Winter Kill Temperatures of Cold-Hardy Vegetables 2019

Baby greens in a cold frame in January.
Photo Southern Exposure Seed Exchange

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).  We had some extremely cold temperatures of -8°F and -9°F (-22°C and -23°C) in early January 2018. The winter of 2018-2019 was not as brutal. Our lowest temperatures were 6°F (-14°C) 1/31/19, 8°F (-13°C) 12/11/18 and a couple of 11°F (-12°C). This year I found that senposai is  more cold-tolerant than I had thought. otherwise I haven’t got much new news here. My results from other years hold up.

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.

Hoophouse Notes

Our double-plastic 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; the plants recover in the warmer daytime conditions (it seems to be the night+day average temperature that counts);

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

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

Hoophouse lettuce Red Tinged Winter and Tango (and senposai) in our hoophouse in December.
Photo Pam Dawling

Outdoor killing temperatures of crops (unprotected unless stated)

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 15°F (-9.5°C)), 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).

Closing rowcovers after a winter spinach harvest.
Photo Wren Vile

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, some turnips (Purple Top).

Using a sturdy digging fork to harvest leeks in December.
Photo Pam Dawling

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), 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.
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 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 (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

Vates kale with a freeze-killed center January 19 2018. Photo Pam Dawling

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

Many of our Vates kale plants survived those cold temperatures Photo Pam Dawling

-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).

A hardy cover crop mix of winter rye, hairy vetch and Austrian winter peas.
Photo Bridget Aleshire

Book Review: Ira Wallace on Vegetable Gardening in the Southeast

Book Review:

The Timber Press Guide to Vegetable Gardening

in the Southeast

by Ira Wallace.

January 2014, 200 pages, $19.95. ISBN-13: 978-1-60469-371-3

 wallace_i books-gardeninginthesoutheast

This book will be valuable primarily to new vegetable gardeners and those moving to the Southeast. Of course, experienced gardeners always want to learn from each other, so I encourage anyone growing food in our region, or its neighbors, to take a look. The writing and format are clear and accessible, the index is excellent, so you can quickly find the topic you want. The scale is backyard, not farm. That said, this book could be useful for new farm interns to learn crop basics, timing, and how things are done without fancy equipment. It has the advantage of clearly setting out the basic information without being overwhelming.

The Southeast has been “traditionally underserved” in terms of vegetable gardening books, so having this compilation of tips for our climate, the names of reliable regional varieties, and the encouragement to start in any month and harvest in every month, is valuable beyond price. There are great gardening writers elsewhere in this big country, but nothing beats learning from someone who really knows the territory. Ira learned gardening in her grandmother’s yard in Florida, and has grown food in Virginia for many years. It is hard to translate northern books which talk of ground frozen solid for months in winter, or curing onions out in the September sun. Beginners will have a much happier time starting out with a book written for the actual conditions they will encounter. Gardeners moving from other climate zones will get useful information about working with “the quirks of our climate”; fall gardening, winter gardening, hot weather crops like southern peas and okra. For instance, did you know why gardeners in hot climates don’t sow in hills? They dry out too much (the gardeners as well as the hills!).

This book is intended mostly for those in the lands where the American Holly grows. This is zones 6-9, with bits of 5 and 10: the Southeastern Coastal Plains, Piedmont, Appalachians, Interior Plateau and Ozarks. Winters are temperate, summers are hot and humid, and there is a wide range of seasonal crops. We need to start spring crops early, to get a harvest before temperatures heat up; we need to add organic matter frequently, as it burns up fast in hot humid conditions; we need to know about shading.

The Gardening 101 section includes many helpful tips on soil tests, garden planning and rotations and cover crops. There are lists of crops by season and by ease of growing in our climate. As the introduction says, “Happily, gardening is a year-round activity in our region”, so there is a month-by-month section listing tasks and possibilities.

The Month-by-month section includes a To Do page for each month with panels on Plan, Prepare and Maintain, Sow and Plant, and Fresh Harvest. There is also a Skill Set panel each month covering topics such as making compost, setting up drip irrigation, building shade structures. Each month has a theme: Sowing Seed Indoors for February, Beating the Heat in July. The tone is encouraging and helpful for beginners and climate migrants alike.

My one quibble is that the drawings don’t always seem to exactly fit the text, and I wish there could have been some photos. Luckily, Ira is from Southern Exposure Seed Exchange and their website www.southernexposure.com has plenty of photos of crops in action and includes a sun symbol for varieties especially suited to the Southeast.

There is a nice description of working against the shortening days and cooling temperatures of fall to get crops established in the “living refrigerator” outdoors – filling it up to draw from later. Plan for crops to mature in late November, before growth slows right down. There are a lot of critters out there, though, so bring in what you can for storage. Also, plan to have some crops reach half-size, be protected over winter, then rapidly grow in early spring for first harvests of the new growing season.

Part 3 of the book is an A to Z of crops, starting with two Planting and Harvesting Charts, one for the Upper South and one for the Lower South. These cover 38 crops or groups of closely-related crops. Then each crop has around one page of details, divided into paragraphs on Growing, Harvesting, Varieties and Seed Saving.

The Resources section  includes seed and plant suppliers, community organizations, weather and climate resources, companies selling tools and supplies, soil testing services and more good books (blush, including mine). There is a Glossary, to demystify any terms you don’t know – soon you will!

If you live in the Southeast and are new to growing food here, you’ll learn a lot from this book and its clearly arrayed information. If you live around the Southeast, you’ll probably also learn some tips (think of Pennsylvania as the “Upper Southeast”!). If you’re a farmer with new interns about to arrive, get this book for your library. If you’re an experienced vegetable grower in our region, take a look too – I bet you’ll find some new ideas.