-black caterpillars up to 45 mm long, with orange-yellow stripes along the sides and pairs of triangular spots on the back of most segments; brown heads with black markings and a white inverted V. They feed similarly to southern armyworms.
  • Potato leafhoppers – Wedge-shaped insects up to 3 mm long; green bodies with yellow to dark green spots. They usually jump rather than fly. They suck sap from the underside of leaves causing yellowing of leaf tips and margins.
  • Fruit or vinegar flies – Small yellowish red-eyed flies about 3 mm long. They hover around overripe or decaying produce. They may be found with their small creamy maggots in cracks in sweet potatoes.
  • Tortoise beetle adults and larvae – Long-oval shaped gold beetles, up to 8 mm long, with various black or red markings on their flattened, shell-like bodies. The larvae have dull yellow, brown, or green bodies up to 12 mm long and black heads, legs, spots, and spines. Long spines on the abdomen hold excrement. Adults and larvae chew the leaves riddling them with holes.
  • Spider mites – Tiny reddish or pale spider-like arthropods that feed on the underside of leaves. Heavily infested plants develop a yellowish, bronzed or burned appearance.
Pests that feed underground on tubers and side roots
  • Sweet potato flea beetle larvae – Thin white, cylindrical larvae, up to 5 mm long, with 3 pairs of legs near their heads. They make shallow, winding tunnels on the surface of sweet potato roots and sweet potatoes. The tunnels darken, split, and leave scars.
  • Sweet potato weevil adults and larvae –Snouted beetles 6 mm long with dark-blue wing cases, orange-red legs and thorax, and fat, legless, 9 mm grubby white larvae with pale brown heads. The beetles make small holes over the surface of sweet potatoes mostly at the stem end. The larvae tunnel inside the tubers, leaving frass, which causes the sweet potatoes to taste bitter.
  • White grubs (spring rose beetles) – Dirty white grubs up to 25 mm long with brown heads and 3 pairs of legs near their heads. They leave large, shallow feeding scars on the sweet potatoes.
  • Wireworms – Thin, tough, wire-like larvae with 3 pairs of short legs near their heads and prolegs at the end of the body. They initially create large shallow cavities in sweet potatoes which they later excavate into deep ragged holes. Three species, with colors from yellowish-brown to cream or yellow-grey. Heads are darker, brownish.
  • White-fringed beetle larvae – Yellow-white legless, 12-segmented grubs, up to 13 mm in length, with small, pale heads. They chew into the roots.
Sweet potato souring.
Photo North Carolina Sweet Potato Commission
Afflictions of sweet potatoes (these are not caused by disease organisms) 
  • Round chunky roots, low yield, purple color: Planted too early, too cold.
  • Low yield: Flooded or crusted soil 6-7 weeks after planting? Planted too early?
  • Rough irregular shaped roots: Heavy clay soils or organic matter above 2%.
  • Rattails – thin, tough, tubers: Hot dry weather, insufficient water.
  • Long, slender malformed roots, reduced yield: Potassium deficiency.
  • Souring – Tissue breakdown caused by poor soil aeration, such as flooding.
  • Water blisters – Small whitish bumps around the lenticels (breathing holes): wet soil.
  • Blister – Small raised bumps appearing several months into storage: boron deficiency.
  • Fine hairline cracks: Another sign of boron deficiency.
  • Cracking: Uneven water supply or too much late-season water.
Sweet potato Ring Rot.
Photo North Carolina Sweet Potato Commission
Sweet potato feathery mottle virus.
Photo North Carolina Sweet Potato Commission
Diseases of sweet potatoes (mostly fungal)

 More info and photos: North Carolina State University Diseases of Sweetpotatoes. 

  • Brownish skin patches, worse in wet years: Scurf fungus, Monilochaetes infuscans. More likely if too much compost was used. Stored roots shrivel.
  • Metallic black surface lesions, maybe covering most of the root: black rot fungus, Certocystis fimbriata. Internal decay is not deep, but the fungus may impart a bitter flavor.
  • Sunken brown lesions that may completely encircle the root: ring rot, Pythium
  • Sunken lesions that dry and may fall out: Circular Spot, Sclerotium rolfsii. May taste bitter.
  • Hard, dry, black, sunken spots developing in harvest wounds: Fusarium. Spots may become larger than 2″ (5 cm) diameter, but damage is not deep.
  • Pitting: Soil rot or soil pox fungus in the presence of water stress. Roots will be small and malformed.
  • Streptomyces root rot bacterium causes a similar rot.
  • Fine or coarse irregular cracks, browning of the surface; dry, corky, dark-colored clumps of tissue scattered throughout the flesh, becoming worse if roots are stored warmer than 60°F (16°C): russet-crack/internal cork, feathery mottle virus (yellow feathery patterns of leaves). Do not use as seed stock.
When to harvest sweet potatoes

Unlike white potatoes, which have the annual plant sequence of vegetative growth, flowering and dying back, sweet potato plants would go on growing forever if the weather remained warm enough. Choose when to dig them up, ahead of cold weather. The longer you wait, the bigger the potatoes, but you are gambling with the weather. Usually sweet potatoes are harvested in the week that the first frost typically occurs in your region. I have written plenty already in previous years about harvesting, so I won’t go into it here. See one of the links to those posts, or my slideshow, if you want to know what comes next, or your climate is considerably colder than mine in central Virginia.

Our sweet potatoes next to our sixth sweet corn planting.
Photo Ezra Freeman

Potato pests and diseases

Potato pests and diseases

These potatoes were almost killed by frost two weeks previously, and have now recovered.
Photo Pam Dawling

This is part of a monthly series on growing potatoes, a dietary staple.

PART ONE: Planting potatoes (April)

PART TWO: Growing potatoes (May)

PART THREE: Potato pests and diseases (this one, June)

PART FOUR: Harvesting potatoes (July)

PART FIVE: Storing potatoes (August)

PART SIX: Planning to grow potatoes again (September)

I have a whole chapter about potatoes in Sustainable Market Farming, where most of this information can be found.

See Root Crops in June for info on digging up new potatoes, if you can’t wait for them to mature!

See The Potato Association of America, Commercial Potato Production in North America 2010 for lots of interesting info, including planting in hot weather. (But hurry up, you need to have enough growing days left in the season to get them to maturity.)

Organic Integrated Pest Management involves tackling pest problems one step at a time with ecologically-based practices, starting with actions to reduce the chances of the pest ever getting a grip on your crops. I recommend the ATTRA online publication Organic Integrated Pest Management. Each of the 22 pages is a poster, complete with good photos and concise clear info. Because nightshades have a lot of fungal, bacterial and viral diseases, it pays to take action to minimize the chance of diseases attacking your plants.

June-planted potatoes in early September
Photo Kathryn Simmons

Integrated Pest Management in Organic Field Crops Webinar from eOrganic

https://www.youtube.com/watch?v=nQXC8SElTMk

Biological IPM disease and pest reduction strategies for potatoes

  1. Cultivate strong crops and provide healthy soil, sufficient space, nutrients and water, suitable temperature, and soil pH.
  • Choose varieties that resist the pests and diseases you most expect. Improve the soil tilth, drainage and aeration. Chisel plow or broadfork to break hardpan, or grow deep-rooting cover crops ahead of your potatoes. Maximize air circulation around the plants. Choose a bright, breezy location (avoid frost pockets as they also collect dew), orient the rows parallel to prevailing winds and give the plants plenty of space.
  • Add compost and cover crops to build fertile soil to support strong plant growth and help increase the diversity of soil microorganisms, building naturally disease-suppressing soil. Use foliar sprays of seaweed extract, microbial inoculants or compost tea to boost general disease resistance. Consult ATTRA for compost tea Brew one part of compost to 5 parts water by volume for 14 days before spraying.
  • Practice crop rotation to reduce the chances of pests and diseases carrying over from one crop to the next. For potatoes, it’s best to rotate away from nightshade crops for at least three years. We don’t manage this ideal of one year in four. In our ten-year rotation, three of our ten years are nightshades (one paste tomatoes and peppers, two plantings of potatoes).
  • Practice good sanitation. Clear old crops promptly, so they don’t act as a breeding ground for pests or diseases. Avoid smoking, especially near nightshades, and have smokers wash their hands with soap or milk before working with potatoes. Tobacco can spread tobacco mosaic virus (TMV) to nightshade plants. Avoid working potato plants while the leaves are wet. Remove and destroy diseased plants, especially for late blight. Clean tools in between use in one field and another. When the harvest is finished, till the tops into the soil to speed decomposition, or remove and compost or burn them if growing on a small scale.
  • Remove nightshade weeds (e.g., horsenettle, jimsonweed and black nightshade), which can be alternate hosts for pests and diseases.
Potatoes, weeds and standing water. Don’t let this happen to you! Provide good drainage, remove weeds especially nightshade weeds.
Photo Bridget Aleshire
  • Prevent soil splash-back onto leaves, to reduce outbreaks of soil-borne diseases. Use drip irrigation rather than overhead sprinklers.
July photo of a mulched June-planted potato.
Photo Wren Vile
  1. Cover or protect the plants physically from the pests
    • mulches to stop soil-dwelling pests (CPB) moving up into your crops
    • netting or rowcover to protect from airborne pests (leaf hoppers, blister beetles)
  2. Provide habitat for natural enemies and other beneficial insects. Farmscaping with sunflowers, peas, vetch, buckwheat or small grains, to encourage ladybugs and lacewings, can make insect control unnecessary in a good year. Ground beetles and bats can consume surface and air attackers before you even need to look.
  3. Monitor your crops regularly at least once a week and identify any pests you see.
  4. Introduce natural enemies of the pest (bacteria, fungi, insect predators or parasites). Try biofungicides for use against some diseases. F-Stop, T-22G Biological Plant Protectant Granules or other forms of Trichoderma can control Rhizoctonia, Fusarium and Sclerotonia. Soil-Gard (Gliocladium virens) can work against Rhizoctonia. Bacillus subtilis works against Rhizoctonia, and Sclerotonia. Mycostop (Streptomyces griseoviridis) can be used against Phytophthora, Alternaria, 35% hydrogen peroxide diluted to a 0.5–1% foliar spray solution may help control early blight. 1% solution = 3.7 oz in 124.3 oz water to make one gallon (1 ml:33 ml). There are commercial products such as Oxidate that are based on hydrogen peroxide, which is corrosive and challenging to handle.
  5. Hand pick (or trap) and kill the pests if the pest population is above the action threshold. Many fruit and root crop plants can take 30% defoliation before suffering any loss of yield. Where the crop is the foliage, this may be too much, but people don’t east potato foliage!
  6. Use biological controls (often derived from natural enemies) if the damage is still economically significant after trying the earlier steps in the process, including Spinosad or Bt.
Colorado potato beetle on an eggplant leaf.
Photo Pam Dawling

Potato Pests

Potatoes can be attacked by more than 150 insect pests. But don’t despair! In each region there are only a few species that could cause unacceptable losses of yield or quality. These losses can result either directly from the insects or indirectly by transmission of diseases.

Colorado potato beetle is the most common pest that potato growers get to deal with. The pink blob-like larvae of this beetle can eat enormous amounts of potato leaves while growing into bigger pink blobs. Left alone they can kill a planting. Acceptable amounts of defoliation without causing loss of yield are surprisingly high: 50%–75% of the top leaves on a young 6″–8″ (15–20 cm) plant, 25% on a 12″–16″ (30–40 cm) plant, a mere 10% at the critical full bloom stage (when the tubers are sizing up), and up to 25% once full grown. As with many pests, having a few of them is not important — it’s all about the numbers. Action to control CPB is only needed if the number of adults or larvae is higher than 1.5 per plant or egg masses exceed one per ten plants.

Crop rotation is effective, because Colorado potato beetles overwinter as an adult in the soil and when they emerge they have to walk around searching for a potato plant. CPB can have 1-3 generations a year. Even where two or three generations are usual, a significant portion of the summer generation adults go directly into the soil and become dormant. Eggs are laid in clusters of 20 or more. They look like ladybug eggs but are a stronger orange color – don’t kill the wrong ones! The beetle can go from egg to adults in as few as 21 days. There are four larval instars, with 75% of the total foliage destruction caused by the final and fattest instar.

Colorado Potato beetle late stage larva or pupa.
Photo Pam Dawling

Mulching with hay or straw can prevent CPB finding your potato plants – we never find them on our summer planting. Our unmulched spring planting is a different matter. I scout that field once a week, counting adults and larvae on a hundred randomly selected plants. As soon as I see more than 50 adults or 150 large larvae or 400 small larvae per 100 plants, I unpack the sprayer. I do a spraying with Spinosad, a fermentation product of a soil bacterium. It kills insects by over-stimulating their nervous systems. Spinosad kills a wide range of helpful and harmful insects too, so spray in the early morning or late evening when bees are not flying. Shake the bottle well, and mix following the instructions. Clean and triple rinse the sprayer. Do not flush in the creek or pond. Repeat in 6 days, but only if needed. Usually one spraying is enough, although I continue weekly checks. In the South, there can be three generations of CPB each year, so stay vigilant.

Prior to using Spinosad, we used Bt. The version of Bt for CPB nowadays is Bacillus thuringiensis var. tenebrionis. The kurstaki strain (such as Crymax) generally available in small quantities previously is genetically modified, so we stopped using it, not wishing to be part of any support for GMOs.  Neem and Beauvaria bassiana can also kill CPB larvae.

Flaming when the potatoes are less than 8” (20 cm) tall, is another effective control measure for CPB. Choose a warm sunny day when the pests are at the top of the plants. Flaming can kill 90% of the CPB adults and 30% of the egg masses, according to Colorado Potato Beetle: Organic Control Options – ATTRA

Young eggplant struggling against lots of aphids.
Photo Pam Dawling

Insects with piercing-sucking mouthparts damage potatoes by physical injury to the leaves, sucking out phloem, injecting their toxic saliva and possibly transmitting diseases. While potatoes can grow new leaves, there is still damage to plant health. Direct injury by sap-feeding insects can kill the plant. Soil-dwelling insects have only minor effects on yield, generally, but can reduce tuber quality and storage life.

Aphid-transmitted viruses cause greater losses than all other insect-related damage together. There are at least 9 aphid-transmitted potato viruses. Aphids can be reduced by farmscaping, planting flowers which attract ladybugs, lacewings and other aphid-eating insects.

Clover flowers attract beneficial insects. Red clover in June.
Photo Pam Dawling

Potato leafhoppers are a bad problem in central and eastern North America. They overwinter on the Gulf Coast. In spring, flying adults are transported north on upper level airstreams. Yield loss can occur before visual symptoms are obvious. Leafhoppers can cause leaves to shrivel and die. The initial effects are reversible if leafhoppers are controlled before leaf tissue is destroyed (“hopperburn”). By reducing the green leaf area, hopperburn affects photosynthesis and growth. The most vulnerable stage is when the tubers are bulking up. Leafhoppers can also transmit diseases. Trichogramma wasps parasitize leafhopper eggs. Garlic with insecticidal soap, sprayed early in the morning, especially on the undersides of the leaves, can control hoppers.

Potato psyllid occurs in the western U.S. Damage to the roots and tubers is caused by feeding nymphs, which can cause psyllid yellows. The first symptoms of psyllid yellows include stunting, loss of green color, leaflet distortion, reddish discoloring of new leaves, and the appearance of aerial tubers. Early action can stop and even reverse the damage. Adults cause little to no damage underground.

Wireworms (click beetle larvae) can tunnel through the tubers. Wireworms can live for 1-3 years, so crop rotation is important. Avoid planting potatoes the first year after turning under pasture or lawn. If you expect to have wireworms, plant small whole seed potatoes rather than cut pieces. Cut slices of potato can be used to trap wireworms (dig up the trap pieces each day and kill the wireworms.

Cutworms can eat the leaves from the bottom of the plant up (the opposite approach from CPB larvae). Once the plants are fully grown, up to 75% loss of lower leaves is unimportant. At earlier stages, if any cutworm damage is seen, dig around the stem, find and kill the cutworms.

Blister beetles can cause trouble later in the season, skeletonizing leaves and spreading a wilt. They contain cantharadin, which can cause blisters on the skin of unwary workers. Blister beetles can be trapped in crops of chard or beets next to the potatoes. The beetles are easier to see and catch in the trap crops than in potato foliage. If there aren’t too many it may be worth putting up with them, as their larvae are carnivorous and eat grasshopper eggs.

The potato tuber moth damages both foliage and tubers during growth, but the biggest losses occur in storage. Larvae inside the potatoes can continue their development in storage, filling the tubers with frass and letting in decay organisms. When commercial infestations are high, the crop is not worth harvesting because of labor costs to cull out the infested tubers.

 Nematodes can be deterred by choosing appropriate preceding cover crops, or by applying 1-2 tons/ac (2240-4480 kg/ha) of crushed mustard seed meal to the soil before planting. This will also reduce early weeds and act as a fertilizer. 

Potato Diseases

Before a plant can become diseased, three conditions must exist: a susceptible host, a disease organism, and a suitable environment for the pathogen. The choice of the disease control method should be based on an accurate identification of the pathogen and the disease.

Late blight on a potato leaf. http://blogs.cornell.edu/livegpath/gallery/potatoes/late-blight/

Late Blight (Phytophthora infestans) is by far the worst disease to afflict potatoes. This is the disease that contributed to the famine in Ireland (caused by the profiteering of the English land-owners, who sold the barley and left the tenant farmers to subsist almost entirely on potatoes). The disease is caused by a species of a fungus-like oomycete or water mold (previously considered a fungus, now reclassified as protozoa) that blows in on the wind. It is worse in warm wet weather with cool nights. Late blight starts as “water-soaked” spots on the leaves. These expand into gray-black “scorched” areas, sometimes with a dotted white mold growth, especially on the underside of the leaves. Cut stems reveal a dark circle of infected tissue. The disease spreads rapidly, turning plants black, as if badly frosted, and can kill an entire planting in ten days unless stopped by hot dry weather.

The best defense is to always remove volunteer nightshades from your fields and compost or bury all crop debris. The disease spreads via cull piles, nightshade plants and petunias — it needs live plant material to survive. If you find volunteer potato plants popping up in early spring, it is best to pull them up! Spores survive winter in warmer climates and then blow north and uphill. Preventive action may be taken with sprays every five days of (toxic) copper products, hydrogen peroxide, Bacillus pumilus or Bacillus subtilis products.

If Late Blight occurs late enough in the season, you can save your crop by mowing off the foliage, raking it off and disposing of it, and leaving the field untouched for two weeks before harvesting whatever potatoes have grown. This prevents the spores getting into the soil and infecting the tubers. Disposing of large amounts of blighted foliage is no easy task. When I had to deal with Late Blight, back in the 70’s, we made a fire and gradually added more tops as the previous ones burned. This was a very smoky fire, polluting, and no doubt contributing to global warming. Digging a big hole and burying it all is probably better.

Early Blight (Alternaria solani) is a common fungal disease, which mostly affects stressed or older plants. It starts as small brown spots on the lower leaves, which conglomerate into brown blotches that are restricted by the leaf veins, and so they can be angular in shape. The lesions have a bullseye appearance – concentric circles with a yellow halo around each one. During warm humid conditions, the fungus steadily defoliates the plants, reducing yields. The disease is seed-borne, soil-borne and airborne, surviving on plant debris and nightshade weeds. Early blight (Alternaria solani) can appear late in the season, not just early, despite the name. The manifestation of blight symptoms can be minimized by growing strong healthy plants, supplying sufficient water, and spraying with compost teas. The beneficial fungus Trichoderma harzianum can give good results.

Black Scurf or Stem Canker fungus (Rhizoctonia solani) is worst in cold wet soils. Early in the season it can cause sprout death. On older plants, red-brown stem lesions develop into cankers, and the infection can spread to the tubers, which then become cracked and misshapen, and may have dead tissue at the stem end. There may be firm black sclerotia (small dried reproductive bodies) on the tuber. In future, get disease-free seed potatoes and wait for the soil to warm a bit before planting.

White Mold (Sclerotinia sclerotiorum) on the vines. If you want to prevent this in future, you could dust the seed pieces with the commercially available fungal antagonists Trichoderma viride and Trichoderma virens.

For a chart with about 30 potato diseases, see The Potato Association of America, Commercial Potato Production in North America 2010

A lady bug on a potato leaf, perhaps producing larvae to help control aphids.
Photo Kathryn Simmons

Hoophouse Musings, Bugs, Okra, Edible Landscaping Workshops in Maryland

Hoophouse beds in December. This is why we have a hoophouse!
Photo Wren Vile

Winter hoophouse posts in Mother Earth News newsletter

Sowing and Transplanting Winter Crops in a Hoophouse

Grow Great Lettuce in Winter

Winter hoophouse lettuce
Photo Kathryn Simmons

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A photo of a Tiny House from Wikipedia

Would you live in a hoophouse?

A reader wrote in:

“I have actually been thinking of building a tiny house and putting it inside a big hoophouse, creating a living area that would include a yard, trees, and gardens – allowing me to snowbird in place in northern New England – but I’m concerned about outgassing, since I’d be there almost 24-7 most days (I work out of my home). Have you done any research on outgassing of hoophouses?”

A Tiny House is generally a residential structure under 400 sq. ft

First off, No I haven’t done any research about hoophouse off-gassing, but I wouldn’t worry about out-gassing from the polyethylene of the hoophouse. Other products  are much closer to your nose: All the materials used to construct, preserve and decorate the house and all the products within the house, such as furniture,  fabrics, soaps, appliances etc.

There are some other things I’d wonder about:

1.      Temperature. When the sun shines, the interior of the hoophouse warms up. When the sun doesn’t shine, it doesn’t. Would you heat the tiny house? You’d have to avoid heating systems that could damage the plants.

2.      Snowfall. When it snows, you need to remove the snow from the roof of the hoophouse. Some snow can be carefully pulled down from the outside. Usually we also walk around inside the hoophouse bouncing a broom on the inside of the plastic to move the snow off. You can’t do that if you have a house in the way.

3.      Humidity. In the winter we grow cold-tolerant hoophouse crops. We are aiming for 65 F (18C). We need fresh air for the plants and to deter fungal diseases. It doesn’t work to keep the hoophouse sealed up and “cozy”!

4.      Strong winds. In hurricanes and gales, hoophouses sometimes collapse or get destroyed. You don’t want to be inside when that happens.

5.      Height. Our hoophouse is less than 14 ft (4 m) at the apex.

In conclusion I’d say it’s better to have a small patio seating area within your hoophouse for suitable sunny days, rather than plan to live inside all the time.
Brassicas in a nematode-fighting hoophouse crop rotation in Hawaii.
Credit Gerry Ross, Kupa’a Farms

Do you value crop rotation in your hoophouse?

A reader in the Pacific Northwest wrote: “This winter I have been re-thinking my crop rotation plan after having some issues (with flea beetle larvae in the soil outsmarting my diligent insect netting of my brassica salad crops). These days I see intensive market gardeners seeming to not worry so much about rotation (i.e. Neversink farm, etc), and yet I’ve always been taught that it is such an important principle to follow. I reviewed your slideshows on crop rotation and also cool crop planning in the greenhouse (which briefly addresses salad brassica rotation with other crops). With how much space I have and the high demand I have for brassicas, for salad mix (mustards) and also the more mainstay cole crops, I had settled on a 2.5 yr between brassica crop rotation (but planting two successions of mustards in the same bed within one year, in the year the bed was in mustards, with a lettuce or other crop breaking up the successions, with the idea that they were very short day and also light feeder crops). Wondering if you think this just doesn’t sound cautious enough, or if this sounds like a reasonable compromise with not having more space to work with (and wanting to satisfy the market demand for brassicas).”

I replied: “Yes, I do think crop rotation is important. I do know some farms seem to have given it up. I think what you are seeing shows one reason why rotation is important. In our hoophouse, we do as you do, allocating brassicas to a space for that winter season and perhaps doing more than one round of brassica crops. Then moving away from brassicas for the next two winters. If doing that doesn’t get rid of the flea beetle problem, and you are being thorough about netting with small-enough mesh netting (sounds like you are, but maybe check the mesh size), then my next step would be spinosad when the flea beetles appear. You can spray the inside of the netting too, and close it quickly. It’s that or a longer rotation, which it sounds like is not financially viable. You could also try farmscaping and/or importing predatory insects (not sure if there are any), Are there beneficial nematodes that attack flea beetle larvae? These are things I don’t know about, but might be worth looking into.”

 

Late sweet corn and sweet potatoes
Credit Ezra Freeman

Sweet Potato fends off bugs

Modern Farmer has this fascinating article about sweet potato plants alerting their neighbors to pest attacks.

Researchers from the Max Planck Institute and the National Taiwan University found that when sweet potato plants are attacked by insects, they emit a bouquet of odors and start production of a protein called sporamin that makes them unappetizing. Neighboring sweet potatoes sense the odors and start their own production of sporamin.


 

A new Tokyo bekana transplant attacked by vegetable weevil larvae October 10
Photo Pam Dawling

Insect damage cause stress-response production of anti-oxidants

In a related piece of news, Agrilife Today from Texas A&M AgriLife Research has found some evidence that wounded plants produce anti-oxidants as a stress response, which may make them healthier for human consumption. Read the report here.

Edible Landscaping with a Permaculture Twist Spring Series

Michael Judd in cooperation with Common Market CO+OP is presenting a combination of hands-on workshops at Long Creek Homestead and evening talks at the Common Market, Frederick Maryland.

Click here for info on Spring Workshops/Talks/Tours

·        Inoculating Mushrooms

·        Fruit Tree Grafting

·        Herb Spirals

·        Creating Growing Beds- Swales and Hugelkultur

·        Edible Landscaping & Straw Bale Home Tour

·  For the Love of PawPaws


 

Fire Ants have reached Toronto

A reader wrote in that the European Fire Ant is now found in Toronto.


“There were two nests of these in my allotment garden 2018.
They actually moved the nest in order to be closer to the zucchini
plants.  Hand on heart: I never had  any cucumber beetles develop past
the instar stage.  The ants did not eat the eggs but they ate the larvae
as soon as they hatched.  Same for potato beetle.  My neighbours had
the best cucumber harvest in history. 
What I’ve read is these Fire Ants kill colonies of native ants.  Summer 2019 I had a Pavement Ant war that went on for days.  Clearly the Fire Ants did not wipe them out.  There are black ants and other smaller red ants
in my garden.  The Fire Ants appear to have moved on for some reason known
only to themselves.   Perhaps they too have enemies.”
“There’s a guy with a Youtube channel who keeps ant colonies.   AntsCanada although he is in the Philippines.  What happened was the feral Pharaoh Ants invaded his colony of Fire ants and killed them.  Pharaoh Ants are much smaller but perhaps that’s what gave them the advantage.   We have Pharaoh ants in Toronto also.   I spend a lot of my time looking at the little critters in my garden.  Like red velvet mites:  there were many in 2016.  Have not seen a single one in two years now. “

Video of Okra Taste Testing

Chris Smith, author of The Whole Okra

Chris Smith, author of  The Whole Okra: Chris has a video of
the taste testing on  Youtube, https://www.youtube.com/watch?v=sAy0pouxlME

Hoophouse Sliding Doors, Vegetable Weevil Larvae – Seasonal Challenges!

September 6 sowing of tatsoi and our August 28 sown catch crop of Tokyo bekana. We harvested the last of this catch crop October 14 and sowed turnips, Photo Pam Dawling

October is the busiest month in our hoophouse! The bed prep, sowing and transplanting keeps us busy for 3 or 4 hours a day. Add in harvesting (peppers, radishes, salad crops) and hand-watering of new plants, and we’re there for a good half of each day. And then there are extra challenges. Yesterday, in tightening one of the strings that mark the bed edges, I managed to hammer a 6” sod staple right through the irrigation main line tubing, which was below soil level. I can hardly believe I did that! I even thought “Be careful not to stab the water pipe!” So I had to dig it up, find a coupler and fix it right away. Because at this time of year, we rely on the irrigation for all the new plants.

And the nights are getting colder. We intend to close the doors every night when the temperature will be below 50F (10C), and the windows if the temperatures will be below 45F (7C). We have been converting the doors at one end from hinged to sliding doors. They’re hanging on their tracks, but one door is jamming in the track, and we need more than a cursory look to fix the problem. So meanwhile, only 3 of the 4 doors close!

One of the new sliding doors on our hoophouse.
Photo Pam Dawling

Hoophouse Doors

My book The Year Round Hoophouse, has a chapter on making end walls, including doors and windows. Writing that helped me decide to change our east doors. Here’s an excerpt from that chapter:

 “For our 30′ (9.1 m) wide gothic hoophouse, we have a pair of hinged double 4′ x 8′ (1.2 x 2.4 m) doors at each end. Our doors open out and have to drag over the grass outside. We have found “rising butt” hinges to be helpful here. As the door opens, it rises on the curved base of the hinge, giving a little extra clearance above the ground. Each door fastens with a hook and eye to the wall when open (it will get windy!).  I recommend considering sliding doors, with the track and hardware on the inside, if the tunnel is wide enough for the track needed to carry the size of doors wanted. This avoids problems in many weathers: rampant grass-growing season, snow season, strong winds. Some people purchase storm doors and use those, but they are not very big. Anyone with basic carpentry skills can make simple door and window frames, as they will be covered both sides in lightweight plastic and not need to be extremely strong.”

View through the old hinged hoophouse doors in a previous December.
Photo Kathleen Slattery
Close up of a roller bracket attaching our hoophouse door to the track.
Photo Pam Dawling
  •  Matt Kleinhenz of the Ohio State University Vegetable Production Systems Laboratory, has a slide show of end wall designs High tunnel end wall and door types images,
  • ·        David Laferney of The Door Garden, in Building Greenhouse Doors,, 2008 has clear photos of hinged door construction for beginner carpenters.
  • ·        Hightunnels.org has various articles on construction, including a slideshow Images of High Tunnel Doors.
  • ·        For more to read, see M D Orzolek, High Tunnel Production Manual. See Chapter 3: High Tunnel Construction

Vegetable Weevil Larvae

A new Tokyo bekana transplant attacked by vegetable weevil larvae October 10
Photo Pam Dawling

Sometimes in the cool weather we have problems with this secretive pest chewing holes in brassica leaves at night. The larvae live in the soil and stay underground or deep in the heart of the plants during the day, so if your leaves are holey, but you can’t find any culprits, you can suspect vegetable weevil larvae. They especially like turnips, pak choy and the flavorful mustardy greens. We sprayed with Spinosad last Monday, then again on Friday, and this week (Monday and Tuesday) I’m not seeing any new holes.

Debbie Roos of Growing Small Farms wrote about this pest in “The Bok Choi Problem” which has good photos and a Description and Biology of the Vegetable Weevil

Vegetable weevil larvae in action. From Growing Small Farms
Photo Debbie Roos

 

Solarization and crop choices to fight nematodes

Solarizing to combat nematodes. Photo Pam Dawling

Solarization

Solarization is a method of killing pests, diseases and weed seeds near the surface of the soil by covering the soil with clear plastic for six weeks or more in hot weather. We use this method to help control nematodes in our hoophouse. Nematodes are only active in warm weather, and we have not had problems with them outdoors, but of course, it’s warmer in the hoophouse!

I’ve written before about solarization to fight nematodes in our hoophouse.

In my Book Review: The Organic No-Till Farming Revolution: High Production Methods for Small-Scale Farmers, Andrew Mefferd, I wrote a little about solarizing:

“Solarization uses clear plastic (old hoophouse plastic is ideal). In a summer hoophouse, solarization can be as quick as 24 hours, Andrew says. When we’ve done this, one of our goals was to kill nematodes and fungal diseases, not just weeds, so we waited a few weeks. Outdoors it takes several weeks. You can see when the weeds are dead. Bryan O’Hara poked a thermometer probe through solarization plastic and found a 50F degree (28C) difference between the outside air and the soil immediately under the plastic; a 10F (6C) difference at 1″ (2.5 cm) deep and little temperature gain lower than that. Solarization does not kill all the soil life!”

Extension offers Solarization and Tarping for Weed Management on Organic Vegetable Farms in the Northeast USA which can, of course, be modified for those of us in other regions.

Solarizing to combat nematodes: Step on a spade to push the plastic down into a slot in the soil.
Photo Pam Dawling

Nematodes

I’ve written here before about our struggles with root knot nematodes in our hoophouse, and you can read everything I know about nematodes in the Year-Round Hoophouse.

My article on nematodes in Growing for Market  in November 2014 describes our discovery of the beasties and our first attempts to deal with them.

My most thorough blogpost about nematodes was for Mother Earth News  Managing Nematodes in the Hoophouse.

Cucumber roots with nematodes (see circles).
Photo Pam Dawling

My post Good news – great hoeing weather! Bad news – more nematodes in the hoophouse August 2014 includes a photo of our first attempt at solarizing – a  bit of a How Not To!

There is info on dealing with nematodes from Garry Ross in Hawaii, where nematodes are a fact of daily life, in my post Cold weather, snow, thinking about nematodes from February 2015.

Cover Crop Choices

French marigolds and sesame to deter Root Knot nematodes in our hoophouse.
Photo Pam Dawling

In June this year I wrote about using marigolds, sesame, Iron and Clay cowpeas as nematode resistant cover crops. We’ve also used winter wheat, and white lupins. See Our Organic Integrated Pest Management . Other cover crops that suppress nematodes include some other OP French marigold varieties (but avoid Tangerine Gem or hybrid marigolds); chrysanthemum; black-eyed Susan; gaillardia (blanket flower, Indian blanket); oats; sesame/millet mix. We decided against sorghum-sudangrass (too big), winter rye (harder than wheat to incorporate by hand), bahiagrass, Bermuda grass (both invasive), castor bean and Crotolaria (sunnhemp) (both poisonous, although newer varieties of Crotolaria have lower toxin levels, and I’ve been rethinking my opposition to using that), partridge pea, California poppy (both require at least one full year of growth) and some obscure vetches that weren’t available locally. We might have included Pacific Gold mustard (B. juncea), if we’d found it in time. Don’t confuse this with Ida Gold Mustard, which kills weeds, and is susceptible to nematodes.

Food Crop Choices

 This list starts with the crops most resistant to Root Know Nematodes and ends with the most susceptible. I’ve included some “bookmarks” between categories, but it can also be read as a continuous list:

Scallions in our hoophouse in late November.
Photo Pam Dawling

Most resistant

Strawberries

Rhubarb

Onion (? not certain)

Corn

West Indian Gherkins

Horseradish

Asparagus

Jerusalem Artichokes

Globe Artichokes

Radishes in our hoophouse in February.
Photo Pam Dawling

Fairly Resistant

Ground Cherry

Some Sweet Potato varieties

Radishes (? not certain)

Rutabagas

Garlic, Leeks, Chives

Cress

Brassica juncea mustards

Brassica rapa var. japonica greens (? Uncertain)

Broccoli, Kale, Collards, Brussels Sprouts

Red Russian kale from Southern Exposure Seed Exchange in our hoophouse in March.
Photo Pam Dawling

Somewhat Susceptible:

Fall Turnips

Peas

Fall Spinach

Swiss Chard

Parsnips

New Zealand Spinach

Very Susceptible:

Lettuce

Cabbage

Cucumbers, Muskmelons, Watermelons, Squash, Pumpkins

Beans, Fava Beans, Soybeans

Okra

Beets

Carrots, Celery

Tomatoes, Eggplant, Peppers, Potatoes, Peanuts

Ruby Streaks, Golden Frills, Scarlet Frills juncea mustards, very resistant to root-knit nematodes.
Photo Pam Dawling

Nematode-resistant winter greens

 We came up with a collection of nematode-resistant winter greens, including radishes, Russian kales, Brassica juncea mustards (mostly salad greens like Ruby Streaks, Golden Frills, Scarlet Frills), and Brassica rapa var. japonica greens, mizuna and Yukina Savoy. We have since learned that Yukina Savoy is a Brassica rapa, not B. juncea as we thought, and that mizuna is Brassica rapa var. japonica with a less certain resistance, or perhaps Brassica rapa var. niposinica, or perhaps B.juncea after all (integrifolia type). We also grow scallions in the nematode-infested areas. Now I am looking for more nematode-resistant cold-weather greens.

Green mizuna in our hoophouse in November.
Photo Pam Dawling

This Year

After the winter greens this spring, we transplanted two beds of tomatoes, one each of peppers, squash and cucumbers, and put two beds into Iron and Clay cowpeas. The eastern ends where we had found evidence of nematodes, we transplanted French marigolds and sesame as stronger fighting forces.

When we pulled up the squash and cucumbers  we found no sign of nematodes on the roots. One of the tomato beds produced no sign either, but the other one did. Our first response was to sow Iron and Clay cowpeas instead of the planned soybeans, but before the plants were even 2” (5 cm) high, we decided to solarize that whole bed. We now have small patches of nematode infestation in almost every bed, calling for a more nimble approach to crop planning.

Brassica juncea mustards to try

According to Wikipedia, Brassica juncea cultivars can be divided into four major subgroups: integrifolia, juncea, napiformis, and tsatsai.  I did some searching for more B. juncea, especially large leafed ones. Some promising looking crops include these:

“Green-in-Snow” mustard, Serifon gai choi type Chinese Mustard, Suehlihung.

Serifon (Suehlihung, Green-in-Snow) mustard. Kitazawa Seeds

“Red-in-Snow” mustard (sorry, no details)

Osaka Purple Mustard. Fedco Seeds

Giant Red, Osaka Purple, Southern Giant Curled Mustards, all quite pungent

 

 

 

 

 

 

Horned Mustard. Wild Garden Seeds

Horned Mustard

 

 

 

Miike Giant mustard. Kitazawa Seeds

Miike Giant

 

 

 

 

Hatakena Mustard. Kitazawa Seeds

 

Hatakena

Yanagawa Takana.
Kitazawa Seeds

 

 

 

 

 

Yanagawa Takana broad leaved mustard

 

 

 

Wasabina baby leaf mustard (wasabi flavor). Kitazawa Seeds

 

Wasabina

Modern Homesteading Interview, Things I’ve Changed My Mind On

Hoophouse squash, tomatoes, and cucumbers.
Photo Alexis Yamashito

In April I did a pleasant phone interview with Harold Thornbro of the Modern Homesteading Podcast  about how year round gardening in a hoophouse can increase yields and the quality of vegetables and extend the growing season.

You can listen to it here:

https://www.stitcher.com/podcast/modern-homesteading-podcast/e/60326060?autoplay=true

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The rest of this post is about the agricultural things I’ve changed my mind on in recent years.

Sowing Leeks
Leek seedlings growing in an outdoor nursery bed.
Photo Kathryn Simmons

The first one that comes to mind is where and how to sow leeks. In Sustainable Market Farming I describe sowing leeks in outdoor nursery seedbeds. We grow leeks for eating from October to March, so even though leeks grow very slowly and need 12 weeks to transplant size, we don’t need to sow them super early in the year. Also because they are so cold-hardy, they don’t need greenhouse conditions. To save greenhouse space, and the bother of watering so many flats, we took to sowing them outdoors. To make this work, you do need weed-free beds. Leeks compete poorly with weeds. Sometimes things went wrong. One year someone decided to “seed-bomb” the fresh bed with poppy seeds. Weeding those tiny leek seedlings was torture! Another time, an overenthusiastic worker ran our new exciting wheel hoe too far onto the bed and eradicated part of a row.

Leek seedlings in flats in April.
Photo Pam Dawling

One year the leek seedling bed wasn’t ready in time to sow, and we sowed rows of seeds in a coldframe, after removing the winter spinach (or maybe we were still growing lettuce in the coldframes then.) This worked well. The next year we tried sowing the seeds in 4” (10cm) deep flats, and putting the flats into the coldframes right away (rather than germinating them in the greenhouse). Still no wasted greenhouse space! On very cold nights, we cover the coldframes, so it was a bit warmer than if we’d just sowed directly into an outdoor bed. The plants grew a bit quicker and we realized we didn’t need to start so early. They were easier to take to the field in the flats, compared to digging up the starts and carrying them in little buckets with water. We had reduced losses of seedlings, so we reduced the amount of seed sown in future years. It’s an easier system, with a more satisfying success rate.

Sunnhemp as a Cover Crop
Sunnhemp cover crop at Nourishing Acres Farm, NC.
Photo Pam Dawling

 Sunnhemp (Crotalaria juncea) is a warm weather leguminous cover crop that I’ve been admiring at various farms in the Southeast in recent years. I’ve been thinking it would be valuable ion our hoophouse and in our gardens. It fights root knot nematodes! I mentioned it recently at a crew meeting, only to be reminded that I previously spoke against growing it as the seed is poisonous! I’d completely forgotten my earlier opinion!

This summer cover crop can grow to 6’ (2m) in 60 days. It thrives in heat, tolerates drought, fixes nitrogen, suppresses nematodes, makes deep roots that pull nutrients from deep in the soil, and it dies with frost. It sounds fantastic, I really want to try it!  It looks a bit like small sunflowers, and according to Southern Exposure Seed Exchange  it  won’t make mature seed above 28 degrees N latitude , so won’t become a self-sowing invasive  in Virginia. Sow in rows 2’-3’ (0.6-1m) apart. If it gets too big, mow when plants reach 5’-8’ (1.5-2.5m) to prevent the stalks from becoming tough and hard to deal with. ¼ lb sows 250 sq ft. (¼ lb = 114 g, 250 sq ft = 23.2m2)

Sowing Sweet Corn
Young sweet corn with a sprinkler for overhead watering.
Photo Bridget Aleshire

I mentioned earlier here, that I’ve changed my mind about the necessity to put up ropes over corn seed rows to keep the crows off. I suppose there are fewer crows these days, sigh. Not needing the ropes makes the benefits of sowing with the seeder greater than the benefits of sowing by hand, so long as we can irrigate sufficiently to get the seed germinated. When we sowed by hand we watered the furrows generously, which meant we did not need to water again until after the seedlings emerged. If we hit a serious drought, the old method could still be best. Overhead watering does germinate lots of weeds, including in the wide spaces between the corn rows, so we need to factor in the extra hoeing or tilling when we weigh up the pros and cons. So, I’m a “situational convert” on this question!

How to Kill Striped Cucumber Beetles
Striped cucumber beetle in squash flower.
Photo Pam Dawling

I wrote about these beasties here. We handpick the beetles in the hoophouse squash flowers, hoping to deal with the early generation and reduce future numbers. One year we had our first outdoor squash bed very close to the hoophouse and the beetles moved there. In desperation I used Spinosad, an organically approved pesticide. It is a rather general pesticide, and harms bees, so I carefully sprayed late in the day and covered the row with netting to keep bees off. It worked brilliantly, taking a fraction of the time that daily handpicking takes. I became a convert to that method, but no one else on the crew did, so we went back to hand-picking.

Pruning Tomatoes
Hoophouse tomatoes in early May
Photo Bridget Aleshire

I used to maintain that life is too short to prune tomatoes, which grow at a rapid rate in our climate, and get fungal diseases, necessitating sowing succession crops. The past couple of years I have removed lower leaves touching the soil, and this year I reduced the sideshoots on our hoophouse tomatoes, which are grown as an early crop here.(We’re about to pull them up in early August, as the outdoor ones are now providing enough). I do think we got fewer fungal diseases, and the diseases started later compared to other years, so I am now convinced that removing the lower leaves is very worthwhile. We also got bigger fruit this year, which logically fits with reducing the foliage some amount.

Is There Such a Thing as Too Much Compost?
Too much compost?? A commercial compost windrow turner.
Photo by Pam Dawling

I used to think the more compost the better. Now I am more aware that compost adds to the phosphorus level in the soil. I wrote about that here. I am not as alarmist as some people about high P in our situation, but I do now think it is worth paying attention and not letting the levels build too fast. I have got more enthusiastic about growing cover crops at every opportunity, and finding legumes to include in cover crop mixtures at every time of year (see above about sunnhemp). I was already a cover crop enthusiast, but as my experience increased, I got my mind round more possibilities.

Using Plastics
Rolling biodegradable plastic mulch by hand
Photo Wren Vile

In my youth I was anti-plastic, If I were growing food just to feed myself, I’d probably still find ways to avoid almost all plastics, but growing on a commercial/professional scale (and getting older) has led me to appreciate plastics. I still don’t want to do plastic mulch, except the biodegradable kind, but I’ve come to accept durable light weight plastics for their benefits. Drip tape saves do much water, reduces weed growth. Plastic pots and flats are so much easier to lift! I do still pay attention and try to make plastics last a long time, and frequently salvage plastic containers others discard. I’m awed by the possibilities of silage tarps or old advertising banners, to keep down weeds without tilling and pre-germinate weed seeds so that when the covers are removed, few weeds grow. This was called by the awkward name of “occultation”, but is now more often referred to in English as tarping.

Hornworms

Lastly, I have a post on Mother Earth News Organic Gardening about hornworms, but you read it here first!

Dealing with Hornworms on Tomatoes

 

A large tobacco hornworm. Note the fake eyes near the horn at the tail end.
Photo Pam Dawling

Two summers ago, I wrote about Hunting Hornworms on Tomato Plants.

Here we are in July again, and here are the hornworms again! Yesterday, in two 80 ft (24.4 m) rows of tomatoes, I found 53 inches (1.35m) of hornworms! There were 24, varying in length from 1” (2.5 cm) to 4” (10 cm). Today I found even more: 42 caterpillars totaling 85” (2.2m)! They are stripping leaves and munching on the green fruit.

In our hornworm photos, you might notice ours are not the same as yours. Ours are tobacco hornworms, not tomato hornworms, but both are bad news and both attack tomato plants. Before Twin Oaks Community started here in 1967, the land was a tobacco farm. Tobacco hornworms have a red (not black) horn, and diagonal white lines, not arrowhead vees.

Hawk moth (mother of hornworms) caught in a web of a zipper spider. Photo Pam Dawling

Hornworms hatch from eggs laid by the night-flying Carolina sphinx moth or  Tobacco hawk moth. This year I did catch one of the moths, and kill it, but we still have plenty of caterpillars. The moths hatch from strange coppery pupae with pipes or spouts attached, which overwinter in the soil. Even our most vigilant caterpillar-hunting seems to miss some, which then drop to the ground to pupate. Another way to break the lifecycle is to close the hoophouse at dusk every night (and open it promptly every morning before it gets too hot), but we’ve decided not to go that route.

Hornworm with parasitic wasp pupae.
Photo Pam Dawling

Hornworms often get parasitized outdoors by a tiny braconid wasp that lays eggs in the backs of the caterpillars. The larvae develop inside the caterpillar and then the pupae develop as white rice-grain-like cocoons sticking out of the back of the hornworm. Usually our friend the parasitic wasp doesn’t come inside the hoophouse and to get parasites into the hoophouse hornworms we have to bring in parasitized hornworms from outdoors. This doesn’t work so well, because the hoophouse tomatoes are a month earlier than the outdoor ones, and the hornworm cycle is well underway in the hoophouse by the time the parasitic wasps are in action outdoors. This year we’ve found several parasitized hornworms indoors in the past few days, and we are very happy.

Hornworms have stripped these tomato leaves.
Photo Pam Dawling

Meanwhile, we are conducting hunting raids every morning. To find where the hornworms are working, first look at the upper leaves of the tomatoes. If they are stripped bare down to the ribs, that’s a good place to look. Hornworms only like the tender upper leaves. If there are intact newer younger leaves, it might mean there was a hornworm, but it’s been removed already, and the plant is recovering. Another sign of hornworms in the area is chewed fruit. Another sign is “pineapple poop” – miniature brown pineapples or hand grenades. If you see fresh poop, look directly upwards – remember the law of gravity. The size of the poop is, naturally enough, in proportion to the size of the hornworm.

Hornworm poop on a tomato.
Photo Pam Dawling
Evidence of hornworms.
Photo Pam Dawling

Having determined there is a hornworm in the vicinity, the next task is to find it. You’d think it would be easy – a big striped caterpillar like that. Not so! They are the exact same shade of green as tomato leaves. Hornworms can look remarkably similar to curled tomato leaves. The white stripes mimic the veins on the undersides of the leaves.

When I find some signs, I gaze at the area, looking for discrepancies in the pattern – bare stems with lumps on them. Usually the caterpillar is on the underside of a chewed stem, and often (but not always) they have their heads raised. When you find one, get a firm grip, pull it off the plant (they have strong legs which hold on tight), drop it on the ground and stomp on it. The skins are quite thick.

Found it! Hornworm on a tomato leaf.
Photo Pam Dawling

If I still can’t see the worm, I stand still and sway a bit from side to side, viewing the plant from different perspectives. It helps if the top of the plant is back-lit, but I do always check both sides of the row, no matter where the sun is. Knowing the signs of hornworm grazing can save you time looking everywhere. Focus your attention on where you are most likely to find them, and you will get the most success in the least time.

A large hornworm eating a tomato.
Photo Pam Dawling

Bt is an organically-approved pesticide spray that kills small caterpillars, without killing other insects. I don’t expect it to work on big hornworms. Hunting seems to be the way to go!

Our goal – tasty ripe tomatoes! These are Five Star Grape.
Photo Pam Dawling

Our Organic Integrated Pest Management

French Marigolds and sesame to combat root knot nematodes in our hoophouse.
Photo Pam Dawling

Organic Integrated Pest Management involves tackling pest problems one step at a time with ecologically-based practices, starting with actions chosen to reduce the chances of the pest ever getting a grip on your crops.

Steps:

  1. Cultivate a good environment for your crops: healthy soil, sufficient space, nutrients and water, suitable temperature, soil pH. Practice crop rotation to reduce the chances of pests and diseases carrying over from one crop to the next. Clear old crops promptly, so they don’t act as a breeding ground for the pest. Choose suitable varieties that resist the pests you most expect.
  2. Cover or protect the plants physically from the pests (mulches to stop soil-dwelling pests moving up into your crops, netting, rowcover, planting diverse crops, and even trap crops)
Young cucumber plant under insect netting in June.
Photo Pam Dawling
  1. Provide habitat for natural enemies and other beneficial insects
  2. Monitor crops regularly at least once a week and identify any pests you see.
  3. Introduce natural enemies of the pest (bacteria, fungi, insect predators or parasites)
  4. Hand pick (or trap) and kill the pests if the pest population is above the action threshold. Many fruit and root crop plants can take 30% defoliation before any loss of yield. Where the crop is the foliage, this may be too much!
  5. Use biological controls (often derived from natural enemies) if the damage is still economically significant after trying the earlier steps in the process.

I recommend the ATTRA online publication Organic Integrated Pest Management.

Each of the 22 pages is a poster, complete with good photos and concise clear info.

Our motion sensor sprinkler and the outer layer of our fence around the sweet potato patch at the end of May. The inner fence was installed later.
Photo Pam Dawling

One of our biggest garden pests is the deer, which are especially fond of sweet potatoes. We use motion-sensor water sprayers initially or in years when the deer pressure is low. For worse years we install an electric fence with a solar-powered charger.  Last year our electric fence didn’t keep the deer out, so this year we have a double layered fence to make sure.

Broccoli bed with alyssum to attract aphid predators.
Photo Pam Dawling

At the other end of the size scale are aphids.  We plant sweet alyssum in our beds of broccoli and cabbage to attract insects that will eat aphids. We sow about 200 plugs for 1500 row feet (450 m) of brassicas planted as two rows in a bed, and pop one alyssum plug in the bed centers every  4ft of bed or about one alyssum per  4 plants. We transplant these the same day that we replace any casualty broccoli and cabbage plants.

Nasturtiums planted in with squash to deter pests. Does it work?
Photo Pam Dawling

We transplant some bush nasturtiums in with our first plantings of cucumber and summer squash. They are said to repel some cucurbit pests such as squash bugs., but I can’t vouch for that. Radishes in cucumber or squash rows are said to repel cucumber beetles and squash bugs. I haven’t tried that. There are a lot of companion planting ideas out there, but most have no scientific evidence for effectiveness.

An insectary circle in early June. The flowers will attract beneficial insects.
Photo Pam Dawling

In late May or early June, we transplant some flowers in our garden to attract pollinators and pest predators. We use circles cut from plastic buckets to surround these clusters of flowers so that inexperienced helpers don’t pull them out as weeds.  We use a combination of sunflowers, dill, borage, cosmos, calendula, tithonia (Mexican sunflowers), zinnias.

Sunflower bee and bug.
Photo by Bridget Aleshire

We also sow sunflowers in our bean beds at each succession. These attract birds and pollinators, while also acting as landmarks for our harvest progress.

In our hoophouse we have been tackling nematodes for several years. This year we have planted the nematode areas in French marigolds and sesame (apparently particularly good in deterring root knot nematodes, the type we have.) Some other nematode areas have been planted with Iron and Clay cowpeas. Unfortunately we now have an aphid infestation on the cowpeas! We are trying blasting the aphids off the plants with a strong stream of water from a hose. Later in the summer we will solarize some of the nematode areas.

We planted Iron and Clay cowpeas to deter nematodes, but got aphids!
Photo Pam Dawling
Flowering sesame in our hoophouse, surrounded by French marigolds. We hope they will fight the root knot nematodes.
Photo Pam Dawling

Squash variety trial for pollination issues and blossom end rot

Golden Glory zucchini in our hoophouse in late May.
Photo Pam Dawling

I wrote about our problems last year with unpollinated squash and our decision to try squash varieties that were less dependent on pollination. So this year we planted some Golden Glory zucchini along with Gentry yellow squash that we had troubles with for several years. The trial is not over, but the early results are promising.

We planted 15 Golden Glory and 25 Gentry and I recorded the number of small rotting squash we removed on 5 dates so far.

Gentry yellow squash in our hoophouse in late May.
Photo Pam Dawling
Date 15 Golden Glory rotted fruit Per plant 25 Gentry rotted fruit Per plant
5/13 2 0.13 12 0.48
5/14 2 0.13 5 0.2
5/17 0 0 32 1.28
5/21 15 1 54 2.16
5/27 9 0.6 39 1.56

The unscientific parts of this trial are

  • we are also removing rotten fruits on other day but not counting them (hey, we’re busy!). But we make sure not to leave rotted squash near the plants and confound the next count.
  • It’s early days still, and we may get different results over the whole of the (short-lived!) hoophouse squash season.
  • Productivity is also important. We won’t be as impressed if we get low yields from the (beautiful) Golden Glories

I’ll keep you posted.Later I learned  that summer squash can get blossom end rot, and wondered if we had that problem rather than a pollination issue.

Is this an unpollinated squash or one with Blossom End Rot? It is shrunken, so I suspect poor pollination.
Photo Pam Dawling

See this excellent article Why Are My Squash Rotting? by Charlotte Glen. Here are some of the points she makes:

  • Blossom end rot is not a disease, but a physiological disorder caused by a shortage of calcium. Many factors can slow the absorption or movement of calcium in the plant, leaving it prone to blossom end rot.
  • Depending on what is stopping calcium from reaching the developing fruits, blossom end rot can be temporary or persistent.
  • Low soil calcium levels are rarely a cause of blossom end rot.
  • Blossom end rot is most usually caused by acidic soil (low pH). In soils with pH below 5.5, nutrients (including calcium) can get chemically locked up and unavailable to the plants (even if abundant in the soil). If low soil pH is the cause of blossom end rot, the problem usually lasts the whole growing season.
  • Too much high-nitrogen fertilizer can also cause blossom end rot. Fast-growing plants often cannot move enough calcium into fruits to support healthy development.
  • Stressors such as unusually cool or hot weather, low nighttime temperatures, drought, or over-wet soil can also trigger BER.
  • Any conditions that cause root damage can lead to poor nutrient absorption and blossom end rot. The most common causes of root damage are wet soils following heavy rain or over-irrigation.
  • Another leading cause of blossom end rot is drought – roots cannot absorb nutrients from dry soil, they need a film of water to convey them.
  • To prevent blossom end rot, the most important thing you can do is to keep the soil evenly moist.
Rotting unpollinated squash. Or some other problem?
Photo Pam Dawling

Here’s how I think our squash stacks up on those factors:

  • Our soil pH and our soil calcium level are both fine, we test the soil every year.
  • Home-made compost is our only source of nutrients in our hoophouse apart from occasional cowpea cover crops. I don’t think we overdid the nitrogen this way.
  • Yes, cool night-time temperatures when we first planted the squash in early April could have been a factor then, but now it’s reliably warm and the problem persists (at least with the Gentry)
  • Root damage? We did transplant these, but carefully. Maybe.
  • Uneven irrigation? We try to run the irrigation every day once it warms up. I don’t think that’s the issue. I don’t think we over-irrigated.
Healthy Gentry squash plants in our hoophouse.
Photo Pam Dawling

Then to help my deliberations, I found this
Squash and Cucurbit Problem Solver with many photos of fruit disorders (also leaf, root, stem, seedling and insect problems)Choanephora RotChoanephora Cucurbitarum is a soft rot of the blossom end, but rapidly developing grey-black fungal spores. I have seen that other times, but it’s not what we have in our hoophouse.Gray MoldBotrytis cinerea is another fungus that enters through the blossom end, leading to yellowed ends with grey furry mold. Bothe these fungal diseases are soil-borne, so more common in fruits touching the soil.Their photo of Blossom End Rot shows margins, and their description says: “The blossom end of the fruit fails to develop normally, turning black-dark brown and eventually shriveling and becoming hard.”Their description of poor pollination is this: “Fruit fail to expand normally, quickly turning brown at the blossom end and falling off the plant.” I’ve seen them fall off sometimes.For further help distinguishing between poor pollination and blossom end rot, I turned to the Garden Mentors (Robin Haglund).  She has helpful photos of these the two cause of problems, and these pointers:

  • Shriveling is due to poor pollination, not blossom end rot. Poorly pollinated fruits become obvious much sooner than fruits with blossom end rot. The fruits start to shrivel and yellow (harder to spot on yellow squash!)
  • If the squash was well pollinated, but has blossom end rot, it will grow plump with a sturdy stem, but rot at the end.

Poor pollination can be exacerbated if you use daytime overhead irrigation when the  female flowers are open. Squash flowers open flowers early in the day and close by early afternoon (and are open for only one day). Daytime overhead watering can discourage bees , leaving  your flowers unpollinated.So, more study needed! We definitely have been having poor pollination (shriveled rotten-ended squash that drop off.) maybe we have some Blossom End Rot too. I’m going to pay more attention! And eat lots of squash!

Date 15 Golden Glory rotted fruit Per plant 25 Gentry rotted fruit Per plant
5/13 2 0.13 12 0.48
5/14 2 0.13 5 0.2
5/17 0 0 32 1.28
5/21 15 1 54 2.16
5/27 9 0.6 39 1.56

Winter Gardening and Farming Conferences and Reading

I’ve just updated my Events page with more details about the events I’m speaking at. Coming up really soon is the Virginia Association for Biological Farming Conference. Here is a flyer for it:

I’ll be offering a half-day (1-5 pm) workshop on Friday 1/11: Year Round Hoophouse Vegetables.

On Saturday 1/12, 2.30 – 4 pm, I’m offering a 90 minute workshop on Cold Hardy Winter Vegetables, and on Sunday 1/13, 4 – 5.30 pm a 90 minute workshop on Crop Planning for Sustainable Vegetable Production.  

I’ll provide handouts for all my workshops, and there will be book signings and sales after each workshop. You can read more details on my Events page or on the VABF website.

I’m having a busy January! The very next weekend, January 17-19 (Thurs – Sat), I’ll be at the 2019 Future Harvest CASAat College Park Marriott Hotel and Conference Center. I’m offering two 90 min workshops, with book sales and signings after the workshops.

Lettuce Year-Round, Friday 1/18 2.00 – 3.30 pm and  Cover Crops for Vegetable Growers, Saturday 1/19 2.00 – 3.30 pm

And then January 23-26, 2019 (Weds to Sat), I’ll be at the Southern Sustainable Agriculture Working Group Conference, Little Rock, AR.

On Thursday January 24, I’m doing a 1 – 5pm Mini-course (4 hours) Hoophouse Production of Cool Season Crops

On Friday January 25, 8 – 9.15 am (75 mins), I’m offering Diversify Your Vegetable Crops.

Book sales and signing Thursday evening.

And then it’s February, and I’m on to the PASA Conference

February 6-9 2019 Pennsylvania Association for Sustainable Agriculture. Lancaster County Convention Center.  I’m offering three 90 min workshops:

Sequential Planting of Cool Season Crops in a High Tunnel, Friday 8.45 am – 10.15 am

Lettuce Year Round, Saturday 8.45 am- 10.15 am

Succession Planting for Continuous Vegetable Harvests Saturday 10.30 am -12 noon.

Handouts, Book-signing, Recordings for sale.

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And if you can’t make it to these events, I have found plenty of good reading on gardening and sustainable agriculture.

Reader Anne Donley sent a link to this fascinating news from the Guardian.com:

Finalists for the Royal Academy of Engineering Africa prize reveal their designs, The team has shortlisted 16 African inventors from six countries to receive funding, training and mentoring for projects intended to revolutionize sectors from agriculture and science to women’s health. Some of the ideas presented include

  • Vertical vegetable growing boxes which grow up to 200 plants and  include a wormery! Wonderful step to more food security.
  • Moisture extraction from the air using silica gels, then solar power to heat up the moisture to provide clean water. Kenyan Beth Koigi  points out that

“There’s six times more water in the air than in all the rivers in the world. With every 1F increase in temperature, water begins to evaporate on the ground but increases by about 4% in the atmosphere, and that’s water that’s not being tapped.”

  • Gloves that translate sign language into speech

  • Smart lockers that dispense medicines.

Growing for Market January issue is out

The Organic Broadcaster is out

Gena Moore at Carolina Farm Stewardship has some great research on the efficacy of various biopesticides.

Modern Farmer is a good online resource for news. They offer a weekly digest via email, or you can go to their site and click on the stories you are interested in. Themes include Farm, Food and Lifestyle, Animal Heroes, News.