At the Organic Association of Kentucky Conference, I gave a a presentation on Winter High Tunnel and Outdoor Vegetable Production. You can see it here. Click on the diagonal arrow icon to see it full screen, then click on the right pointing triangular arrow
Winter-Kill Temperatures of Cold-Hardy Vegetables 2020
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 2019-2020 has been mild, with our lowest temperature being a single night at 12°F (-11°C). The Koji became completely unmarketable but did not completely die. Yukina Savoy is indeed hardier, being OK down to 10°F (-12°C). We had one night at 13°F (-10°C) and two each at 17°F (-8°C), 18°F (-8°C also) and 19°F (-7°C).
This winter I noted the death of rhubarb stems and leaves at 25°F (-4°C), rather than 22°F (-6°C), as I noted a year or two ago.
In early January 2018, we had some extremely cold temperatures of -8°F and -9°F (-22°C and -23°C). The winter of 2018-2019 was not as brutal. Our lowest temperatures were 6°F (-14°C) in late January, 8°F (-13°C) in December 2018 and a couple of 11°F (-12°C). I found that senposai is more cold-tolerant than I had thought. Averaging our winter low over those three winters gives 3.2°F (-16°C), completely within the zone 7a range.
My other results from other years still hold up.
I also learned that there is more damage when the weather switches suddenly from warm to cold. And that the weatherman in Raleigh, NC says it needs 3 hours at the critical temperature to do damage.
Radicchio seeds from Seeds from Italy
Notes on Chicories and Endives
We gave up growing chicories and endives because we really didn’t like the bitterness. I decided to do some more online research to make sure I wasn’t spreading untruths. 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. If you know differently, please leave a comment.
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?
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. Your own experience with your soils, microclimates and rain levels may lead you to use different temperatures in your crop planning.
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 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 bibb, Oscarde, Outredgeous, Pirat, Red Cross bibb, Red Sails, Red Salad Bowl, Red Tinged Winter, Revolution, Rouge d’Hiver, Salad Bowl, Sylvesta bibb, Tango, Winter Marvel, Winter Wonderland.
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)), 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 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).
15°F (-9.5°C): Some beets (Albina Verduna, Lutz Winterkeeper), beet leaves, some broccoli, 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, 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 10F), 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 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.
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 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), 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, 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 slightly colder
-5°F (-19°C): Leaves of overwintering varieties of cauliflower, Vates kale survives although some leaves may be too damaged to use.
-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).