Winter-Kill Temperatures of Cold-Hardy Vegetables 2021

Our pond iced over.
Photo Ezra Freeman

Winter-Kill Temperatures of Cold-Hardy Vegetables 2021

I keep records of how well our crops do in the colder season, both outdoors and in our double-layer hoophouse. I note each increasingly cold minimum temperature and when the various crops die of cold, to fine-tune our planning for next year. We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C).

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). Averaging our winter low over those four winters 2017-2021 gives 4.8°F (-15°C), within the zone 7a range.

Georgia Cabbage Collards, good down to 20F (-7C) Photo Southern Exposure Seed Exchange

New Info this winter

I’ve added in some temperatures for collard varieties (Georgia Cabbage collards, McCormack’s Green Glaze, variegated collards) from the Heirloom Collards Project, and also gained some info on spinach (Long Standing Bloomsdale), kales (Rainbow Mix Lacinato) and mustards (Chinese Thick-Stem) from Southern Exposure Seed Exchange. I’ve added in their suggestions on cold-tolerant early spring lettuces, Crawford, Simpson Elite, Susan’s Red Bibb and Swordleaf.

My results from other years still hold up.

Swordleaf lettuce on the right with another lettuce and radishes in spring.
Photo Bridget Aleshire

Using the List

Unless otherwise stated, these are killing temperatures of crops outdoors without any rowcover. All greens do a lot better with protection against cold drying winds. Note that repeated cold temperatures can kill 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. Crops get more damage when the weather switches suddenly from warm to cold. If the temperature drops 5 or more Fahrenheit degrees (about 3 C degrees) from recent temperatures, there can be cold damage. The weatherman in Raleigh, NC says it needs 3 hours at the critical temperature to do damage. Your own experience with your soils, microclimates and rain levels may lead you to use different temperatures in your crop planning.

Reflect spinach in the open got damaged but not killed at -9F.
Photo Pam Dawling

Outdoor killing temperatures of crops (unprotected unless stated)

35°F (2°C):  Basil.

32°F (0°C):  Bush beans, some cauliflower curds, corn, cowpeas, cucumbers, eggplant, limas, melons, okra, some pak choy, peanuts, peppers, potato vines, squash vines, sweet potato vines, tomatoes.

27°F (-3°C): Many cabbage varieties, Sugarloaf chicory (takes only light frosts).

25°F (-4°C): Some cabbage, chervil, Belgian Witloof chicory roots for chicons, and hearts, Chinese Napa cabbage (Blues), dill (Fernleaf), some fava beans (Windsor), annual fennel, some mustards (Red Giant, Southern Curled) and Asian greens (Maruba Santoh, mizuna, most pak choy, Tokyo Bekana), onion scallions (some are much more hardy), radicchio, rhubarb stems and leaves.

22°F (-6°C): Some arugula (some varieties are hardier), Bright Lights chard, endive (Escarole may be a little more frost-hardy than Frisée), 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), some cauliflower varieties, celeriac, celtuce (stem lettuce), some collards (Georgia Cabbage Collards, variegated collards), some head lettuce, some mustards/Asian greens (Tendergreen, 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).

Large oat plants will get serious cold damage. Oats seedlings die at 17°F (-8°C)

Canadian (spring) field peas are hardy to 10-20°F (-12 to -7°C).

Ruby chard, good down to 15°F (-9.5°C). hardier than Bright Lights, but less hardy than green chard varieties.
Photo Kathryn Simmons

15°F (-9.5°C): Some beets (Albina Verduna, Lutz Winterkeeper), beet leaves, some broccoli and cauliflower leaves, some cabbage (Kaitlin, Tribute), covered celery (Ventura), red chard, cilantro, fava beans (Aquadulce Claudia), Red Russian and White Russian kales, kohlrabi, some lettuce, especially medium-sized plants with 4-10 leaves (Marvel of Four Seasons, Olga, Rouge d’hiver, Tango, Winter Density), curly leaf parsley, rutabagas (American Purple Top Yellow, Laurentian), broad leaf sorrel, most covered turnips, winter cress.

12°F (-11°C): Some beets (Cylindra,), some broccoli perhaps, some Brussels sprouts, some cabbage (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, some turnips (Purple Top).

10°F (-12°C): Covered beets, Purple Sprouting broccoli for spring harvest, a few cabbages (Deadon), chard (green chard is hardier than multi-colored types), some collards (Morris Heading can survive at least one night at 10°F), Belle Isle upland cress, some endive (Perfect, President), young Bronze fennel, Blue Ridge kale, probably Komatsuna, some leeks (American Flag (Broad London), Jaune du Poiteau), some covered lettuce (Pirat, Red Salad Bowl, Salad Bowl, Sylvesta, Winter Marvel), Chinese Thick-Stem Mustard may survive down to 6°F (-14°C), covered winter radish (Daikon, China Rose, Shunkyo Semi-Long survive 10°F/-12°C), Senposai leaves (the core of the plant may survive 8°F/-13°C), large leaves of savoyed spinach (more hardy than smooth-leafed varieties), Tatsoi, Yukina Savoy.

Oats cover crop of a medium size die around 10°F (-12°C). Large oat plants will die completely at 6°F (-17°C) or even milder than that.

Garlic shoots poking through the mulch in January. Survive down to 5°F (-15°C), and if killed, will regrow from underground.
Photo Pam Dawling

5°F (-15°C): Garlic tops even if small, some kale (Winterbor, Westland Winter), some leeks (Bulgarian Giant, Laura), some bulb onions, potato onions and other multiplier onions, smaller leaves of savoy spinach and broad leaf sorrel. Many of the Even’ Star Ice Bred greens varieties and the Ice-Bred White Egg turnip are hardy down to 6°F (-14°C), a few unprotected lettuces if small (Winter Marvel, Tango, North Pole, Green Forest).

0°F (-18°C): Chives, some collards (Blue Max, Winner, McCormack’s Green Glaze), corn salad (mâche), garlic, horseradish, Jerusalem artichokes, Even’ Star Ice-Bred Smooth Leaf kale, a few leeks (Alaska, Durabel, Tadorna); some bulb onions, yellow potato onions, some onion scallions, (Evergreen Winter 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)

-5°F (-19°C): Leaves of overwintering varieties of cauliflower, Vates kale survives although some leaves may be too damaged to use. Lacinato Rainbow Mix kale may survive this temperature.

A cover crop mix of winter rye, hairy vetch and crimson clover.
Credit Kathryn Simmons

-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 cabbage 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).

Hoophouse Notes

Winter crops snug in our hoophouse in a December snowstorm.
Photo Pam Dawling

Our double-plastic hoophouse keeps night time temperatures about 8F (4.5C) degrees warmer than outdoors, sometimes 10F (5.5C) degrees warmer. Plus, plants tolerate lower temperatures inside a hoophouse. The soil stays warmer; the plants recover in the warmer daytime conditions (it seems to be the night+day average temperature that counts);

In the hoophouse (8F (4.5C) degrees warmer than outside) plants without extra rowcover can survive 14F (7.7C) degrees colder than they could survive outside; with thick rowcover (1.25oz Typar/Xavan) at least 21F (11.6C) degrees colder than outside.

For example, salad greens in our hoophouse can survive nights with outdoor lows of 14°F          (-10°C). Russian kales, lettuce, mizuna, senposai, spinach, tatsoi, turnips, 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.

Lettuce Notes

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 Rumpled Wave, 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.

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

Notes on Chicories and Endives

Verona Red radicchio, hardy to about 20°F (-7°C).
Photo Southern Exposure Seed Exchange

Chicories and endives fall into two groups, but they are confusing because the common names sometimes suggest the opposite group than they are botanically. Here’s the best info I have.

Cichorium intybus, commonly called chicories, are mostly heading crops. The group includes radicchio, both Treviso and Chioggia – hardy to about 20°F (-7°C). Belgian Witloof endive (the kind for forcing chicons) is also a chicory. It dies at 25°F (-4°C). Sugarloaf chicory is the least hardy chicory, and dies at 27°F (-3°C).

Cichorium endivia, commonly called endives, are mostly loose-leaf crops, less cold-hardy than intybus types (chicories). This group includes Frisée types and escaroles, which are also known as Batavian endives. They generally survive down to 22°F (-6°C), although Perfect and President endives can survive down to 10°F (-12°C) – can anyone confirm or deny this?

© Pam Dawling 2021


Go to my Events Page for more information on these online events

Mother Earth News Fair Online: Food Independence Course Part Two was released on 3/26/21.

It consists of eight video presentations, most of which come with pdf handouts. My contribution is Growing Asian Greens, and pairs nicely with Guide to Asian Vegetables with Wendy Kiang-Spray, author of The Chinese Kitchen Garden: Growing Techniques and Family Recipes from a Classic Cuisine. Other topics include Dandelion Wine, Homemade Teas, Food Conversations, Passive Solar Greenhouse Design, Productive Growing from Home, and Growing Your Own Spices.

You can subscribe to the All-Access Bundle for $2.99/month (or $35 for a year).

My previous contribution is an 8-part Garden Planning Course

I also did a workshop on Winter Cover Crops for Gardeners as part of the Winter Gardening Course.

All these and many more videos and handouts are available as part of the All-Access Bundle.

 

Cooking Greens in November

A cabbage, with curled back leaf on the head, showing maturity.
Photo Bridget Aleshire

Cooking Greens to Harvest in Central Virginia in November

Beet greens – we get our last chance for greens as we harvest all our beets for storage. Sometimes the greens are in too poor shape to eat. Beets are hardy down to 15-20°F (–7 to –9.5°C) outside without rowcover.

There’s also cabbage, chard, Chinese cabbage (perhaps), collards, kale, komatsuna, senposai, , spinach, tatsoi, and Yukina savoy. Eat-All Greens harvests can continue, if you sowed some in September. When we sowed some on September 16, we got several harvests in November.

From the hoophouse we continue harvesting spinach, tatsoi thinnings and leaves, as well as leaves of Tokyo Bekana and Maruba Santoh. We can start to harvest chard, senposai, Yukina Savoy leaves and perhaps kale, although it is a slow grower.

At the end of November we keep a close eye on the Tokyo Bekana and Maruba Santoh, for signs of bolting. Normally these will bolt in December, so we harvest the whole plants that month. But we have sometimes needed to terminate the plants November 26 or so.

Cooking Greens to Sow in Central Virginia in November

Young spinach plants (and henbit) in our hoophouse in December.
Photo Pam Dawling

Outdoors

we sow spinach (for spring harvesting) in early November if we have not been able to do it already.  Hopefully we will have got this done during October. Here it’s too late for any more outdoor sowings till spring, although there will be garlic planting.

In the hoophouse

on November 9 we sow spinach #3 to fill any spinach casualties that happen during the winter, and “Frills“ #2 (mizuna, Ruby Streaks, Scarlet Frills, Golden Frills). This is one of our favorite winter crops to suppress nematodes. We sow tatsoi #2 on November 15. We could sow Eat-All Greens in hoophouse in November, but so far we haven’t tried that.

No Cooking Greens to Transplant in Central Virginia in November!

Other Cooking Greens Tasks in Central Virginia in November

While watching the temperature forecasts, we continue to harvest the hardier greens, such as chard, yukina savoy, collards, kale, spinach and tatsoi.

Pak Choy outdoors should be harvested before night temperatures of 25°F (–4°C) or covered with thick rowcover.
Photo Ethan Hirsh

As night temperatures drop, we clear some crops

In this order:

25°F (–4°C) Most broccoli, some cabbage, Chinese Napa cabbage, Maruba Santoh, mizuna, most pak choy, Tokyo Bekana.

22°F (–6°C): Bright Lights chard.

20°F (–7°C): Less-hardy beets, broccoli heads (some may be OK to 15°F/-9°C), Brussels sprouts, some cabbages (the insides may still be good even if the outer leaves are damaged),  cauliflower, most turnips.

15°F (–9.5°C): The more hardy beet varieties and their greens, some broccoli, some cabbage, red chard (green chard is hardy to 12°F (-11°C)),  Russian kales, rutabagas if not covered, turnip leaves, most covered turnips.

Each winter I update my Winter-kill Temperatures of Cold-Hardy Vegetables. This year I’m watching the Koji carefully, to get some good data.

Washing Cylindra beets for storage.
Photo Wren Vile

Killing temperatures outdoors

Here are some more numbers for killing temperatures outdoors (without rowcover unless otherwise stated). In my Cooking Greens in October post, I gave the Veggie Deaths in the  35°F (2°C) to 15°F (–9.5°C) range. Here’s the next installment, which I am prompted to post by the forecast 16°F (-9°C) here for the night of Friday November 8. This list only includes the cooking greens. Your results may vary!  Let me know!  Click the link above to see the complete list.

12°F (-11°C): Some beets (Cylindra,), some broccoli, Brussels sprouts, some cabbage (January King, Savoy types), most collards, covered rutabagas (swedes), some turnips (Purple Top).

10°F (-12°C): Covered beets, Purple Sprouting broccoli for spring harvest (too cold here for us to grow that), a few cabbages (Deadon), chard (green chard is hardier than multi-colored types), some collards (Morris Heading can survive at least one night at 10°F/-12°C), probably Komatsuna; Senposai leaves (the core of the plant may survive 8°F/-13°C), large leaves of savoyed spinach (more hardy than smooth-leafed varieties), Tatsoi, Yukina Savoy.

5°F (-15°C): some kale (Winterbor, Westland Winter), smaller leaves of savoyed spinach and broad leaf sorrel. Many of the Even’ Star Ice Bred greens varieties are hardy down to 6°F (-14°C).

0°F (-18°C): some collards (Blue Max, Winner), Even’ Star Ice-Bred Smooth Leaf  kale, some spinach (Bloomsdale Long Standing, Bloomsdale Savoy, Olympia).

 Reminder: The temperatures given are air temperatures that kill those outdoor unprotected crops.

Ruby chard.
Photo Kathryn Simmons

Overwintering chard

To keep chard in good condition overwinter, either cover with hoops and rowcover (in milder areas, Zone 6 or warmer), or else mulch heavily right over the top of the plant, after cutting off the leaves in early winter.

Covering spinach

Once the frost has killed the galinsoga we go ahead and put rowcover over the spinach beds. That happened this weekend (November 2 and 3) – we got temperatures of 27°F (–3°C) and 25°F (–4°C). Spinach will make growth whenever the temperature is 40°F (5°C) or more, which happens a lot more often under rowcover than exposed to the elements. We don’t want to provide rowcover for the galinsoga!

Weeding rowcovered spinach in winter.
Photo Wren Vile

Special Cooking Greens Topic for November: Seed Inventory

November is a good month for us to start our big winter planning process. For all the crops, not just cooking greens! The first step is the Seed Inventory, in preparation for ordering the right amounts of the right varieties of seeds for next year. We do ours fairly accurately, because we also use the process to fine tune the amount of seed  to buy for each row we plan to sow. Some growers simply buy plenty and throw away all the leftover seed each season, but for us the time spent paying attention to what we need is very worthwhile. See the Planning section in my book Sustainable Market Farming for step by step details on how we do it.

We use a spreadsheet and a cheap little digital scale (for the small amounts, up to 100g). Ours is an AWS-100. It’s not legal for trade, but we are not using it to weigh seeds for sale, just to give ourselves a good idea of what we have left. For large quantities, we use our business shipping scale.

We take a few seed buckets and the scale into a pleasant-temperature room, and take out a bundle of seed packets of a particular crop. First we weigh a packet at a time and write down the amount. The scale can be tared for the empty packet.

Seed Viability

Next we assess whether the seed will be viable next year. Storage conditions make a big difference, the best storage being cool, dark, dry and airtight. Make your own decisions based on how carefully you stored the seeds, the information on each packet about percentage germination when you bought it, and the economic importance to you of that particular crop.

We have a simplified chart:

  • Year of purchase only: parsnips, parsley, salsify, scorzonera and the even rarer sea kale;
  • 2 years: corn, peas and beans of all kinds, onions, chives, okra, dandelion and
    martynia;
  • 3 years: carrots, leeks, asparagus, turnips and rutabagas;
  • 4 years: spinach, peppers, chard, pumpkins, squash, watermelons, basil, artichokes and cardoons;
  • 5 years: most brassicas, beets, tomatoes, eggplant, cucumbers, muskmelons, celery, celeriac, lettuce, endive and chicory.

If the seed is still recent enough to grow well, we keep it. If it is too doubtful we “write it off” on the spreadsheet and consign the packet to a special “Old Seeds” bucket, which we keep for a year in case of mistakes or desperation!

This is the time we adjust the “seed rate” (seed/100′ or /30 m) column on our spreadsheet using our new information from our year.

After completing the inventory we have our annual Crop Review which we combine with popping garlic cloves for planting.

Popping garlic cloves in preparation for planting
Photo credit Southern Exposure Seed Exhcange