I’ve just updated my “Winter-kill” list, adding information from last winter to update for this winter’s planning. For several years, 2015, 2104, 2013, 2012 my friend and neighboring grower Ken Bezilla of Southern Exposure Seed Exchange and I have been keeping records of how well our crops do in the colder season. Ken provided much of the original information, and has suggested the morbidly named Death Bed idea: set aside a small bed and plant a few of each plant in it to audition for winter hardiness. Note each increasingly cold minimum temperature and when the various crops die of cold, to fine tune your planting for next year (and send me an email!) This year I have added some cover crop hardiness temperatures. We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C).
It’s worth noting that in a hoophouse plants can tolerate lower temperatures than those listed here: they have the pleasant daytime conditions in which to recover. Salad greens in a hoophouse in zone 7 can survive nights with outdoor lows of 14°F (-10°C). All greens do a lot better with row cover to protect them against cold drying winds.
Also 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. Here are some starting numbers of killing temperatures, although your own experience with your soils, microclimates and rain levels may lead you to use different temperatures:
35°F (2°C): Basil.
32°F (0°C): Bush beans, 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, Sugarloaf chicory (takes only light frosts), radicchio.
25°F (-4°C): Chervil, chicory roots for chicons, and hearts, Chinese Napa cabbage (Blues), dill, endive (hardier than lettuce, Escarole more frost-hardy than Frisée), annual fennel, some mustards and Asian greens (Maruba Santoh, Mizuna, most Pak Choy, Tokyo Bekana), onion scallions (some much more hardy), radicchio.
22°F (-6°C): Arugula (may survive colder than this), large leaves of lettuce (protected hearts and small plants will survive even colder temperatures).
20°F (-7°C): Some beets, broccoli heads (maybe OK to 15F), some cabbage heads (the insides may still be good even if the outer leaves are damaged), celeriac, celtuce (stem lettuce), some head lettuce, some mustards/Asian greens (Tendergreen, Tyfon Holland greens), radishes, most turnips with mulch to protect them (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 cabbage (Kaitlin, Tribute), celery (Ventura) with rowcover, cilantro, endive, fava beans (Aquadulce Claudia), Russian kales, kohlrabi, some lettuce, especially medium-sized plants (Marvel of Four Seasons, Olga, Rouge d’hiver, Tango, Winter Density), curly leaf parsley, flat leaf parsley, large leaves of broad leaf sorrel, turnip leaves, winter cress.
12°F (-11°C): Some beets (Cylindra,), some cabbage (January King, Savoy types), carrots (Danvers, Oxheart), most collards, some fava beans (not the best flavored ones), garlic tops if fairly large, most fall or summer varieties of leeks (Lincoln, King Richard), large tops of potato onions, rutabagas (if mulched), Senposai leaves (the core of the plant may survive 10F), some turnips (Purple Top), winter radish including daikon (may survive colder).
10°F (-12°C): Beets with rowcover, Purple Sprouting broccoli for spring harvest, Brussels sprouts, chard (green chard is hardier than multi-colored types), a few varieties of cabbage (Deadon), some collards (Morris Heading can survive at least one night at 10F), Belle Isle upland cress, some endive (Perfect, President), young stalks of Bronze fennel, probably Komatsuna, some leeks (American Flag, Jaune du Poiteau), some head lettuce under row cover (Pirat, Red Salad Bowl, Salad Bowl, Sylvesta, Winter Marvel), large leaves of savoyed spinach (more hardy than flat 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): Brussels sprouts, Garlic tops if still small, some kale (Winterbor, Westland Winter), some leeks (Bulgarian Giant, Laura, Tadorna), some bulb onions, potato onions and other multiplier onions, some winter radishes (Daikon, China Rose, Shunkyo Semi-Long survive 10°F/-12°C), 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): Chives, some collards (Blue Max, Winner), corn salad (mache), garlic, horseradish, Jerusalem artichokes, a few leeks (Alaska, Durabel); some bulb onions, some onion scallions (Evergreen Winter Hardy White, White Lisbon), parsnips (probably even colder), salad burnet, salsify, some spinach (Bloomsdale Savoy, Olympia, Tyee). Crimson clover is hardy down to 0°F (-18°C) or slightly colder
Even Colder: Vates kale survives 0°F (-18°C), although some leaves may be too damaged to use. Leaves of overwintering varieties of cauliflower are hardy down to -5°F (-19°C). Walla Walla onions sown in late summer are hardy down to -10°F (-23°C). Narrow leaf sorrel, Claytonia and some cabbage (January King?) are said to be hardy in zone 3, -30°F to -40°F (-34°C to -40°C).
Austrian Winter Field Peas and Crimson clover (used as cover crops) are hardy down to -10°F (-23°C). Hairy vetch is hardy to -15°F (-26°C), some say down to -30°F (-34°C). Dutch White clover cover crops are hardy down to -20°F (-29°C) or even -30°F (-34°C). Winter wheat and winter rye (cover crops) are hardy to -40°F (-40°C).