Category: hoophouse

Hoophouse Solarization Ponderings

 

Our hoophouse on solarizing day, August 5. Note we kept the big shadecloth over the roof until we had done the work of installing the plastic over the soil. Photo Ken Bezilla

Solarization Benefits We Hope For

I wrote Solarization of Our Whole Hoophouse on 8/12/26. This summer we decided to solarize the whole hoophouse. We hope to reduce fungal diseases in our winter crops, draw down the weed seed bank, and kill lots of nematodes. We planned to solarize for the whole of August (4 weeks). We started that on August 5, and solarized until September 3, a full 4 weeks later.

A One-Month Break from Usual Hoophouse Work

It was a nice break to do nothing in the hoophouse for a month except run the drip irrigation twice a week. We left the drip tape in place so we could water the soil and keep it alive below the hot zone. It was an unearthly sight to just have a sea of plastic in there, and one of the crew popping in there to turn on the irrigation reported that to was quite surreal to walk barefoot on the hot plastic.

We agreed to use the time to do some infrastructure repairs, and did get a new window made and the other one repaired.

We repaired our big all-encompassing shadecloth and reset the grommets to be a better fit with the spacing of the hooks in the baseboards.

We researched horizontal airflow fans to replace our hippy-rigged box fans, but we balked at the $1000 price tag. Maybe EQIP grants would help?

We did our crop planning for the winter and made a map. We cross-checked the schedule against the map and made sure everything fit. We did some perimeter weeding and laid down landscape fabric.

A Bad Choice Narrowly Avoided

We had the notion that if we had thought of it in time, we might have spread mature compost on the beds before spreading the plastic and killed the weed seeds that landed on the compost pile while we ignored it all summer. We got behind with the mowing, and we know grass and spiny amaranth seeded somewhat on to the pile. Fortunately, we didn’t do that! Sterilized compost has most of its microorganisms killed! Instead, we re-composted the weed-seed-infested pile and got a new pile in the meantime.

Hoophouse soil thermometer showing 100F (38C). Photo Ken Bezilla

De-Solarizing Day

On de-solarizing day we allowed a little lead time (not enough!) to disperse some of the 120F (49C) heat by flinging open the doors and windows. The soil temperature was 100F (38C). The peak air temperature during the month may have reached 140F (60C) although no one wanted to linger in there long enough to really study the thermometer. It had been 80F (27C) before we solarized and is back down to that temperature in as I write this in late September.

Normally we leave the big shadecloth in place over the whole hoophouse until mid-September or so, but this year we wanted maximum heating for the solarization, so we removed it a couple of days after solarizing (when it stopped raining).

We had bought 32’x100’ (9.7×30.5m) plastic which was just right for the footprint of our 30’x 96’ (9 x 29m) gothic hoophouse. We unrolled it down the center bed (it was folded in several concertina folds from each edge to the middle). We mimicked that kind of folding to pack it away.

The first step of removing the hoophouse solarizing plastic: folding the edges to the middle. Photo Ken Bezilla

We started by removing the buckets of rocks we had set round the edges to hold the plastic down. Next we folded both sides to meet in the middle. We got old towels to wipe the underside of the plastic dry. We used some of the drying time to teach broadforking and cultivation and raking to new crew members. We noticed how warm the soil was to our hands.

Wiping the solarizing plastic dry between folds. Photo Ken Bezilla

Next we brought the outer folds in to meet in the middle again, and repeated the mopping operation. Later we thought a sponge mop might have been a useful tool. We used more drying time to start to return things we had stored in a shed during Solarization Month.

Removing hoophouse solarizing plastic: second fold of edges to the middle. Photo Ken Bezilla

We repeated the folding until a simple folding in half made a sensible size for rolling, and ended up with a fat roll of plastic tied up with rope. We wrote on it to deter anyone who might think of using that plastic for any other purpose.

The last fold of the the hoophouse solarizing plastic, making it ready to roll once dried. The dried stems are from a buckwheat cover crop we covered with the plastic. Photo Ken Bezilla.

Disadvantages of Solarizing

I was sad to say goodbye to the hoophouse bell peppers as they were the best we’ve had in years. Usually we keep the peppers until October and do our winter crop planning so that the pepper area is the last planted in winter crops, but we made that sacrifice! For several years our peppers didn’t do well, as the salinity of our soil had got too high. We now do salt wash-downs in September and March. This year the peppers were spectacular! Big plants, huge peppers! Rather disappointing to have to pull them at the beginning of August.

Bullnose pepper in our hoophouse June 27. Photo Alexis Yamashita

Several days into the solarizing I realized with dismay that we had killed off all our zipper spiders and probably their egg cases. The spider corpses were still attached to the framework. Zipper spiders seem to be homebodies. Surely they could have found a way to leave? But mostly they do die at the end of summer, just not on open display. In recent years we have not had any of these spiders outdoors, but many in the hoophouse. They do good work consuming the insects they trap.

Zipper spider on the pepper plants in our hoophouse on September 10 a previous year.
Photo Pam Dawling
Zipper spider egg cases hanging from the hoophouse plastic.
Photo Wren Vile

One big disadvantage of our choice was that the work of bed prep and planting in there after de-solarizing was dreadful! We had hot sunny weather, and the allocation of each day’s prep work took 3 hours or more. By lunchtime it was 120F (49C) in there on many days. Exhausting work! But it didn’t seem worth putting the big shadecloth back in place as we normally remove it around September 20.

Hoophouse fall bed prep. Broadforking in plastic sandals is not recommended! Photo Ken Bezilla

Apart from the workers suffering the effects of the heat, the soil also dried out a lot faster, and we had to keep working (no rest!) to break up the big lumps of soil before they became bricks.

We prepped one bed and got ready to sow. We decided to use wire hoops and a 6’ (2m) roll of shadecloth like we do outdoors for lettuce in hot weather. That worked well, although the bed edges did dry out fast, maybe because the paths were still holding heat from the solarizing. So we had to handwater the bed edges daily for a week or so.

Spinach seedlings from pre-sprouted seed in the hoophouse in September.
Photo Pam Dawling

One of the first crops we sow is spinach, using seed pre-sprouted in the fridge for 6 days. No way would it have germinated in that hot hoophouse soil! On sowing day, the spinach seed showed little sign of sprouting, but we sowed, watered well, shaded it, and it came up great! The extra attention paid off. This spinach will feed us all winter until about March, as we just harvest leaves. By September 21, all the early crops in that bed had emerged (scallions, Bulls Blood beets, tatsoi, radishes as well as the spinach). We needed to remove the shadecloth from the bed once everything had emerged, so the seedlings didn’t get leggy. 12 hours of daylight isn’t so much.

The perennial weeds growing in under the baseboards did not die. Once we removed the plastic the wiregrass (Bermuda grass) raced across the edge bed and started colonizing the next bed. Fortunately juvenile wiregrass is relatively easy to pull up.

De-solarizing our hoophouse. Note the vigorous wiregrass along the south baseboard. Photo Ken Bezilla

Most years we grow summer cover crops in the hoophouse, thus keeping the soil-life active and adding organic matter. This summer offered no gain of organic matter, which I do regret.

Advantages of Solarizing

We have sowed an outdoor seedbed with things to transplant into the hoophouse in October when we have more beds prepared. Work is progressing well, although we’ve had to scramble some days to stay on track.  Because using transplants from outdoors is part of our usual plan, we did not have to make any changes to the beds after the first one, om account of high temperatures.

The advantages of solarizing will become apparent in the next 6 months. Two weeks after uncovering the soil we did find a tiny tomato seedling, but the seed could have been dropped from our clothes after the solarizing month, rather than being a survivor. It was interesting to see it.

For myself, I’m unsure if I’d want to do this again. The advantages are going to have to be huge – like no more nematodes. I’ll write more as the next year unfolds.

Success with Growing Lettuce in Winter

 

Ezrilla, a favorite cold-hardy lettuce.
Photo Wren Vile

Lettuce Seed Inventory and Ordering in September

We’re doing a seed inventory and ordering seeds of winter and early spring lettuce varieties, to get us through to the main lettuce growing season starting in early March. By then we will have received our big seed order for the year.

I’ve updated our Lettuce Varieties List, after perusing catalogs and choosing a few promising varieties new to us.

Lettuce Varieties 2027

We’ve decided not to grow romaine lettuces in our hoophouse this winter (and maybe never again), due to finding too many frozen or at least cold damaged stems last winter. We already gave up growing bibbs in our hoophouse some years ago (tip burn and restricted airflow encouraging fungal diseases.). So we’re left with leaf lettuces. We particularly like the Multileaf types, also variously known as One-Cut, Eazyleaf, Salanova, or Cut and Come Again lettuce.

Red Hawk Farm hoophouse planted wall to wall with multileaf lettuces.
Photo Pam Dawling

These produce “heads” composed entirely of small and medium-sized leaves – the outer leaves do not grow very big. They are bred for uniformly small leaves, with more texture, loft and flavor than baby mixes and faster harvesting. Transplanted 6″–8″ (15-20 cm) apart they produce 40% more than baby leaf mixes. (We transplant at 10″-12″ (25-30 cm), to improve airflow, so our production might not be as much as 40% more than baby leaf mixes.)

The full-size plant can be harvested as a head, providing a collection of bite-sized leaves.  But you get higher yields, particularly in winter, if you cut just one side of the plant (or the outer leaves) and let the plant regrow for future harvests. Growing multileaf heads takes 55 days in spring weather, compared to 30 days for baby lettuce.

Lettuce with sclerotinia drop.
Photo Pam Dawling

One of our winter lettuce challenges is sclerotinia lettuce drop, a fungal disease we call Solstice Slime. The Missouri Integrated Pest Management site linked here has lots of excellent photos and info on using an IPM approach to tackling this disease.

https://Image of red and green lettuces including some with Sclerotinia Lettuce Drop. <ipm.missouri.edu/MEG/2016/2/recovered/index.cfm>

Our instructions to ourselves say: “Sow 10 winter varieties, 2′ of each. We want to include Multileaf types. Best slime resistance: Buckley, Ezrilla, Hampton, Brentwood, Merlot, Oscarde, Panisse, Red Tinged Winter, Revolution, Rhone, Tango. Next best: Red Salad Bowl, Salad Bowl. (NO: Hyper Red Wave, Outredgeous, Romaines or Bibbs)” Of our eleven favorite slime-resistant lettuces, eight of them are currently available and in-stock from Johnny’s, High Mowing, or Fedco.

Rhone lettuce.
Photo High Mowing Seeds

Two varieties on our wish list seem to be discontinued now: High Mowing is out of Buckley and Brentwood and won’t be restocking.  They recommend Rhone as a substitute for Brentwood (we grew that one last winter and liked it a lot).  They recommend a new Eazyleaf variety called Ezflor as a sub for Buckley, and we hope to try that.

I have notes from last year to try Frisygo, Rubygo, Ezpark from Johnny’s.

Merlox Red Oakleaf lettuce (resistant to Sclerotinia Lettuce Drop) from Fedco

Merlox Red Oakleaf (resistant to sclerotinia) from Fedco sounds very promising – we hope to try that. It’s not a multileaf type, but any winter-hardy lettuce resistant to sclerotinia, especially a red one, is worth trying.

We’re ordering more Oscarde and Red Tinged Winter seed now in case there’s none available next year. We may have enough, but don’t want to take big risks. Red Tinged Winter is a particularly productive and beautiful variety.

In 2026, we had bad germination on everything except Salad Bowl and Oscarde in our 9/15 sowing. I think this was due to inadequate nursery seedbed care. We’ll try to do better this year.

Lettuce Log

We are getting to the end of sowing lettuces for growing outdoors this year (it’s early September). Those lettuces will be harvested in the second half of November.

Lettuce Log by Pam 2024 onwards

Click the + button to enlarge to readable size!

I’ve just updated my Lettuce Log, which is a work in progress, aiming to provide transplants for continuous lettuce year round for up to a hundred people. We tweaked some of the sowing and transplanting dates in hopes of a more even supply. Of course, every year is different, and all are unpredictable. Part of my strategy for resilience is to sow four different lettuce varieties each time, hoping several will thrive. It’s worked out very well this year. We had a slight shortage in August. Some bolted and we got no food from those ones.

As you can see from the log, the intervals between sowing dates get smaller as the year progresses, starting with 14 day intervals in January and February, decreasing rapidly in March and April to land at 8 days for early May, then 7 days and 6 days until mid-June when we start sowing every 5 days. In mid-August we sow every 4 days, then 3. We no longer grow winter lettuce in our coldframe (as the hoophouse provides a much better environment), so we can take a break from lettuce sowing for the first half of September. In the second half of September we sow for transplanting in the hoophouse and the greenhouse in late October.

Looking next at the column of transplanting dates and the column just before it (Days to transplant from sowing), you can see the length of time it takes a lettuce seedling to grow to transplanting size decreases from 51 days in January/February to mid-20s in May, then 20 or below right through to the end of August. You might well ask why we don’t just start sowing later in January, or even wait till February, and that would be a reasonable question. Probably the later-sown ones would catch up! We’re just enthusiastic to get sowing!

The number of days between transplanting one sowing and the next also decreases from 10 days at the beginning of the year down to 4 days in September. September plantings will each take longer than the preceding one to grow to harvest size.

The Harvest Start and Harvest Finish columns have the least reliable data. I hope to pay more attention to this part in 2027! My general goal is to harvest at least 100 good lettuces (from a planting of 120) every week from late April to the end of November.

The columns to the right include details about when we need to shade the transplants, when we put ice cubes on top of the seedbed, and when we watch out for cutworms. Those are topics for a different time of year.

I wrote about our late summer lettuce transition in August 2025, and about winter lettuce in December 2022. Also about Lettuce All year in a Changing Climate in September 2021. I wrote about our Fall Lettuce Transition in my September 2022 post Preparing your Hoophouse for Fall and Winter.

If you want to start from scratch figuring out sowing and transplanting dates for a continuous lettuce supply in your climate see this post, which includes my slideshow Lettuce Year Round and our graph of succession planting dates.

Solarization of our whole hoophouse

 

Solarization plastic spread across our hoophouse. Next we weighted down the edges with small buckets of rocks. Photo Ken Bezilla

I wrote in August 2019 about Solarization and Crop Choices to Fight Nematodes. Previously we solarized part of a bed in our hoophouse, or one or two beds. I’ve written before about our first efforts at solarization to fight root knot nematodes in our hoophouse. Back then we shoveled soil on the plastic edges to make a good seal between the plastic and the soil. Later we developed a good technique of pushing the plastic edges down into soil using spades, which gives a much better seal, and if you work both sides of the bed at the same time you can avoid wrinkles.

Solarizing a single bed in 2019 using a spade to push the plastic edge into the soil..
Photo Pam Dawling

This summer we decided to solarize the whole hoophouse. We hope to make progress on reducing fungal diseases in our winter crops, drawing down the weed seed bank, and killing lots of nematodes. We plan to solarize for the whole of August (4 weeks). Here’s the story so far.

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 in hot weather. 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!

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!”

Preparation for Solarizing

Last time we changed the plastic, we saved our old hoophouse plastic in one large sheet, for just this option of solarizing the whole hoophouse. Unfortunately, someone used it for some other purpose, and we ended up having to buy new plastic to carry out our plan. Ouch! Label your old plastic very thoroughly! We bought 4 mil Tufflite Nursery Clear plastic from Nolts Greenhouse Supplies, Ephrata, PA (only rated for one year outdoors), thinner than the 6mil we buy for covering the frame. Be aware that shipping can be a big part of the cost, unless you are ordering lots of things.

Clearing hoophouse tomatoes and sunn hemp cover crop.
Photo Pam Dawling

We pulled up our tomatoes and cucumbers at the end of July, a week earlier than usual. The squash had already gone. I was sad to say goodbye to the bell peppers as they were the best we’ve had in years. Usually we keep the peppers until October and do our winter crop planning so that the pepper area is the last planted in winter crops, but we made that sacrifice! For several years our peppers didn’t do well, as the salinity of our soil had got too high. We now do salt wash-downs in September and March. This year they were spectacular! Big plants, huge peppers!

Lady Bell peppers ripening on 7 July.
Photo Pam Dawling

We decided to keep the big shadecloth in place for the solarizing day, expecting it to be very hot. It wasn’t, but it was a wise precaution. We planned to remove the shadecloth the next day, but it was raining and we need the shadecloth to be dry to pack it away. So we waited a couple of days for our little monsoon season to pass. Normally we leave the shadecloth in place until mid-September or so, but this year we want maximum heating for the solarization.

We are leaving the drip tape in place so we can water the soil and keep it alive below the hot zone. If we had thought of it in time, we might have spread compost on the beds before spreading the plastic and killed the weed seeds that have landed on the compost pile while we ignored it since early April. We were stretched thin and got behind with the mowing, and we know grass and spiny amaranth seeded somewhat on to the pile. We did have it partly covered with a tarp.

Unrolling the Plastic

Unrolling plastic to solarize the hoophouse.
Photo Pam Dawling

We bought plastic 32’x100’ which was just right for the footprint of our 30’x 96’ gothic hoophouse. We unrolled it down the center bed (it was folded in several concertina folds from each edge to the middle). We gradually unpacked the folds and pulled the plastic out to the edges. It is important to get as smooth and tight a fit as possible (few airgaps between soil and plastic).

We designated one person to take their sandals off and walk on top of the plastic, while the rest of us gathered small buckets and put some rocks in them, to anchor the edges. This all went very smoothly and took less time than we expected. We used about one bucket of rocks every 20’ down the length. We rolled our painters’ pole that we use for our SnoBrum in the plastic across the west doorway, the direction the strongest winds come from, for add extra security in preventing wind getting under that edge of the plastic. We did not bury the plastic edges in the soil, as that seemed difficult and we figured a large area would suffer less edge cooling/heat escape than a single bed.

Closing Up the Hoophouse for a Month

We closed the doors and windows and relaxed into enjoying a summer month off from harvesting in the hoophouse. We have made a list of infrastructure repairs, landscape care and, of course, the winter crop planning to do in t he next month. And we’ll be irrigating maybe twice a week and checking the soil thermometer periodically. It showed 80F last week, before we solarized.

Hoophouse solarization at Dripping Springs Garden, Arkansas.
Photo Pam Dawling

More Resources on Solarizing

eOrganic’s Organic Vegetable Production Systems Topics list offers publications on plant disease management, weed management and management of soil-borne pathogens using Anaerobic Soil Disinfestation. Here’s a 2025 webinar about On-farm Anaerobic Soil Disinfestation for the Management of Soilborne Pathogens in High Tunnel Production Systems:

 

https://www.youtube.com/watch?v=2pEf26ae6XI

The University of California Solarization site and their Small and Organic Farm Advisor have good information, including some in Spanish and Hmong

More Resources on 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. It’s free for MEN members.

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.

Winter-Kill Temperatures of Cold-Hardy Vegetables 2025 revision

My scheduling program lost my post. Sorry. I won’t use that one again.

Here’s my table of winter-kill temperatures of cold-hardy vegetables (2025 revision). Not a lot has changed in the past year. This is the time of year to plan winter crops and even start them (apart from leeks and parsnips, which need sowing in spring).

I’ve added a few other resources people often want when planning winter crops: Four Ranges of Cold Hardiness; Rowcover Notes; Hoophouse Notes; Cold-Hardy Lettuce.

I compiled this as a handout to accompany my presentation for the Seedtime Winter Garden Challenge event.

Winter Garden Challenge Handout   

© Pam Dawling 2026.  Twin Oaks Community, Central Virginia (zone 7a)

Author of Sustainable Market Farming, 2nd edition and The Year-Round Hoophouse.

SustainableMarketFarming.com

Overwintered Acadia spinach, with weeds, March 16, Pam Dawling

Winter-Kill Temperatures of Cold-Hardy Vegetables 2025 revision

Here are some starting numbers of outdoor killing temperatures of unprotected crops (except where stated). Note that repeated cold temperatures can kill off crops that can survive a single dip to a low temperature, and that cold winds, or cold wet weather can destroy plants quicker than simple cold.  All greens do a lot better with protection against cold drying winds.  Your own experience with your soils, microclimates and rain levels may lead you to use different temperatures in your crop planning. This version includes cover crops.

  • 35°F (2°C): Basil
  • 32°F (0°C): Bush beans, cauliflower curds, corn, cucumbers, eggplant, lima beans, melons, okra, some pak choy, peanuts, peppers, potato vines, southern peas, squash vines, sweet potato vines, tomatoes.
  • 27°F (–3°C): Many cabbages, sugarloaf chicory (takes only light frosts)
  • 25°F (–4°C): Some cabbages, chervil, chicory roots for chicons and hearts, Chinese Napa cabbage (Blues), dill (Fernleaf), some fava beans (Windsor), annual fennel, some mustards and Asian greens (Red Giant, Southern Curled, maruba santoh, mizuna, most pak choy), some onion scallions (many varieties are hardier), radicchio, rhubarb stems and leaves.
  • 22°F (–6°C): Some arugula (some varieties are hardier), Bright Lights chard, some endive, large leaves of lettuce (protected hearts and small plants will survive colder temperatures).
  • 20°F (–7°C): Some beets (Bulls Blood, Chioggia), broccoli heads (maybe OK to 15°F/−9.5°C), some Brussels sprouts, some cabbages (the insides may still be good even if the outer leaves are damaged), celeriac, celtuce (stem lettuce), some collards (Georgia Cabbage Collards, variegated collards), some head lettuce, some mustards and Asian greens (Tendergreen, Tokyo bekana, Tyfon Holland greens), flat leaf parsley, radicchio (both Treviso and Chioggia), radishes (Cherry Belle), most turnips (Noir d’Hiver is the most cold-tolerant variety). Canadian (spring) field peas are hardy to 10-20°F (–12 to –7°C).

    Georgia Cabbage Collards Photo Southern Exposure Seed Exchange
  • Large oat plants will get serious cold damage, and oats seedlings die at 17°F (–8°C).
  • 15°F (–9.5°C): Some beets (Albina Verduna, Lutz Winterkeeper), beet greens, some broccoli, some cabbages (Kaitlin, Tribute), some cauliflower leaves, rowcovered celery (Ventura), red chard, cilantro, some fava beans (Aquadulce Claudia), Russian kales, kohlrabi, some lettuces (Marvel of Four Seasons, Rouge d’Hiver, Winter Density), especially small plants with 4–10 leaves, curly parsley, rutabagas (American Purple Top Yellow, Laurentian), broad leaf sorrel, most covered turnips, winter cress.
  • 12°F (–11°C): Some beets (Cylindra,), some broccoli, some Brussels sprouts, some cabbages (January King, Savoy types), carrots (Danvers, Oxheart), most collards, some fava beans (mostly cover crop varieties), garlic tops if fairly large, Koji greens, most fall or summer varieties of leeks (Lincoln, King Richard), large tops of potato onions, covered rutabagas, senposai leaves – the core of the plant may survive 10°F (–12°C), Purple Top turnips.
  • 10°F (–12°C): Covered beets, Purple Sprouting broccoli for spring harvest, a few cabbages (Deadon), green chard (hardier than other colors), some collards (Morris Heading), Belle Isle upland cress, some endive (Perfect, President), young bronze fennel, Blue Ridge kale, probably komatsuna, some leeks (American Flag, Jaune du Poiteau), some covered lettuce (Pirat, Red Salad Bowl, Salad Bowl, Sylvesta, Winter Marvel), covered Asian winter radish (Daikon, China Rose, Shunkyo Semi-Long survive 10°F/–12°C), big leaves of savoyed spinach (more hardy than flat leafed varieties), tatsoi, Yukina Savoy. Aprovecho Hardy turnip is hardy to 8°F (–13°C) and Ice-Bred White Egg is hardy to 6°F (–14°C), many Even’ Star greens varieties are OK down to 6°F (−14°C), Oats cover crop of a medium size die around 10°F (–12°C). Large oats will die completely at 6°F (–17°C) or even milder.

    Morris Heading collards.
    Photo Kathryn Simmons
  • 5°F (–15°C): Garlic tops even if small, some kale (Dazzling Blue, Winterbor, Westland Winter), some leeks (Bulgarian Giant, Tadorna), some unprotected small lettuces (Winter Marvel, Tango, North Pole, Green Forest), some bulb onions (Walla Walla), multiplier onions if not well-mulched, smaller leaves of savoyed spinach and broad leaf sorrel.
  • 0°F (–18°C): Chives, some collards (Blue Max, McCormack’s Green Glaze, Winner), corn salad (mâche), garlic, horseradish, Jerusalem artichokes, Even’ Star Ice-Bred Smooth Leaf kale, a few leeks (Alaska, Durabel), some bulb onions, mulched yellow potato onions, some onion scallions (Evergreen Hardy White, White Lisbon), parsnips (probably even colder), salad burnet, salsify, some spinach (Bloomsdale Savoy, Long Standing Bloomsdale, Olympia).
  • Walla Walla onions sown in late summer are said to be hardy down to –10°F (–23°C), but I don’t trust below 0°F (–18°C)
  • Crimson clover is hardy down to 0°F (–18°C) or perhaps as cold as –10°F (–23°C)

    Vates kale – our winter outdoor favorite.
    Photo Bridget Aleshire
  • –5°F (–19°C): Vates kale survives although some leaves may be too damaged to use. Lacinato Rainbow Mix kale may survive.
  • –10°F (–23°C) Austrian Winter Field Peas and Crimson clover (used as cover crops).
  • –15°F (-26°C) Hairy vetch cover crop – some say down to –30°F (–34°C)
  • –20°F (–29°C) Dutch White clover cover crops – or even –30°F (–34°C)
  • –30°F to –40°F (–34°C to –40°C): Narrow leaf sorrel, claytonia and some cabbages (January King) are said to be hardy in zone 3. I have no personal experience of this.
  • –40°F (–40°C) Winter wheat and winter rye (cover crops).

Four Ranges of Cold Hardiness

  1. Crops to harvest before frosty weather (32°-25°F) and use or store 
  2. Crops to keep alive in the ground into winter to 22°-15°F (-6°C to -9°C), then harvest to use or store.

Use Soon: Asian greens, broccoli, cabbage, chard, lettuce, radishes

Store: Beets, cabbage, carrots, celeriac, kohlrabi, winter radish (including daikon), rutabagas, turnips

  1. Hardy crops to store in the ground and harvest during the winter. In zone 7, they need to be hardy to 0°-10°F  (-17.8°C to -12.3°C): Collards, horseradish, Jerusalem artichokes, kale, leeks, parsnips, scallions, spinach
  2. Overwinter crops for spring/early summer harvests before the main season: Cabbage, carrots, chard, collards, garlic and garlic scallions, kale, multiplier onions (potato onions), scallions, spinach

    Garlic scallions in April.
    Photo Kathryn Simmons

Rowcover Notes

Rowcovers on hoops will help keep crops marketable, by improving the microclimate for better growth rate. Thick rowcover (Typar, 1,25 oz/sq yd) can keep crops 6F degrees, 3C degrees warmer. Hoops prevent crops freezing onto the rowcover. Polypro outlasts polyester. Consider Quick Hoops and caterpillar tunnels if they suit your climate and crops.

Closing rowcovers after a winter spinach harvest.
Photo Wren Vile

Hoophouse Notes

In a double-layer hoophouse (staying 8F/5C warmer than outside at night) plants can survive 14F/8C colder than they can outside, without extra rowcover; with thick rowcover (1.25 oz Typar/Xavan) at least 21F/12C colder than outside without. Salad greens in a hoophouse in zone 7 can survive nights with outdoor lows of 14°F (-10°C). Lettuce, mizuna, turnips, Russian kales, senposai, spinach, tatsoi, yukina savoy survived a hoophouse temperature of 10.4°F (-12°C) without rowcover, -2.2°F (-19°C) with; Bright Lights chard got frozen leaf stems. We only add rowcover when the outdoor low is forecast to be 8°F (–13°C) or lower.

Hoophouse mizuna harvest in January. Photo Wren Vile

Cold-Hardy Lettuce

Lettuce varieties for a solar-heated winter greenhouse or hoophouse in zone 7a: (hardiest are in bold) Buckley, Ezrilla, Green Forest, Green Star, Hampton, Hyper Red Rumple Waved, Marvel of Four Seasons, Merlot, New Red Fire, North Pole, Oscarde, Outredgeous, Pirat, Red Cross, Red Sails, Red Salad Bowl, Red Tinged Winter, Revolution, Rouge d’Hiver, Salad Bowl, Sylvesta, Tango, Winter Marvel, Winter Wonderland.

Lettuce Mix in our hoophouse 20 December. Photo Pam Dawling

Particularly cold-hardy for outdoors:  Brune d’Hiver, Buckley, Ezrilla, Green Forest, Hampton, Lollo Rossa, Merlot, North Pole, Red Tinged Winter, Revolution, Rouge d’Hiver, Tango, Winter Marvel.

Cold-tolerant outdoor early spring lettuces: Buckley, Crawford, Green Forest, Hampton, Merlot, New Red Fire, Revolution, Simpson Elite, Susan’s Red Bibb and Swordleaf.

Early spring lettuce Ezrilla. This variety also does well in the winter with some protection.

Winter Gardening Challenge

Digging compost into our coldframes for a winter spinach crop.
Photo Wren Vile

Winter Gardening Challenge

The Seedtime annual free Winter Gardening Challenge is just around the corner!
Dates: Sunday July 19-Wednesday July 22 2026

Time: 7:00 PM CDT every evening (with additional sessions at 10am (paid), 12pm and 3pm (free). It kicks off Sunday 7/19 at 7pm Central (8pm Eastern, 6pm Mountain, 5pm Pacific).

If you’re out in the garden right now, sweating through the heat, and you’d love to still be picking fresh food out of your yard once the frost hits, take a look at this.

A free, 4-day, live online Winter Gardening Challenge, hosted by Edwin and Paul Dysinger, the father and son team behind Seedtime, on how to keep a continual harvest of fresh food through the winter. With optional additional paid morning sessions.

Over-wintered Acadia spinach emerging undamaged from under the ice and snow. Ken Bezilla

Over the four days, Paul and Edwin (with co-host Luke Hammond and an awesome line up of guest speakers) will walk you through how to keep fresh food coming out of your garden all the way through winter. Yes, even when there’s snow on the ground, with the right cold-hardy crops and protection to hold off the cold.

And because it’s live, you can ask your real questions and build your winter garden plan right alongside them, so you walk away with an actual plan you can put in the ground, not just a pile of notes.

This year The Winter Gardening Challenge has three of the most respected fall and winter growers around joining them to teach, live:

Melissa K. Norris, the voice behind the Pioneering Today podcast and a fifth-generation homesteader who teaches people to grow, preserve, and cook their own food, will present Your Climate-Based Plan, choosing crops for your location and season, how to choose the right winter growing strategy based on climate + goals + tools available (none, row covers only, tunnels/hoophouse) on Monday at noon Central (1pm Eastern, 11 am Mountain, 10 am Pacific).

Pam Dawling, who wrote the book Sustainable Market Farming and grew food year-round managing a 3.5 acre garden for 25 years. I’m presenting Habits of a Winter Gardener (Temperature thresholds, frost strategies, crop-specific protection) on Tuesday at noon Central (1pm Eastern, 11 am Mountain, 10 am Pacific).

Nicole Burke, founder of Gardenary, who has helped so many people start a kitchen garden that actually feeds their family, is presenting Timing and Crop Selection (Fall & Shoulder Season success) on Wednesday at noon Central (1pm Eastern, 11 am Mountain, 10 am Pacific).

Every one of them is showing up live to help you grow through the winter. You get to be in the room, ask your questions, and learn from people who have actually been doing this year after year.

It’s completely free to join.

Save my spot
Takes about 10 seconds.

Overwintered Acadia spinach, weeds, March 16, Pam Dawling

Grow your best cool season garden ever.
Act fast because this event is coming up right around the corner!
On registration, users have the option to purchase the replays and VIP experience for $19 plus some optional bump and upsell offers.

Cold winter weather damage to our vegetable crops

Over-wintered Acadia spinach emerging undamaged from under the ice and snow. Ken Bezilla

Cold winter weather damage to our vegetable crops

Like people and gardens in many parts of the US, we had a very cold February snap. We had at least two nights down to 2°F (-17°C). In December 2025 we had one night at 13°F (-11°C), one at 14°F (-10°C), most were in the 20s. We had one 20°F (-7°C) night in January (1/4-1/5), one at 10°F (-12°C) (1/15-1/16), one at 7°F (1/23-1/24), then two nights at 3°F (-16° C) (1/26-1/27 and 1/30-1/31). In early February we got at least two nights at 2°F (-17°C). Our weather station was the victim of a hit and run, which broke the pole, so we haven’t been consistent at recording our outdoor temperatures. We need the soil to melt before we can dig a new post hole.

Our garlic beds before the snow Feb 2026. The tops can be seen poking through the hay mulch. Photo by Ken Bezilla
Our garlic beds totally covered in snowcrete February 2026. Ken Bezilla
Our garlic beds after the thaw Feb 2026. The tops are all still alive! Ken Bezilla

Luckily we had thick snow before it got really cold and the snow froze in place, forming what is now called “snowcrete”. Crops completely covered, like our spinach and garlic, seem to have survived. Ones that had tops out in the air suffered much more. Our Tadorna leeks and our Vates kale don’t look much good, but they are not dead.

Tadorna leeks damaged by exposure to cold weather Feb 2026 Pam Dawling
Vates kale exposed to cold above the snow Feb 2026 Pam Dawling

I keep records of how well our crops do in winter, both outdoors and in our double-layer hoophouse. I record this as Winter-Kill Temperatures of Cold-Hardy Crops, and I note each increasingly cold minimum temperature and when the various crops die of cold, to fine-tune our planning for next year.

Winter-Kill Temperatures of Cold-Hardy Vegetables 2021

Cold-hardiness zone map, heat zone map and fruit chill hour requirements

Watching the Weather

We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C). According to https://weatherspark.com our temperature is rarely below 15°F (-9.5°C). Here’s screenshot from their site for Winter 2025-2026 so far.

The winter of 2024-2025 was fairly average in temperature

The winter of 2023-2024 was more like the average temperature, with extra precipitation in December and January.

The winter of 2022-2023 included a spell with a deep dive in temperature in late December, and again a cold March. Precipitation was fairly even.

Reflect spinach in the open got damaged but not killed at -9°F (-23°C), in a previous year Photo Pam Dawling

See the Dec 2022 post How cold can leafy greens and salad crops survive?

The winter of 2021-2022 was colder than the 90th percentile average several nights each month, particularly in November and April, meaning it felt like a long winter.

The winter 2020-2021 was mild, with our lowest temperature being a single late January night at 10°F (-12°C). We had one night at 11°F (-12°C) one at 17°F (-8°C), three at 18°F (-8°C also) and one at 19°F (-7°C). Very little snow or ice, similar to temperatures in the 2019-2020 winter.

The winter of 2018-2019 had lowest temperatures of 6°F (-14°C) in late January 2019, 8°F (-13°C) in December 2018 and a couple of 11°F (-12°C). In early January 2018, we had some extremely cold temperatures of -8°F and -9°F (-22°C and -23°C). Our Vates kale got freeze-killed centers. Averaging our winter low over the four winters 2017-2021 gave 4.8°F (-15°C), within the zone 7a range.

Posts on aspects of winter growing, to help you be better prepared next winter!

Resilience: Survive, thrive and farm another season! Note that Carol Deppe’s website has been taken over since this post. Instead, go to https://www.caroldeppe.com/bio-2016-1-15.html which has links to much of her writing

Success with Spinach for Fall, Winter and Spring

Winter lettuce and other salad crops

The Winter Greens Trifecta: Kale, Collards and Spinach

Success with winter kale and collards

Dazzling Blue kale is beautiful, tasty and cold-hardy!

Dazzling Blue kale.
Photo High Mowing Seeds

Posts on responding to the weather you have, even as our climate changes

What can you do if fall sowings fail?

Signs of winter, signs of spring, includes some phenology (robin migration, crocus, Lenten Rose)

Vegetable Seed Germination Temperatures and Phenology

Winter Preparations for Vegetable Gardens, includes an article Autumn’s first frost is falling later. For farmers, the consequences are wide-ranging by Sarah Vogelsong, November 3, 2020 from the Virginia Mercury.

Growing vegetables in a winter hoophouse

Lettuce in our hoophouse in winter. Pam Dawling

No crops died in our double-layer hoophouse, where we spread inner rowcovers when the outdoor night temperature is forecast to be 8°F (-13°C). We did get some cold damage to lettuce stems and chard stems but not much, considering.

Another aspect of early spring in our hoophouse, is bolting of the overwintered greens. See Success with Asian Greens in Spring. In spring the order of bolting of some Asian greens is: tatsoi, Maruba Santoh, Tokyo bekana, Koji, Napa cabbage, pak choy, Yukina Savoy, Komatsuna, mizuna, leaf radish, senposai. I wouldn’t even try sowing tatsoi after the winter solstice. Yesterday (2/17/26) I was researching bolting in order to find something to sow in some short-term open space we have. From memory I wasn’t sure which of Yukina Savoy and senposai bolted first, so I counted bolting plants in the plots which were sown on the same day. Yukina Savoy had 15 bolters among 67 plants (22%). Senposai had 9 bolters among 220 plants (4%). Then I considered that the senposai grow enormous quickly and I was sowing in the outer thirds of beds that would be interplanted with tomatoes, squash or cucumbers transplanted down the middle on March 15 or April 1. The shorter, smaller Yukina Savoy was my choice. We can eat the thinnings small in salads, and then harvest larger leaves as the plants grow bigger. In milder weather, it takes 21 days to baby size and 45 days to full size. I’m hoping we’re moving into milder weather now!

Yukina Savoy in our hoophouse in late October

Posts on growing vegetables in a winter hoophouse

Planning winter hoophouse crops

Winter hoophouse growing includes a roundup of links to earlier winter hoophouse blogposts

Dealing with Winter Weather in Your Hoophouse

Back-up plans for winter hoophouse crops

Mid-winter hoophouse harvests

Hoophouse tomatoes interplanted with spinach. April photo. Pam Dawling

Pests of the Month in January– Vegetable Weevil Larvae in the hoophouse

Mystery cocoon (not chrysalis!) on Tokyo bekana in our Virginia hoophouse in November. Photo Pam Dawling

Pests in December started with a “Mystery Chrysalis,” and confidently claimed I now knew the difference between a chrysalis and a cocoon. Well! I sent the photo to our Virginia Entomology department and they told me it is a cocoon, not a chrysalis! And indeed, now I know, it is plain to see that this structure is spun fibers, and therefore a cocoon. The shape is not typical of the long oval cocoons I have seen previously. The entomologists told me they thought it was some kind of parasitic wasp. I rushed back to see if I could collect it and save it until the creature hatched out. But I was too late. Something had either already left, or died in the attempt.

My monthly series on pests 

I feature the pests I’m most familiar with here in central Virginia. If you are in a different climate, you can use the Search Box on this site. All the crop chapters in the new second edition of Sustainable Market Farming also have sections about dealing ecologically with pests. If you have other good strategies, please leave a comment on this post.

Pests in October discussed squash bugs, squash vine borers, spinach webworms, squirrels and groundhogs (not insects!)

Insect Pests in September discussed brassica caterpillars, blister beetles and allium leaf miners.

Incidentally, while searching online for a picture of the vegetable weevil larva, I found that many of the images were pure fiction, generated by AI. When my cursor hovered over the bottom left corner of an image I saw the admission. This meant I had twice as many pages to trawl through as I would have if I’d known and had filtered for NO AI. I recommend searching directly in BugGuide.Net or Bugwood Images.

Vegetable Weevil Larvae

Vegetable Weevil. Pest and Diseases Image Library , Bugwood.org

This month I’ll focus on a pest that is a particular trouble in our hoophouse and greenhouse in January: the vegetable weevil larva. Both the larvae and the adults feed on leaves and roots of chosen plants at night, hiding during the day. They eat a wide range of plants: brassicas, tomatoes, potatoes, carrots, beets, beans, many weeds and some ornamentals. They especially like turnips, pak choy and the flavorful mustardy greens.

Vegetable Weevil larva, Debbie Roos, N.C. Cooperative Extension, Chatham County Center

Virginia Tech has a two page Description and Biology of the Vegetable Weevil

This pest, Listroderes difficilis Germain is native to South America and is now found in southern and western parts of the US. The adults are gray-brown 6-8mm (1/4-1/3 inch) splotchy bristly snouted creatures. Newly hatched larvae are slender pale c-shaped grubs with darker heads. Mature larvae are about 14 mm (0.6 inch) long and are chubby whitish grubs with pale brown heads.

Damage by vegetable weevils of Listroderes spp. John C. French Sr., Retired, Universities:Auburn, GA, Clemson and U of MO, Bugwood.org (The light in this picture is a bit purple.)

They 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, or even any frass (insect poop) you can suspect vegetable weevil larvae. Close inspection of the plants may reveal small larvae in the crown of the plant, at the base of the leaves. Often I don’t see any. The feeding holes start as small round holes, but can grow to irregular shapes, and leaves can even become skeletonized. Young plants can be killed by stem girdling.

Stem girdling damage by Vegetable Weevil. Ronald Smith, Auburn University, Bugwood.org

Lifecycle of the Vegetable Weevil

There is only one generation of vegetable weevils each year and adult weevils live 1-2 years.

No known male vegetable weevils exist. The females are parthenogenetic (reproducing young from unfertilized eggs). They lay eggs near the soil line of the plants in late summer, and sometimes early spring. Eggs and larvae develop slowly and the mature larvae pupate underground in the winter, with adults emerging the following spring.

Adult weevils have been known to feign death when disturbed, even dropping off the host plant.

Adults aestivate (it’s the opposite of hibernate) during the summer. They don’t fly much, and so undisturbed crop debris at the field edges is a good off-season location for them. Remember to mow!

Vegetable weevil larvae hiding in bok choi leaves. Debbie Roos, N.C. Cooperative Extension, Chatham County Center

Biological Management of the Vegetable Weevil

Removing crop debris can reduce their numbers. Crop rotation away from susceptible hosts is helpful, as is soil cultivation and tilling after crop harvest.

Natural enemies include ground beetles and rove beetles which search for prey at night. Plant insectary flowers and create habitat to encourage generalist predatory insects.

We have successfully used Spinosad against them. We sprayed with Spinosad on a Monday, then again on Friday, and the next Monday and Tuesday I couldn’t see any new holes.

Neem oil and pyrethrins are other organic control options. Insecticidal soap can be used against the larvae.

Debbie Roos of Growing Small Farms wrote about this pest in “The Bok Choi Problem” which has very good photos.

Resources

Also see resources in earlier Pest of the Month posts

ATTRA, Farmscaping to Enhance Biological Control. 2000. Rex Dufour.

ATTRA, Biointensive Integrated Pest Management. 2001. Rex Dufour

eOrganic Biological Control of Insect Pests Mary Barbercheck, Penn State University

eOrganic Biological Control Options in Vegetable Production  webinar

Organic Pest Management. Debbie Roos, NC State Extension: Growing Small Farms. Photos of beneficial insects.

Insect Pests in December: Wildlife in our Hoophouse

 

Mystery chrysalis on Tokyo bekana in our Virginia hoophouse in November. Photo Pam Dawling

Mystery Chrysalis

Last week (November 18) I found this chrysalis attached to the center of a Tokyo bekana plant. I don’t yet know what it is. If you know, please leave a comment at the end of the post.

I did learn the difference between a chrysalis and a cocoon. In case you, too, are confused, a chrysalis is the quiescent insect pupa of a butterfly. The pupa is the stage between a larva (caterpillar) and an adult. The chrysalis is a tough-ish, smooth, hardened skin of the butterfly pupa, made from shed skin cells of the caterpillar, and its own exoskeleton. Only butterfly species make a chrysalis. When fully grown, the caterpillar makes some silk to fasten its body to a leaf or a twig. This may be a sling, as in Swallowtail butterflies, or a simple attachment and suspension rod as in Monarchs. Next, hanging down from the attachment point, the caterpillar sheds its skin for the final time, and under the old skin is the thicker-skinned chrysalis. When the pupa transforms into the adult it sheds the chrysalis, to make its way out. Most internet searches seem to focus on Monarch chrysalises/chrysalides, which are definitely very beautiful, but not what I was looking at.

Moths make only cocoons. Some other creatures, such as mud dauber wasps, also make cocoons. Moth caterpillars spin a cocoon from silk and wrap it around themselves while they transform via the pupal stage into adult moths. Cocoons are soft, fibrous and usually tan or beige in color.

Buck Moth (Hemileuca maia), aka saturn moth

In October I found a group of buck moths in one corner of our hoophouse. These were new to me and I spent a bit of time identifying them. At first, websites were telling me they were rare, but I soon found out this is not so. Most of our moths flew into webs of the zipper spiders (garden spiders) and ended their days feeding the spiders. Buck moth caterpillars feed on oak leaves and other host plants, but rarely cause much damage to the hosts. Their caterpillars have venomous stinging spines. There is one generation each year. The eggs are laid in the fall, and overwinter and hatch in the spring.

If you get stung by one of these caterpillars, immediately wash the affected area to remove any lingering hairs and venom. An icepack will reduce the swelling, but discomfort may remain painful for a few days.

Imported Cabbage Worm (Pieris rapae), aka imported cabbage white butterfly

Lifecycle of the Imported cabbage worm. Clockwise from top left: adult butterfly, egg, caterpillar, chrysalis. Utah State University

Among our luxuriant Tokyo bekana plants I found all too many of these velvety green caterpillars, feasting on our greens and leaving lots of frass. Numbers are low enough that we can probably hand pick them. The imported cabbage worm is the offspring of the cabbage white butterfly, and you may be able to fend them off by making decoy butterflies. They are territorial and will avoid crowding in one spot. Here’s the link to a template: http://goodseedco.net/blog/posts/cabbage-butterfly-decoy

There are several cabbage caterpillar pests and although they can all be dealt with in the same ways, it’s good to know what you are up against.

Four types of caterpillars that feed on brassicas. Clockwise from top left Imported cabbage worm, cabbage looper, diamondback moth, cross-striped cabbageworm.

Another caterpillar pest of brassicas is the cross-striped cabbageworm (Evergestis rimosalis).  I was quite surprised to see that the adult moth looks so different from the butterfly of the imported cabbage worm. As a child I was told they looked similar. Not so!

Another is the diamondback moth, (Mamestra brassicae). This one also feeds on tomatoes and sunflowers.

Here’s a link to info on the cabbage looper, (Trichoplusia ni), which is pale green with side stripes of yellowy-green.

Beet Webworm (Spoladea recurvalis)

Adult spinach/beet webworm (Mark Dreiling,
Bugwood.org).

We found quite large numbers of this moth (also known as spinach webworm) on our hoophouse senposai last November. As described by Virginia Tech Extension, the adult moths are typically active in late summer. The caterpillars feed on spinach, beets, Swiss chard and amaranth (including pigweeds and lambsquarters).

There’s a nice source of information on Insectidentification.org

Grasshoppers (Melanoplus spp.) and Crickets (Gryllus spp.)

There are still a few grasshoppers in our hoophouse, but the numbers are not damaging. The ones I’m seeing now are Differential Grasshoppers (Melanoplus differentialis, the ones with chevron marks on their thighs), which eat plants. We also have some Field Crickets. Crickets eat plants, especially seeds and fruits, but also insects.

Biointensive Integrated Pest Management

I have written in another post about biointensive IPM, which is a series of steps to manage pests, starting with the actions that cause least damage to the soil or to other species.

  1. Start by trying to prevent pest outbreaks by growing strong plants, caring for the soil, practicing crop rotation, providing habitat for diverse natural predators,
  2. Next, by using physical controls (also called cultural controls) such as removing weeds that are host plants, using rowcover or netting, handpicking the pests if numbers are small enough.
  3. Monitor pest levels and assess if further action is needed. You can do this while hand picking. Many crops can take 30% defoliation without any yield reduction. In general, monitoring should begin at the seedling stage and continue until after harvest.
  4. When pest levels exceed the Action Level, and prevention and physical control have proved insufficient, biological, microbial, botanical and mineral control measures can be used to reduce pest damage of crops to an economically viable level, while minimizing environmental risks. There are four types of sustainable bio-intensive control measures to choose from, starting with the least toxic solution:
    1. Biological control involves either introducing beneficial predators or parasites of the pest species, or working to boost populations of existing resident predators and parasites.
    2. Microbial controls refer to the use of fungi, bacteria, and viruses to kill pests. Bacillus thuringiensis (Bt) and the Entrust formulation of spinosad are acceptable if you have USDA Organic certification.
    3. Botanical control uses plant-based products for pest control. An example is neem oil.
    4. Inorganic (mineral) controls, also known as biorational disease controls, make use of oils and soaps.

 

Soil Test Results: Hoophouse Soil pH

Five Star Grape tomato in our hoophouse. Photo Alexis Yamashita

Soil Test Results: Hoophouse Soil pH

Last week I promised to say more about soil tests, pH and what we are doing to improve our hoophouse soil.

The pH is a measure of how acidic or alkaline (basic) a solution is, assessed on a scale from 0 to 14. It is based on the concentration of hydrogen ions. Think of the “p” as standing for Potential, and the “H” as standing for Hydrogen ions. The lower the number, the higher the concentration of H ions (less Potential left!) and the more acidic. Pure water is neutral (neither acidic nor alkaline), at the midpoint of the pH scale, with pH rated as 7.0. Substances with a pH less than 7 are acidic; those with a pH greater than 7 are alkaline.

Battery acid has a pH of 0; lemons measure about 2, and milk measures between 6 and 7. To dig deeper it is important to understand that the pH scale is a logarithmic scale, meaning that each whole number change in pH represents a tenfold increase or decrease in acidity or alkalinity. A solution with a pH of 4 is ten times more acidic than one with a pH of 5. A pH of 3 is 100 times more acidic than a pH of 5 (10 x 10).

Image from epa.gov

A good place to start learning more about soil testing is

Conventional Chemical Soil Testing in Organic Farming Systems

There you can find an explanation about the value of soil testing and info on choosing a soil testing lab. The one nutrient that is often not assessed is nitrogen, which combines readily within the soil with other materials, making it difficult to measure and interpret results. See Soil Microbial Nitrogen Cycling for Organic Farms for more information.

For information on converting conventional fertilizer recommendations to ones for organic growing, see How to Convert an Inorganic Fertilizer Recommendation to an Organic One from the University of Georgia Extension.

Interpreting Soil Test Buffer pH Reports from Commercial Labs. (Alabama)

Lime recommendations made by soil testing labs use two different soil tests: soil pH and buffer pH. Buffer pH measures the soil’s resistance to changes in pH, which affects how much lime is required to increase soil pH to a target pH (or sulfur to lower it). Soils high in OM and clay need more lime to raise pH than sandy, low OM soils do. For example, a sandy soil with pH 5.0 may need 1 ton of ground limestone to raise the pH to 6.5, while a clay soil with the same pH may need 4 tons. Each soil testing lab has its own buffer solution and tables or formulas used for calculating amendment requirements.

Our hoophouse beds after the nights at 2F (-17C) and 8F (-13C). Looking amazing!
Pam Dawling

Soil Test Recommendations for Virginia

Virginia Tech recommends writing in a crop code number on the form you send in with your sample, even if you will be growing a mix of vegetables next year. This enables them to give a recommendation, which you can modify as needed. You could choose the crop which will occupy the most space in the area, or a representative of the main crop family. For example, for brassicas, choose Cabbage. Their document linked in the title gives the numerical equivalents of the L- to VH ratings for P, K, Ca, Mg results, and soil test calibrations related to soil pH for Zn and Mn. There is also a list of Critical Soil Test Levels for various factors, giving you a heads-up when action is needed to remedy a problem. Also included are fertilizer recommendations for NPK. You will need to get the equivalent information from your local extension office or the organization that did your soil tests, as testing procedures vary.

Dealing with Alkaline Hoophouse Soil

Outdoors, we more often need to make the soil less acidic, so we add lime. Adding sulfur to the soil can make soil more acidic (bring down the pH). This is commonly done before growing blueberries for example.

Blueberry bush with buds in March. Photo Pam Dawling

The Clemson Soil Acidification Calculator lets you enter your data and learn how much sulfur to add. (And if you need a lime calculator instead, to reduce the acidity of your soil, they have a calculator for that too.) Sulfur additions can take many months to show results. For example, to reduce the pH of loamy soil from 7.2 to 6.5, the calculator informs me I would need to add 12.6lbs/1000ft2 of elemental sulfur, and that this is too much to add all at once, so would be best divided into at least two applications with a gap of 2-3 months between applications. They further recommend re-checking the pH before subsequent applications. They recommend only applying S when temperatures are cool (<75°F), and to water in immediately after application.

Elemental Sulfur from Seven Springs Farm Supply, Floyd, VA

Water-soluble products can be a way to bring down the soil pH in hoophouses.

Hoophouse soils can drift towards alkaline over time (partly due to the pH of the irrigation water) and need a little acidification. The pelleted elemental sulfur used outdoors is unsuitable for use in covered spaces that have drip irrigation, as the pellets will not readily break apart.

Magnesium Sulfate (Epsom salts) from Seven Springs Farm Supply, Floyd, VA

Aluminum sulfate (28.1% S) could be used in the above example at 84lbs/1000ft2 dissolved in water. But it can damage plants and is not OMRI listed. Instead, use OMRI listed Epsom salts (magnesium sulfate), which is 12.9% S. Consider that this practice may actually lead to more problems, as the acids used to bring the pH down can add additional salts to the soil, in this case aluminum or magnesium.

Your irrigation water should have a pH of between 5 and 7. Once you know the pH, determine the amount of acid you’ll need to add to neutralize it. For more about fertigation (fertilizer-irrigation), see https://extension.umn.edu/nutrient-management-specialty-crops/soil-health-and-nutrient-management-high-tunnels

Fertilizer injector a Red Hawk Farm.
Photo Pam Dawling

and Fertigation in Organic Vegetable Production Systems

Fertilizers for Hoophouses

See the U of Minnesota Soil health and nutrient management in high tunnels. Hoophouse soils have particular nutrient management needs. The yield potential of hoophouse crops is usually 2-4 times more than outdoors. At higher yield potential, the need for nutrients naturally also increases. Adding organic matter (OM) to your soil as compost can improve its aggregate stability, and nutrient and water retention. Clay soils and those with high OM allow moisture to spread out from the drip emitters and the roots can extend farther. For this reason, all hoophouses need generous amounts of compost added to the soil at planting time. This is particularly important in sandy soils due to low water-holding capacity.

One of many wheelbarrows full of compost we spread every year.
Photo Wren Vile

However, too much compost can cause problems with water retention, especially if compost is left on the surface rather than incorporated, and can lead to high levels of phosphorus accumulation. Incorporate compost and manure into the soil rather than leaving it on the surface. This helps prevent compost from drying out on the soil surface and resisting water absorption. Some soil testing labs report finding granular fertilizers included in hoophouse soil samples submitted to them! Granular products do not get absorbed into hoophouse soil where drip irrigation is used.

Many growers start with a balanced fertilizer (a product containing equal amounts of nitrogen, phosphorus, and potassium). They are often used alongside fertilizers providing only nitrogen, as relying only on balanced fertilizers will result in phosphorus build-up. Use plant-based composts rather than animal-based ones in hoophouses unless your soil is deficient in phosphorus. Even if no animal composts are used, phosphorus can build up.

Nitrogen Fertilizers for Organic Hoophouses

Here are three options for supplying adequate concentrated nitrogen to organic hoophouse crops. All provide readily available nitrogen, but can lead to salt build-up if applied in excess or if not mixed thoroughly into the soil.

  • Feather meal (13-0-0): This insoluble product does not work for fertigation or foliar applications.
  • Bloodmeal (12-0-0) and Fish emulsion (5-1-1): can be applied as a foliar spray, but may clog drip lines if applied via fertigation. Fish meal has a ratio of about 10-6-0. Some forms are water soluble, others not.

Legume Cover Crops for Hoophouses

Crimson clover cover crop with bumblebees.
Photo Bridget Aleshire

In addition to adding fertilizers, consider using cover crops and reducing tillage. Legume cover crops can supply nitrogen without increasing phosphorus levels. Legumes fix nitrogen from the air and gradually release it into the soil when the cover crop residues break down.

The University of Minnesota recommends the following legumes for their climate:

  • Fall planting: Austrian winter peas, hairy vetch
  • Winter planting (before your spring/summer crop): field peas, crimson clover
  • Summer planting: southern peas, crimson clover

Potassium Sources for Organic Hoophouses

Once fruiting starts, the need for potassium increases dramatically. Potassium sulfate or potassium magnesium sulfate are common in organic systems, and you can apply most of the potassium at planting and then side-dress the rest midway through the season. Potassium nitrate and potassium chloride are water-soluble and work well with fertigation systems, but are usually not allowed in organic systems.

Calcium and Magnesium Sources for Organic Hoophouses

If the soil pH is maintained in the optimum range of 6-7, calcium and magnesium are usually in sufficient amounts. Dolomitic lime supplies both Ca and Mg. If the soil pH is above 6, lime is not required, and gypsum (Ca-sulfate) and Ca-nitrate can be used to supply Ca.

Epsom salts (Mg-sulfate) and potassium magnesium sulfate will supply Mg without changing pH. Various sulfate compounds, such as gypsum, Epsom salts, potassium sulfate, and potassium magnesium sulfate, are sources of the secondary nutrient sulfur, and may be the amendment of choice if your soil needs acidification.

During the growing season, determine if your crops have enough nutrients. Learn more about tissue testing for fruit and vegetable growers. Expect to need to side-dress mid-season for long season crops like tomatoes.

 

ATTRA logo

ATTRA Soil Health Indicators and Tests, Table 1. Soil Health Tests

More Resources on Soil Health

eOrganic https://eorganic.org/

GFM Podcast: Episode #148 with Vern Grubinger from the University of Vermont.

Virginia Cooperative Extension Explanation of Soil Tests publication 452-701

ATTRA Soil Health Indicators and Tests

SARE Building Healthy Living Soils for Successful Organic Farming in the Southern Region

SARE Building Soils for Better Crops 4th edition

OFRF Soil Health and Organic Farming Building Organic Matter for Healthy Soils: An Overview

Pasa Soil Health Benchmarks Report 2021

Soil Test Results: Leaf Yellowing, High Salts

Yellowed over-wintered spinach outdoors in spring.
Photo Pam Dawling

What to do with Soil Test Results

Here I’m addressing some of the soil test results from our hoophouse, and of course the same principles apply outdoors too.

This fall we took multiple samples of soil in our 96’ x 30’ (29 x 9 m) hoophouse, because of curiosity as well as two specific concerns. The first concern was that our twice-a-year salt wash-downs maybe weren’t doing a good job of reaching the edge beds. The second was that one particular area of one of the beds produced a patch of poor yellow spinach last winter, and we wondered why. In a normal year we just take one soil sample from the whole hoophouse, mixing several cores. After all, agriculturally speaking, 2,880 square feet (268 m2) is a small area! Last year we didn’t do any soil tests. Normally it isn’t necessary to test every year, but if you think something has changed, it’s a good move.

Soluble Salt Levels

Our salt levels (after our salt wash-down) were reassuring. All were low (230-269 ppm), except the north edge bed which had 474 ppm, medium. The south edge bed had 358 ppm, but that’s still classed as low. “Soluble Salts are not high enough to cause salt injury”. We decided to do nothing right now, except buy a new heavy-duty lawn sprinkler, so we always have a spare on hand.

Why Salt Wash-Downs Become Necessary in Hoophouses

Because hoophouse soil is protected from rain and snow, soluble salts do not get washed down or away, as happens outdoors. Like many hoophouse growers, we use drip irrigation, not overhead watering. This is a much more efficient use of water, although it makes salt build up (salinization) more likely.  Think about salt pans where salt is harvested from seawater: a large flat area is covered with seawater and left undisturbed while the water evaporates in the sun. The (no-till) hoophouse soil has water (including soluble salts) spread through the surface layers. It is left undisturbed. The heat of the sun draws the water up to the surface and evaporates it, leaving salts to build up and damage crops in hoophouses, especially if your soil doesn’t drain well or if you use animal manures or synthetic fertilizers. Salinization can cause symptoms like drought stress, interfere with seed germination, and inhibit plant growth. It can also encourage some pests.

How We Do Salt Wash-Downs

Twice a year, we do a multiday salt wash-down treatment with an oscillating lawn sprinkler. In the fall we time the wash-down for October after we’ve planted the first bed of winter crops, before we start prepping the other beds, or in early November after they are all planted. We don’t want to be broadforking heavy, sticky wet soil! In the spring we do the wash-down in early March before we start transplanting the tomatoes. We don’t want overhead watering on the crops that are prone to fungal diseases.

Here is a 3-slide explanation. Click the bottom of the slide to advance to the next.

Salt Build-up

 

Our lawn sprinkler covers a rectangle about 30 ft (9 m) wide and about 24 ft (7.2 m) long. We flag 4 spots along the center bed, the first 12 ft (3.7 m) from one end wall, and then every 24 ft (7.4 m). In the simple version, we start at the easternmost flag and run the sprinkler for two hours. Then we move the sprinkler to the next flag and run it there for two hours. Doing all 4 flagged spots takes an 8-hour day. Before watering the western spot, we gather all the tools, logbook, tubs of labels, and put them outside in a garden cart (covered with a tarp) for the night.

Next day we start at the west end and once we have moved the sprinkler to the second spot we can return the tools to their accustomed hooks. In the spring we can only manage two days of wash-down, but in the fall we do three days. According to the soil tests, this is enough to give an OK reading on soluble salts.

It is tricky to get the edge beds adequately watered, so we usually run the sprinklers offset one bed either side of the center-line, giving the edge beds a better chance. This year, the south edge bed got as good a reading as the middle beds, and the north bed got a slightly poorer (higher) reading. Our hydrant gives 1” (2.5 cm) of water every hour. You can test with a shallow straight-sided container, such as a tuna can. So the soil gets 4” (10 cm) of water in the spring salt wash-down and 6” (15 cm) in the fall one.

As a guideline, to leach out soluble salts from the top 12” (30 cm) of soil, apply 6” (15 cm) of water to wash down 50% of the salts, apply 12” (30 cm) to wash down 80% of the salts and 24% to wash down about 90% of the salts (California Fertilizer Association, Western Fertilizer Handbook, 8th Ed.)

See Dealing With High Soluble Salt levels in High Tunnels PA

 Soil Nutrient Levels

Our levels of nutrients were all Sufficient, High or Very High, except potassium in one test, which was Medium. Various nutrient deficiencies can cause leaf yellowing and other symptoms. Nothing I saw in the nutrient test results worried me. I’ve written previously about high phosphorus levels. I did note that the phosphorus was highest in the area where we’d had the very yellow spinach last winter (and where the pH was highest). More on this next week.

Causes of Leaf Yellowing (Chlorosis)

I wrote about causes of yellow spinach here in 2019. Here’s a recap:

Yellow over-wintered spinach in spring. Note that it is the outer leaves that are yellow, while the younger inner ones are green. Note that even the veins are yellow.
Photo Pam Dawling

Possible causes of yellow spinach leaves include poor drainage, compacted soil, damaged roots/poor root growth, high soil pH, too much or too little water, too low or too high a temperature, or perhaps cold temperatures followed abruptly by very warm temperatures, 80°F (26.7°C) or greater; nutrient deficiencies or disease. Our roots grew OK, and we don’t tend towards alkaline soil. But this year’s soil test revealed exactly that! The problem area had a pH of 7.2! Other areas within the hoophouse had 6.6-7.1, all buffered to about 6.6. I’ll say more about pH and what we did to improve it in a week or so. Today I’ll focus on salinity and causes of leaf yellowing.

Nutrient deficiencies can occur due to low amounts in the soil or because high soil pH has made those nutrients unavailable. The most common nutrient problem associated with yellowing is lack of iron, but it may also be caused by deficiencies of manganese, magnesium, boron, zinc, or nitrogen. Iron deficiency starts on young leaves and works towards the older leaves (which initially had enough iron, as a transplant). It can occur in water-logged soil. The veins can remain green. Not the problem we have – our older leaves were yellower.

Deficiencies in manganese, zinc or nitrogen develop on older leaves first and then progress upward/inward. Within older leaves, magnesium is transported from the leaf’s interstitial areas to the veins, resulting in yellowing of the areas between leaf veins. This creates a marbled appearance, a typical symptom of magnesium deficiency. Our leaves were yellow all over.

Nitrogen deficiency. Overall yellowing (including veins). The lower, older leaves appear yellow first as the plant moves the available nitrogen to the more important newer leaves. Spinach is sensitive to inadequate nitrogen. Our main suspect.

Soil auger, form and sample box from Virginia Tech and our soil tests results file.
Photo Pam Dawling

Boron deficiency also yellows the leaves and stunts spinach plants. We do tend to run short of boron, and my approach was to add boron before brassicas each winter. We haven’t added any for several years and the only brassicas in these beds were turnips in spring 2017 and fall 2018. I don’t remember if we added boron in 2016/2017, and the records are tucked away.

Spinach is a heavy feeder. Recommendations are to feed with compost tea, manure tea, or fish emulsion when plants have four true leaves. Side-dress with compost tea every 10 to 15 days. Mix 1 tablespoon of fish emulsion and 2 tablespoons of kelp extract per gallon of water; use about one cup per foot (30 cm) of row on a weekly basis until plants are about 4″ (10 cm) tall; then feed twice more before harvest. Add mature compost to planting beds twice each year.

Fusarium wilt or fusarium yellows, Fusarium oxysporum f. sp. Spinaciae (also called spinach yellows) is a fungal disease which infects plant vascular tissues. Fungal spores live in the soil for at least 10-15 years and can be carried by cucumber beetles. We certainly have lots of striped cucumber beetles! But our plants that year were not wilting. Liming to ensure the soil is not acidic can help prevent this disease.

See Identifying nutrient deficiency in plants and Guide to Symptoms of Plant Nutrient Deficiencies

Nematodes (we do have peanut root-knot nematodes) can cause yellow leaves.