Planting Sweet Potatoes

Fresh sweet potato slips for sale from Southern Exposure Seed Exchange.
Photo Pam Dawling

Are you thinking about crops to grow that will feed a crowd next winter? Sweet potatoes are an easy crop to grow, provided your climate gives enough warm days. They store very well at room temperature, for a long time. In mid-late May, we are still eating sweet potatoes we grew last year, and they are delicious! Well grown and cured, sweet potatoes reach their peak in flavor during January and February. One baked sweet potato of 114gm (4 oz) has 185% the RDA of vitamin A, 28% the RDA of Vitamin C, 100% of vitamin E, lots of anti-oxidants, and 160 calories, none from fat.

Order some slips online and get the ground ready. You can order from Southern Exposure Seed Exchange 11 varieties, all organic, in bundles of 6–100. For more varieties but smaller orders, go to Sand Hill Preservation Center, Iowa. Heirlooms, 225 Varieties, all Organic. Some limits on how many you can buy. Another supplier I recommend is the Steele Plant Co, 10 varieties (not Organic) of slips in small (12) and large (500) quantities, good prices, great service. Slips are young shoots with stem, leaves and, usually, some roots. Sweet potatoes are not grown from seed or from chunks of potato.

I have known people grow sweet potatoes in hoophouses if their climate isn’t warm enough outdoors. This can fit with winter use of the hoophouse for greens and roots. My book Sustainable Market Farming contains a whole chapter on growing this crop, including growing your own slips, but it’s too late to start that this year.

Sweet potatoes fit easily into the crop schedule. Planting out comes later than most spring crops. It’s late enough to precede them with a mature cover crop mix (including legumes to the flowering stage), providing all the nitrogen needed. Or add an organic fertilizer this year and plan ahead for next year. Sweet potatoes do not need lots of organic matter, or high fertility levels. Fitting sweet potatoes into a rotation is easy because it is unlikely that you are growing anything else in that family. As with other crops, a three-year gap (or more) before planting sweet potatoes in the same beds helps control disease.

Modern varieties of sweet potatoes grow to a good size in as little as 90 days, so they are not just for the South! The further north you are, the longer the daylight at midsummer and the more photosynthesizing the plants can do.

Sweet potatoes thrive in hot weather and are fairly drought-tolerant. After vining they need little care during the summer (apart from irrigation) until harvest. Their extensive vines smother most weeds, and they have few pest or disease issues. Most of the labor is the harvest in early October, in-between most other intensive harvests of summer and fall crops. I’ll never complain about a crop which has most of the work be the harvest!

Understanding sweet potatoes

 Sweet potatoes are tuberous roots (Ipomoea batatas), related to morning glory, in the genus Ipomoea. Sweet potatoes or sweetpotatoes are native to the tropical regions in central and south America. The root is long and tapered, with a smooth skin, and color ranging from orange, yellow, beige, white, red, pink, violet and purple. Sweet potato varieties with white or pale yellow flesh are less sweet and moist than those with orange, pink or red flesh. The young leaves and shoots are sometimes eaten as greens. Hot weather greens are hard to find!

Sweetpotatoes are not yams, even though they’re often called yams! True yams are a tropical species of tuber (genus Dioscorea). They come from Africa and the Caribbean. Some are huge! They have rough and scaly skin. The flavor is starchy, and usually not very sweet (more like regular potatoes). Sweet potatoes will not even cross with yams.

Sweet potatoes are sometimes mistakenly thought to be a type of potato but they do not belong to the nightshade family, Solanaceae. Sweet potatoes are only distant cousins of the nightshade “Irish” (more accurately Peruvian) potatoes (Solanum tuberosum). Unlike Peruvian potatoes, which have the annual sequence of vegetative growth, flowering and dying back, sweet potato plants continue growing as long as the weather is warm enough. They are frost-tender herbaceous perennials.

A well-established young Georgia Jet sweet potato plant.
Photo Bridget Aleshire

Read more

 I have a Sweet Potato slideshow.

And I have written a lot of other posts on sweet potatoes (mostly about propagating our own slips, or about harvesting). Very little on actually growing them all summer. My posts include:

Starting Sweet Potato Slips

Sweet Potato Propagation and Yields

Transplanting Sweet Potatoes (biodegradable plastic; deterring deer)

Harvesting Sweet Potatoes (2016); Sweet Potato Harvest (2015) and again (2015) and earlier (2014) and in 2013, 2012

What Makes Sweet Potatoes Sprout (during storage)

I also have a blog post for Mother Earth News Organic Gardening blog, about growing sweet potatoes. And I have several articles in Growing for Market, which you can access online if you have a Full Subscription.

The vines of sweet potatoes start running after the roots have made good growth
Photo Bridget Aleshire

When to plant sweet potatoes

Figure out your ideal planting date. Planting is usually done about 2 weeks after the last frost. You need settled warm weather. Heat is vital. The soil temperature should reach at least 65˚F (18˚C) at 4” (10 cm) deep on 4 consecutive days—don’t rush into planting early. Plants set out too early will struggle with skin fungi, and produce uneven yields. Their growth will be stunted. Forget about climate zones—those are about winter-hardiness of perennials. Sweet potato growing is all about warmth and light. Your sweet potato plants can remain ignorant about your winter cold!

We plant May 10–20, between pepper, okra and watermelon transplanting dates. It takes 7–8 weeks to grow your own slips using our method, so buy slips for this year and learn how to save seed stock when you harvest, to grow your own slips next year.

Planning aheadhow many to plant

Decide how much space you want to plant, or how many pounds (tons?) you want to grow. One slip will produce a cluster of 4–10 roots, each weighing 3–17 oz (80–500 g). The yield range is 2.5–6.8 lbs (1–3kg) per plant, 276–805 lbs/1,000 ft² (14–40 kg/10 m²), or 6–17.5 tons/ac

The in-row planting space is 6–18” (wide spacing gives more jumbo roots). We do 15″ (38 cm) as we like to get some jumbos. If unsure, try 12” (30 cm). Climate, spacing, and the length of the growing season all affect yields.

The space between the rows could be 32–48” (0.8–1.2m). Calculate how many slips you’ll need and add 5–10%. For an acre you’ll need around 15,000.

Newly planted sweet potato slips with a motion sensor sprinkler to keep deer away. the wilting of newly-planted slips is common. Don’t despair!
Photo Pam Dawling

Sweetpotato crop requirements

 Sweet potatoes prefer loose, well-drained soil with pH of 5.8–6.2. They will tolerate pH from 4.5–7.5. Enough potassium (K) is important for drought-resistance, but too much K makes them taste bitter. Sweet potatoes do not benefit from high nitrogen (N). They can get plenty from high-biomass cover crops, organic mulch, and soil life.

Once they are established, sweet potatoes are fairly drought-tolerant. Critical times to maintain sufficient moisture are after transplanting and for at least the first 20–40 days while the roots are developing.

Tips to increase sweet potato yields

Compacted, heavy, lumpy soils can result in misshapen, undersized tubers. If you have clay soil or drainage problems, work in lots of compost and make raised beds or ridges 8”–12” (20–30 cm) high. Ridges help heat up the soil and reduce flood damage.

Black plastic or silage tarps set out 3 weeks before planting warm the soil considerably and increase the growth rate. In colder climates, plant under low tunnels of clear plastic. Ventilate in hot weather.

If it’s too cold to plant out your slips when they arrive, keep them indoors, with water covering the roots (remove any newspaper or other packing material). If you are planting in hot dry weather, water the soil first, and keep the roots enclosed in damp or wet compost as you plant.

We like to do two plantings a week apart, using the older slips first, and then do a third session to replace any casualties.

 

Young sweet potato plants with drip tape and ripped plastic mulch.
Photo Pam Dawling

Planting out sweet potato slips

 Prepare your beds or ridges. Get ready with your irrigation system. If you are covering the beds with biodegradable plastic mulch lay it out just before you will plant. Set out stakes and ropes to mark where the rows will be planted, and gather a measuring stick, trowel and watering can.

To grow the biggest roots, plant the slip vertically, which steers the plants to develop roots only from one node. To grow average size roots but many of them, plant slips horizontally 2–3” (5–7 cm) deep, encouraging tubers to develop at several nodes. Ideally, have 3–5 leaf nodes underground and only the tips above the ground—this gives the plants a second chance if a late frost strikes. Many growers have success with slips with few or no roots, but we like a good root system.

If you are using drip irrigation, run it while you plant. If the emitter spacing matches your plant spacing, plant in the damp spots without measuring. Otherwise, use a measuring stick or a double hand-span to get the plants evenly spaced and not waste plants or land, by diverging from your planned spacing. If you have driptape under mulch, feel for where the tape is, and avoid stabbing it with your trowel.

Firm the soil around each plant, so that it is not left sitting in an air pocket, unable to reach water.

If not using drip irrigation, stop every few plants and water from a watering can. Err on the side of too much water on planting day. Keep newly transplanted slips well-watered.

If it turns cold or windy soon after you transplant, you could use rowcover to protect the plants.

This motion sensor sprinkler has been working well to keep deer out of the sweet potato patch.
Photo Wren Vile

Sweet potato development

I usually reckon on the first month after planting being focused on root growth, the next month on vines and the rest on roots. 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. Give 1” (2.5 cm) of water per week, and cultivate to remove weeds. The second month or so is the vine growth stage. The roots begin to store starch and sugar close to the stem base. Cultivate until the vines cover the ground, after which very little weeding will be needed.

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.

There are some pest mammals and insects, afflictions and diseases to watch out for, but usually sweetpotatoes will grow relatively untroubled until harvest time. Do watch out for deer, rabbits, and other rodents, or anything that appears to be reducing the leaf cover substantially. I’ll write more about these issues in a few weeks.

For more info, see ATTRA Sweet Potato: Organic production

North Carolina State University, Pests of Sweetpotato
North Carolina State University Diseases of Sweetpotatoes.

Growing Potatoes

Potato plant emerging in spring.
Photo Kathryn Simmons

This is the second part of a monthly series on potatoes. Last month I talked about Planting potatoes. Now they are in the ground, we turn our attention to growing healthy plants and doing all we can to maximize the yield.

Frosted potato plants May 10., after two nights at 30F and 29F. I hope this didn’t happen to you! If you know a frost is coming, try to get the potatoes hilled, which protects more of the stem and leaves under the soil. This increases the likelihood the plants will be able to grow back.
Photo Pam Dawling

Potato development stages and crop requirements

  1. First the plant produces roots, stems and leaves. This vegetative state lasts 30–70 days. Bigger plants have more yield potential, so the goal for this stage is to produce robust large plants. Vegetative (leafy) growth of potatoes is favored by warm, 80°F (27°C) moist weather, but tuber growth is favored by cooler soil conditions of 60°F–70°F (15.5°C–21°C). This combination can be achieved either by planting in spring, when the soil temperature lags behind increasing air temperatures and is still cool enough for tuber formation, or by adding organic mulches to keep the soil cool if planting in early summer.
  2. Tuber formation (a two-week process) and branching of the stems comes next. All the tubers (potatoes!) that will grow on that plant are formed in those two weeks. The number of tubers produced per plant depends on hours of daylight, temperature and available water in that short period of tuber initiation. Watering stimulates the production of more tubers. 5 gal/yd2 (22.8 l/m2) is a good amount to supply when tuber formation begins. Short day length is optimal, with a night temperature of 54°F (12°C). If temperatures at night are 68°F (20°C), initiation will be reduced; and at 84°F (29°C), will be inhibited. High nitrogen also inhibits initiation. During this stage, leaf growth continues. Flowering can happen too, but it’s not essential, so don’t worry if you get few or no flowers. Hilling adds soil to the stems, encouraging stem growth and providing sites for tubers to form.
  3. Third, the tubers grow larger, but don’t increase in number. When the leaves start to turn pale, the plant has finished its leaf-growing stage and will be putting energy into sizing up the tubers under the ground. Adequate water and nutrients are important during this critical stage which lasts until the plant reaches maturity for that variety, up to 90 days. Try to ensure at least one inch (2.5 cm) of water per week, up until two weeks before harvest. Avoid very uneven watering, or overwatering, as hollow heart could result. The size of the tubers depends on various growing conditions. Two or three weeks after flowers appear (if they do), the baby potatoes will be 1–1.6″ (2.5–4 cm) across. The best temperature is around 65°F (18°C), and I’ve read that potato size decreases by 4% for every Fahrenheit degree (7% per Celsius degree) above the optimal. Spacing is another factor — we got large potatoes one summer because we had poor emergence and therefore wide spacing! The heat of the summer didn’t stop them.
Three flat-tined potato forks to the left, four round-tined digging forks to the right. Flat tines do less damage, usually.
Pam Dawling

Potatoes dug during this tuber-sizing-up period will be “new” potatoes, and not have the thick skins necessary for storage. If you dig your potatoes during this stage (or “snitch” some, leaving the rest of the plant growing), you will be happy short-term, but your final total yield will be less than if you grew all the tubers to full size. If you planned for this, and grew plenty, you will be happy now, and happy later!

  1. Finally, the tops naturally yellow and die. The skins of the tubers thicken, which makes them suitable for storage. No more growth is possible. Sometimes there are reasons to terminate potato growth early – see Early Harvest below for more about this.

Hilling potatoes

Start hilling (pulling soil up over the plants in a ridge) when the plants are 6” (15 cm) tall. Hill again two or three weeks later and two more weeks after that, if the plant canopy has not already closed over, making access impossible. Hilling also provides an opportunity for dealing with weeds, so if possible do this task in sunny breezy weather which won’t let the weeds re-root.

Rows of potatoes after hilling.
Photo Wren VIle

On a small scale, use a rake or standard hoe to pull soil up from the side of the row opposite to where you are standing. If you are sharing the job, one person can work each side of the row at the same time. If you are alone, turn round and work back when you get to the end of the row. Don’t be tempted to twist your arms around and move the soil up the side nearest you. You will damage your body by this distortion of your spine and shoulders!

At the next scale up, use a rototiller with a hilling attachment, or perhaps a wheel hoe with a hiller, if your soil and stamina allows. We have used a semi-manual planting method, making single furrows with our BCS walk-behind tiller, planting by hand in the furrows, then using the tiller again to cover the seed pieces and hill. Nowadays we use a tractor-mounted furrower that can make two furrows in each pass, and disks turned inwards in pairs to ridge the soil.

Alternatives to hilling potatoes

If you can’t hill, you can increase the effective depth of planting by covering the rows with thick straw or hay mulch. This is easiest to do immediately after planting, before the plants emerge. We don’t mulch our spring-planted potatoes because we want the soil to warm up some from its winter temperatures.

Potato plant emerging through hay mulch in early July.
Photo Pam Dawling

When we plant in June, we cover the seed pieces, then hill, then unroll round bales of spoiled hay immediately, like wall-to-wall carpeting. We choose this method to help keep the soil cooler through the summer. In warm conditions, deeper planting, hilling and thick organic mulches all help keep the plants cooler, as does irrigation.

Weed control for potatoes

Potatoes are sometimes said to be a “cleaning” crop, as if they did the weeding themselves. Not so! Any cleaning that takes place is a result of cultivation. As with many plants, the initial growth stage is the most critical for weed control. Hilling in sunny weather can deal with lots of weeds in a timely way, especially if the machine work is followed up by the crew passing through the field hoeing. Organic mulches also reduce weeds. Potatoes later in life produce a closed canopy that discourages more weeds from growing until the tops start to die. Mary Peet reports that potato yields were decreased 19% by a single red root pigweed per meter of row left in place for the entire season.

Flame weeding can be used for potato plants while they are small, as well (as for carrots and beets before emergence as we see here).
Photo Brittany Lewis

In wet weather it can be impossible to hill when you’d like to, and this is where flaming can save the day. Flaming is not an alternative to hilling, but it can be a way to buy time and deal with rampant weeds if the soil is too wet to hill. Potatoes may be flamed at 6″–12″ (15–30 cm) tall. Beyond that, flaming is not recommended. See ATTRA for more on flame weeding. Flaming when the potatoes are less than 8” (20 cm) tall is also an effective control measure for Colorado potato beetles (More on pests next month). Choose a warm sunny day when the pests are at the top of the plants. Flaming can kill 90% of the adults and 30% of the egg masses, according to ATTRA.

Early harvest of potatoes

Sometimes there are reasons to terminate potato growth early. If you need storable potatoes, cut, flame or mow the tops of the plants, and wait two weeks for the skins to thicken up. To test for storability, dig up two potatoes and rub them together, or rub them firmly with your thumb. If the skins rub off, wait a couple of days before trying again. If the skins are strong, go ahead and harvest. You might do this if you have a fast crop turnaround after spring-planted potatoes, such as we used to do when following our spring potatoes with our fall cabbage and broccoli. Another time I’ve brought potatoes to a rapid end was in England, when we got Late Blight. We cut and removed the diseased tops (so no spores went down into the soil), and were able to salvage the potatoes two or so weeks later. Back then, the recommendation was to burn the green tops. This would probably not be recommended these days. It made for a smoky fire we kept going for several days. Terrible air pollution!

For the earliest possible crop in a dry climate (but not the highest yield), plant “old” seed (ones with lots of hairy sprouts) in early spring, hold off on watering until the tubers are marble size, then give a single good watering at 5 gal/yd2 (22.8 l/m2).

While it’s tempting to dig up potatoes early, the yields will be higher if you wait till the tops die.
Photo Kathryn Simmons

Next month I will write about potato pests and diseases.

Root Crops in May

A tidy bed of young carrots in May. Photo Kathryn Simmons

Root Crops to Plant in Central Virginia in May

Potatoes: It’s OK to plant any time in May: See my post on planting potatoes

Sweet potatoes: We generally plant sweet potatoes around May 10, 16″ (40 cm) apart, with 4-4.5′ (1.2-1.4 m) between ridges, allowing 5’ (1.5 m) space at the edges of patch. We install drip irrigation on the ridges when using plastic mulch. It is ideal if the soil temperature is 65°F (18°C) for four consecutive days before planting.  Plant 2-3” (5-7 cm) deep, with at least 2 nodes in ground, and at least 2 leaves above ground.  If the slips are long, plant them diagonally or horizontally, rather than going into a deep vertical hole, where the soil will still be cold. See my post on sweet potato planting

Carrots: We direct sow our sixth bed of carrots in mid-May. We sow our first carrots in mid-February, and then sow every 2-3 weeks after that. Our soils are cold in February, but the seed comes to no harm in the ground, and it’s a job we can get done early. May is our last month for sowing carrots that we know will be sweet. If we need to, we also sow once a month in June and July, but the hot weather impairs the flavor. Our big storage carrot sowing in early August will taste good, because the weather cools as they grow.

Here in central Virginia, zone 7, on a sandy clay loam, we grow Danvers 126, a sturdy open pollinated variety suited to high production of bulk carrots. In the past I have grown Chantenay Red Core (65 days), a blocky variety with a blunt tip, 5″ (13 cm) long and 2″ (5 cm) at the shoulder. It resists splitting, and can deal with clay.

Any decent soil will grow some carrots, but the best ones grow in deep, loose, and fertile sandy loams with good moisture-holding capacity. Old books warn against using manure before carrots as it will make them fork. This refers to uncomposted manure, not to compost. Compost will increase yields, and even reduce the culls with some varieties. (Research by Daniel Brainard at Michigan State University.) Compost not only increases the organic matter in the soil, but also suppresses some diseases and nematodes (which can cause forked carrots).

Thin carrots promptly to get good yields.
Photo Bridget Aleshire

Sow carrots whenever the soil is below 95°F (35°C), so long as you can keep the surface damp. Aim to sow 30 seeds/ft (1/cm), 0.25-0.5″ (0.6-1.2 cm) deep. Some people sow in single rows 8-10” (20-25cm) apart. Others sow in bands 2″ (5 cm) wide, at 8” (20cm) apart, with one length of drip-tape serving two bands in one 16-24″ (40-60 cm) bed. Carrots do well on raised beds, because the soil stays loose and the roots can easily grow deep. Hard rain in the first 3 or 4 days after planting can dry to a crust which could stymie the emergence. To prevent this, if you get heavy rain, irrigate for half an hour each day afterwards until the carrots emerge. Some people use shade cloth to help keep the soil surface moist. There are precision seeders which save you from thinning, but most growers I know use an EarthWay seeder, and then thin. Some people mix inert materials with the seed to help get a spaced stand. Sand at 1 quart (1 liter) to 0.5 teaspoon (2.5 ml) seed per 25’ (8 m) of row, is one recipe, although I worry that sand will destroy the plastic parts of the Earthway seeder before long.  Some people bake old carrot seed to dilute the good new seed.

Carrots do very poorly with competition, so try to start early carrots in a bed that had only light weeds the year before. Later sowings can make use of the Stale Seedbed Technique, where the bed is prepared ahead of time, and one or more flushes of weeds are germinated and flamed or hoed off. We flame weed our carrot beds before the carrots emerge. See the Special Topic below.

Carrots thinned to 1″
Photo Kathryn Simmons

Get to the initial thinning as soon as you can, spacing to about 1” (2.5 cm) apart, weeding at the same time. We usually have someone with good eyesight and hand-eye co-ordination hoe between the rows the day before the hand-weeding. If you are in an area with Carrot Rust Fly (Carrot Root Fly), you will want to remove all thinnings and broken foliage from the field, so you don’t lure the low-flying pest with the wonderful smell of the broken leaves. We do a second thinning, to 3” (8cm) at the stage when the baby carrots can be used for salads. If we get more weeds, we might do another round of weeding before harvesting the full size carrots. If carrots are spaced too widely, they will be more likely to split, and the overall yield will be reduced.

Flats of celeriac seedlings.
Photo Kathryn Simmons

Celeriac: An eye-catching winter storage root vegetable. Celeriac is sometimes called “turnip-rooted celery”. Its flavor is starchier and sweeter than celery, with hints of parsley, and a nutty taste. Celeriac is slow growing, but easy to care for once established.

If you have a long growing season, you could direct sow celeriac 6-7 months before the first fall frost date, for a late fall harvest. Perhaps put a board over the seed row to keep the soil damp and cool until the seedlings emerge. For most of us, we don’t have that much time. So if you didn’t already start some celeriac plants, May is too late for this year. Plan to order seed next winter.

Transplant celeriac when plants are 2.5-3″/6-7.5 cm tall, once the weather seems settled and warm, after your last frost date. If the weather is cold, just wait. Falling apple blossom is a phenology sign that conditions are suitable. We transplant celeriac around May 7 (our last frost is expected April 28). Use rowcover if a cold spell arrives after you have planted them out, or if you know cold weather is likely to return.

Celeriac gets 12”/30 cm spacing, with 4 rows to a 4’/120 cm bed –  that’s about 10”/25 cm between rows. We found closer spacing doesn’t work in our humid climate, as poor air-flow encourages rot. The Virginia climate is actually on the warm side for this crop, it prefers cooler areas, but we have good success if we pay attention at a few critical times. Celeriac requires long steady growth, so the task of the grower is to prevent checks to growth (such as weeds!). It can tolerate frost quite well, so there is no hurry to harvest in the fall. It can benefit from side-dressing with compost during the growing season, or giving seaweed as a foliar spray. A pH of 5.8-6.7 is ideal.

Root Parsley: A less well-known member of the umbelliferae family, also known as Parsley root, Hamburg parsley, Dutch parsley and turnip-rooted parsley. The flavor is a cross between carrot, celery and parsnip.  Like celeriac, it is slow to germinate and slow to grow. 70-90 days to maturity from direct sown seed.

Root Crops to Harvest in Central Virginia in May

Young beet plants
Photo Wren Vile

Beets: We like the long Cylindra/Formanova/Forono ones which are 6’’ (15 cm) long, very tender and are easily cut into regular slices, for pickles or cooking (55 days to maturity, OP). Among round ones we like Ace (50 days to maturity, F1 hybrid), and Detroit Dark Red (60 days, OP). Detroit Crimson Globe is said to maintain better flavor in hot weather than most others, which can develop off-flavors.

Young bunched beets can be stored for 10 days at 32°F (0°C) and 95% humidity. Mature beets can be stored for 6 months or more at 32°F (0°C) and 95% humidity. Trimmed beets keep well in perforated plastic bags under refrigeration.

Carrots: Our first carrots (sown in mid-February) will be ready to harvest in early May. Harvesting carrots barely needs describing. You need to loosen the soil to the depth of the carrots, pull them out, trim and wash. Some growers remove the tops first, but then it can be harder to remove the roots from the ground. Some people store them without washing, but then cleaning them is harder than if done before the soil dries on them. Carrots store very well in a refrigerator, in a perforated plastic bag. If you have lots to store, it is best to sort them, ensuring no scrawny ones or damaged ones get stored. Don’t store with apples or other fruit, or large amounts of cut flowers, or sprouting crops. The ethylene these crops give off can spoil the taste of carrots, removing the sweetness and leaving them tasting a bit soapy.

Harvested kohlrabi.
Photo McCune Porter

Kohlrabi: An unusual vegetable, sure to attract attention and be a discussion piece. It is tender with a flavor between a cabbage and a turnip. This tasty, crunchy root-like vegetable is easy to grow and doesn’t wilt as soon as you harvest it. It is actually the swollen stem, rather than a genuine root, but it behaves like a root vegetable. It can be eaten raw (sliced or grated) or cooked. The kind most commonly grown is a pale green or purple globe with long-stemmed leaves. When the leaves are cut off leaving stubs it resembles a sputnik. In addition, the leaves are also edible.

Conditions for growing kohlrabi are much the same as for other brassicas. It does best in cool weather. In our zone 7 climate, kohlrabi, like other brassicas, can be grown in spring or fall.

Harvest when the kohlrabi are 2-3” (5-7.5 cm) in diameter or even up to softball size. If left growing for too long the swollen stem becomes woody. Cut them from the ground with a sturdy knife. The base of the globe can be quite fibrous, so cut either the wiry root just below the soil surface, or cut higher, leaving a small disc of the globe behind, attached to the root. Snip or lop off the leaves, perhaps leaving a small top-knot if the kohlrabi will be sold immediately. We harvest in spring from around May 10 to June 30, and in the fall from October 20 to November 15.

Kohlrabi stores well in perforated plastic bags in a walk-in cooler, offering flexibility about when it is used, which is always an advantage.

Radishes: Early May brings an end to our spring radishes. Our last sowing is April 10. After then it is too hot for radishes where we are. In July or early August we sow winter storage radishes, including daikon.

Turnips ready for harvest.
Photo Wren Vile

Turnips:  A reliable root vegetable in the brassica family. They are among the fastest growing crops other than leafy greens. In zone 7, we sow a small crop of turnips outdoors under rowcover March 15, or earlier if spring is mild. Although they grow best in cool weather, turnips have no trouble germinating at high temperatures, as when grown for a winter storage crop.

Turnips are also available in gourmet varieties, to be eaten small, young and tender, 35-50 days after sowing, up to 2” (5 cm) in diameter. The delicious F1 hybrid Hakurei, 38 days, a smooth white flat-round shape, with crisp sweet flesh, and hairless leaves, is the most famous of the gourmet varieties. Although best harvested small, they do retain quality for a short storage period.

Young turnips can be pulled, banded, washed and sold with tops intact. Prompt cooling is important to keep the leaves from wilting. Small spring turnips can be pulled by hand, without digging – ours are ready May 20, and we clear the last of them in early June, refrigerating them till mid-July if we have enough.

For manual harvest, loosen the roots with a digging fork as needed, then pull. Trim tops and tails in the field (or move to the shade if it’s hot). All foliage should be removed for successful long term storage. Cut cleanly between the leaves and the root. Then wash, drain and store. Prompt washing before the soil dries on the roots will make them easier to clean.

Storage in perforated plastic bags under refrigeration works well for us. Turnips will keep for about 4 months at temperatures close to freezing and humidity of 90-95%. Higher humidity will make them rot.

Cut and damaged roots do not store well. If you haven’t enough humans to feed them to, but you have milking animals, you could chop them (to prevent choking) and feed them to your livestock. Even moderate quantities will not flavor the milk.

Other Root Crops Tasks in Central Virginia in May

See my post next week on growing and hilling potatoes, including alternatives to hilling for wet conditions. Flamers are intended to kill small weeds, not big ones, but we successfully used our wand-type flamer to kill weeds in the potato patch one spring when it was too wet to hill the potatoes.

Special Root Crop Topic for May in Central Virginia: Flame-weeding

When we sow carrots, we sow about 12″ (30 cm) of beet seeds at one end of the bed – these are “Indicator Beets”. When the beets germinate, we know the carrots will be up the next day and today is the time to flame weed the carrot beds. Flame-weeding is a great way to get rid of millions of fast-growing weeds and leave the field free for the slow-growing carrots. We still have to weed and thin once or twice as the carrots (and weeds) grow, but it is much easier to see the carrots, and they grow better if the first flush of weeds has been flamed off.

Soil thermometer in a beet bed.
Photo Bridget Aleshire

As well as beets, I use a soil thermometer and a chart of days to germination of carrots and beets at various soil temperatures. The table shows that beets are always a bit quicker than carrots in germinating. This information is in Sustainable Market Farming, Knott’s Vegetable Growers’ Handbook and Nancy Bubel’s New Seed Starter’s Handbook.

Days to Germinate 50°F (10°C) 59°F (15°C) 68°F (20°C) 77°F (25°C) 86°F (30°C) 95°F (35°C)
Carrots 17.3 10.1 6.9 6.2 6.0 8.6
Beets 16.7 9.7 6.2 5.0 4.5 4.6

Figure out which day you will probably need to flame. As soon as you see the red loops of the indicator   beet seedlings breaking the surface, flame the carrots. (But look for carrots too, just in case). In summer we flame carrots on day 4 after sowing, because we have found that carrots can emerge on day 5 in summer temperatures. One snag we hit once was that the carrots were mistakenly sowed an inch deep, instead of near the surface. Of course, this delays emergence, so by the time the carrots made it through that inch of soil, many new weeds had sprung up too.

Once you get over the hesitation about using a fiercely hot propane burner, flame weeding is quick and easy. And boy, it saves so much hand weeding! We use a Red Dragon backpack flame weeder (without the backpack frame).  We use the hand-held flamer attached to a propane cylinder that is in a wheelbarrow pushed by a second person behind the first. This person also acts as a safety monitor, looking out for unwanted things (like hay mulch burning). Some growers mount the propane on a backpack frame, and work solo, but we prefer to include a second person (and in this picture, a third!).

Flame weeding a carrot bed.
Photo Kati Falger

The operator walks along the aisle between beds, and wafts the wand diagonally back and forth across the bed. It takes about 10 minutes for a 100’ (30 m) bed. Flame weeding can reduce hand-weeding to one hour/100’ (30 m). Flame weeding plus stale beds 3 or 4 times can reduce hand weeding to 6 minutes/100’ (30 m).