Here’s the long version of one of the slideshows I presented on January 13 at the Future Harvest CASA conference. Since I got home, I updated my Winter-Kill Temperatures list, which appears in the slideshow. Compared to my list for 2016, there are a few differences, nothing major. We had some extremely cold weather, as I reported last week with some sorry pictures of lettuces. Now I have some photos of the outdoor crops too. The Vates kale had mixed survival, the rowcovered Reflect and Avon spinach are damaged but OK, the Tadorna leeks are battered but hanging in there (so are we!).
For several years I have been keeping records of how well our crops do in the colder season. I note each increasingly cold minimum temperature and when the various crops die of cold, to fine tune our planting for next year. We had some extremely cold temperatures of -8°F and -9°F (-22°C and -23°C) in early January 2018. We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C).
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, micro-climates and rain levels may lead you to use different temperatures in your crop planning.
Our double-skin hoophouse keeps night time temperatures about 8F (4.5C) degrees warmer than outdoors, sometimes 10F (5.5C) warmer. Plus, plants tolerate lower temperatures inside a hoophouse. The soil stays warmer and the plants recover in the warmer daytime conditions (it seems to be the night+day average temperature that counts).
In the hoophouse (8F warmer than outside) plants without extra rowcover can survive 14F colder than they could survive outside; 21F colder than outside with rowcover (1.25oz Typar/Xavan).
For example, salad greens in a hoophouse can survive nights with outdoor lows of 14°F (-10°C) without inner rowcover. Lettuce, mizuna, turnips, Russian kales, Senposai, Tyee 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.
Lettuce hardy enough for a solar heated winter 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, 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.
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 varieties, Sugarloaf chicory (takes only light frosts).
25°F (-4°C): Some cabbage, chervil, chicory roots for chicons, and hearts, Chinese Napa cabbage (Blues), dill (Fernleaf), endive (Escarole more frost-hardy than Frisée), 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.
22°F (-6°C): Some arugula (some varieties are hardier), Bright Lights chard, large leaves of lettuce (protected hearts and small plants will survive colder temperatures), rhubarb stems and leaves.
20°F (-7°C): Some beets (Bulls Blood, Chioggia,), broccoli heads (maybe OK to 15F), Brussels sprouts, some cabbages (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), flat leaf parsley, 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, endive, 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) if not covered, broad leaf sorrel, most covered turnips, winter cress.
12°F (-11°C): Some beets (Cylindra,), some broccoli, 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, 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), 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, 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), 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): 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 savoyed 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 (mache), garlic, horseradish, Jerusalem artichokes, 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, Tyee). 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 die, 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
-40°F (-40°C) Winter wheat and winter rye (cover crops).