Winter-Kill Temperatures of Cold-Hardy Vegetables 2018

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!).

Vates kale which survived temperatures of -8F and -9F outdoors, uncovered.
Photo Pam Dawling

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

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.

Hoophouse Notes

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.

Spinach under rowcover, with our hoophouse in the background – crop protection pays!
Photo Pam Dawling

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).

Tadorna leeks, struggling but not dead, after -9F.
Photo Pam Dawling

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.

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

-40°F (-40°C) Winter wheat and winter rye (cover crops).

A cover crop of winter wheat untroubled by -9F.
Photo Pam Dawling

Asian Greens for January: Chinese Cabbage

Sorry for the delay this week – technical problems.

Young Chinese cabbage transplants in our hoophouse in October. Photo by Bridget Aleshire

If you have Chinese cabbage in your hoophouse, January is the month to harvest it in zone 7. We do not harvest leaves from this crop, but wait for it to form full-size heads and then harvest those mature plants. We sometimes start harvesting as early as December 4, if the plants have reached full size and we “need” to harvest them. Otherwise we wait till December 15. If we have planted enough we can harvest until January 23, or sometimes as late as February 9.

Other Asian greens we are also harvesting at this time of year include pak choy, mizuna and the frilly mustards, tatsoi, Senposai, Tokyo Bekana, Maruba Santoh and Yukina savoy.

Chinese cabbage has very tender, light green savoyed leaves and is excellent for stir-fries, or pickling (sauerkraut or kimchee).

Chinese cabbage (both the Napa kind and the Michihli or Michihili kind) are Wong Bok types (Brassica rapa var. pekinensis) along with the “celery cabbages” – the non-heading Tokyo Bekana and Maruba Santoh.

We like Blues, an open-pollinated “barrel-shaped” Napa cabbage, shown in the photo above. Kasumi has the best bolt tolerance and is larger: 5 lb (2.3 kg) compared to 4 lb (1.8 kg); Orange Queen is a colorful but slower-growing variety (80 days in spring).

Napa and Michihili Chinese cabbages in October.
Photo Wren Vile

The Michihili types are taller and narrower, can be transplanted closer (8″) and might make more sense in terms of space use, although Napa cabbages do store better under refrigeration than michihli types. Jade Pagoda and the O-P Michihli both take 72 days from sowing to harvest in spring – considerably slower than Napa types. Michihili are more stress tolerant and resistant to bolting and black speck than Napa cabbage.

Blues takes 52 days from sowing to harvest in spring, but of course, takes longer in fall and winter. We sow September 15 in an outdoor nursery seedbed, and transplant into our hoophouse at 2-3 weeks old (October 2). It is very fast-growing in those temperatures and conditions. If we start harvesting December 15, it’s 3 calendar months from sowing, 91 days. The minimum germination soil temperature for Chinese cabbage is 50F (10C), and the ideal soil temperatures are 68F (20C) to 86F (30C). Under the ideal conditions the seedlings will emerge in 4 days. The maximum soil temperature to get any germination is 95F (35C).

We plant 52  plants for 100 people, with 4 staggered rows in the 4ft bed, 10.5″ apart (every 7th tine on Johnny’s row marker rake) and plants 10″ apart. With a harvest period of 5-8 weeks, 6-10 heads per week is about right for us.

We have not had many disease or pest problems with our hoophouse Chines cabbage. We do pay attention to using insect netting over the outdoor seedbed in the fall, but once we transplant indoors, our pest troubles are usually over. Vegetable weevil larvae have caused trouble in January. They come out of the soil at night and make holes in the leaves. They tend to prefer pak choy and turnips. We have used Spinosad against them with some success.

Chinese cabbage.
Photo Ethan Hirsh

Tipburn (brown leaf margins, including internal leaves) is caused by quick drying of the soil, when the weather makes a sudden switch to bright and sunny from overcast. Be ready to irrigate when the weather suddenly brightens.

The winter-kill temperature of Chinese cabbage outdoors without protection is 25F (-4C). Our hoophouse crop has taken outdoor temps of 8F without inner rowcovers, and -8F with added thick rowcover. It is more cold-hardy than most varieties of pak choy, and less cold-hardy than Komatsuna, Senposai, tatsoi, Yukina savoy. Mizuna, Maruba Santoh and Tokyo Bekana have a similar level of cold-tolerance.

Once past the winter solstice, the order of bolting of Asian greens is something like: Tokyo Bekana and Maruba Santoh, pak choy, Chinese cabbage, tatsoi, Komatsuna, Senposai, mizuna, Yukina Savoy, leaf radish, frilly mustards.

When it’s time to harvest, we lever and pull the plant out of the soil, then cut off the root. This helps with the next task of replanting the space. It is much easier than cutting the plants at the base and then digging up the root.

After the Chinese cabbage are all cleared, we might follow with kale or collards on January 24 to transplant outdoors as bare root transplants in March. If we have no plans for a follow-on crop that early in the year, we fill gaps in the Chinese cabbage plot until January 25, using “filler” Asian greens we sowed in October. After that date we fill all gaps with spinach transplants until February 20, and from then on we only fill gaps on the edges of beds, leaving the bed centers free for tomatoes, etc in mid-March.

Close-up of Chinese cabbage in our hoophouse in late November. Photo Pam Dawling

Resources

  • Grow Your Own Chinese Vegetables, Geri Harrington, 1984, Garden Way Publishing. Includes the names for these crops in different cultures.
  • Growing Unusual Vegetables, Simon Hickmott, 2006, Eco-Logic books, UK.
  • Oriental Vegetables: The Complete Guide for the Garden and Kitchen, Joy Larkham, revised edition 2008, Kodansha, USA
  • The Chinese Kitchen Garden: Growing Techniques and Family Recipes from a Classic Cuisine, Wendy Kiang-Spray

What’s still standing after two nights below 0F?

Recently I reported on which crops were still alive after two nights at 14F (-10C) and What’s still alive after two nights at 4F?  We’ve now had the Polar Vortex, which brought us two nights at 4F, on 1/6 and 1/7. Then it got even colder.We got the Big Round 0F 1/22-1/23, then a few nights at 5F or 6F, and then the big insult: -4F on the night of 1/29-30.

What’s still standing?

The Tyee spinach under thick rowcover has sustained big damage, showing as patches of beige dead cells. It will recover. Meanwhile we can eat from the more-protected spinach in the coldframes and the hoophouse.

The Vates  kale without rowcover is still alive, but badly damaged. The big leaves are crunchy and brown round the edges, and some of the inner leaves are dead. I hope it will grow back, but we won’t be able to pick that for a while. The Beedy’s Camden kale looks worse – the big leaves have died and flopped over. Not sure if it will recover.

Many of our strawberry plants look dead – very disappointing!

Our hardneck garlic and Polish White softneck tops are killed back to about one inch up from the mulch. Equally hardy, it seems. 

We had the remains of a lettuce nursery bed, still holding surplus transplants from September sowings that we didn’t need for our greenhouse or hoophouse. After the 4F assault we still had life in the centers of the Winter Marvel, North Pole, Tango, Green Forest. Now only the Winter Marvel shows any signs of life. So that variety gets the prize for cold-tolerance here!

Red Round Turnip. Photo Baker Creek Heirloom Seeds

Red Round Turnip.
Photo Baker Creek Heirloom Seeds

In the hoophouse, we covered all the beds with thick rowcover every night it looked like dropping below 10F inside. Almost everything survived – we only got some minor stem freezing on some turnips and Asian greens. We have been eating Pak Choy, Tokyo Bekana, Yukina Savoy, various turnips and their greens (Hakurei, White Egg, Oasis, Red Round), also plenty of lettuce leaves, radishes, scallions, and some spinach. We lost our second sowing of spinach in there to over watering and flooding, and we are really noticing the lack right now. We’re short on spinach. We have small amounts of mizuna, Ruby Streaks, Bright Lights chard, Bulls Blood beets to add to salad mixes, and Red Russian and White Russian kale growing slowly.

In January we have taken to sowing spinach, kale and collards in a hoophouse bed to transplant outdoors in early spring. We back this up with sowing some in flats if we don’t get good emergence for some reason. This year emergence is late. Is it just late, or is there a problem? We’re holding our breath for a few more days. . .

GFM_February2014_cover_300pxWe are not the only people tracking the effects of the unusually cold weather. The February Growing for Market magazine opens with an article by Ben Hartman “Testing the Limits of Cold Tolerance”. He farms in Goshen, Indiana, using two double-layer plastic greenhouses heated to 30F (yes. I said heated!) and two unheated. They planted kale, carrots, spinach, salad greens and arugula in their greenhouses for winter harvest. Their outdoor temperatures fell to -16F on 1/6 and 1/7. I imagine they’ve had worse since. They used mid-weight rowcover over their beds. Ben reports that baby greens and young spinach survived, as did their rosemary and their 3 fig trees (all farmers deserve some thrills!). They lost baby salad greens that had already been cut previously (all those cut edges didn’t do well). Crops in the outer beds were lost. The tips of full-grown kale leaves froze, but the plants survived.

In their unheated, single-skin plastic hoophouses, the soil froze down to 4″. They used two layers of mid-weight rowcover suspended over the crops. Despite this cold,  tiny salad greens less than 1″ tall survived. Spinach survived under just one layer of rowcover. The carrot tops froze and the roots may or may not be marketable. The (uncovered) fully mature kale looks dead. The mature salad with two layers of rowcover didn’t survive.

From this experience, Ben points out that salad greens and spinach less than 1″ tall are very cold-tolerant. Spinach and kale once larger, benefit from more protection than they got this time. Beware the outer beds!

My own article in this issue is about matching crop spacing with desired goals, such as maximum yield, optimum size, or convenience for cultivation.

Andrew Mefford has written some greenhouse tips for hoophouse growers, including tomato grafting, trellising. Chris Blanchard has written the second part of his piece on growing herbs – this is about harvest and maintenance. Erin Benzakain has undertaken a 59-variety trial of celosia.

 

What’s still alive after two nights at 4F?

Recently I reported on which crops were still alive after two nights at 14F (-10C) and several others in the teens. We’ve now had the Arctic Vortex, which in our part of central Virginia, meant two nights at 4F, last Monday 1/6 and Tuesday 1/7 nights. How did it go?

Before the Prelude to the Big Chill, when we got 9F, I harvested the odds and ends of small cabbages left in our main patch. Quite worthwhile, I got two 5-gallon buckets. Between the 9F and the 4F nights, I decided to gather the Deadon cabbage, which we grew with January harvests in mind. There was some freeze damage, so in future I’ll say that Deadon is good down to 10F, but not lower. I got two full net bags and two more buckets of small ones. I left one smaller and one larger cabbage as sacrificial victims in the cause of better information for next year. When we got 4F, the smaller one died and the larger survived.

Deadon cabbage Credit Johnnys Selected Seeds

Deadon cabbage
Credit Johnnys Selected Seeds

One of the other gardeners harvested the last of the outdoor senposai. Another couple of buckets of tasty food.

Senposai, the Thousand Wonder Green, Credit Kathryn Simmons

Senposai, the Thousand Wonder Green,
Credit Kathryn Simmons

I took another walk round the frozen garden after the Big Chill, to see what is still alive. We have Tyee spinach under rowcover, and Vates and Beedy’s Camden kale without rowcover. They are all still alive! There’s some freeze damage in spots on the spinach leaves, but plenty of good meals still to come!

Our hardneck garlic tops suffered some damage but didn’t get killed back to the mulch level. The Polish White softneck tops are considerably smaller and they too are still alive. They will grow back if they have died. 

Garlic planting in November. Credit Brittany Lewis

Garlic planting in November.
Credit Brittany Lewis

We had the remains of a lettuce nursery bed, still holding surplus transplants from September sowings that we didn’t need for our greenhouse or hoophouse. A good chance to see which ones are hardiest! Here’s the scoop:

Still alive in the centers – Winter Marvel, North Pole, Tango, Green Forest.                  No longer alive – Salad Bowl, Red Salad Bowl, Winter Wonder, Red Tinged Winter, Merlot, Red Sails, Outredgeous, Roman Emperor, Revolution.

At nearby Acorn Community, the home of Southern Exposure Seed Exchange, they had some young but mature heads of cabbage outdoors. The Late Flat Dutch, Early Flat Dutch and Chieftain Savoy all survived one night at 6F. (It’s usually two degrees warmer there than at Twin Oaks on winter nights).

Meanwhile I’m tracking the Blue Ridge kale grown by Clif Slade in his 43560 project at Randolph Farm, VSU. The Blue Ridge survived. It got down to 9F there. Not as cold as Louisa County! Blue Ridge is taller than the Vates we grow, and I’d like to try it here, if it can survive our winters. Otherwise not!

In the hoophouse, we covered all the beds with thick rowcover on Monday afternoon, and didn’t roll it up till Thursday, after the warmer weather returned. There was a tiny bit of freeze injury on some turnip greens that poked out the side of the rowcover, and some on some stems of Tokyo Bekana. I think the rowcover saved the crops! Also, a bad thing happened. it was very windy Monday night and the west window blew open. Argh! Of all the nights to have an open window. Memo: fix the latch to make it stronger.

I didn’t enjoy the really cold weather. I was anxious about the crops and the plumbing! But I can see two silver linings: I now have more information about cold-hardiness of various crops, and hopefully some pests will have died. Now we’re getting ready for another two cold nights, tomorrow and Wednesday.

When we placed our seed orders we gave up for this year on our quest for a reliable red cabbage of at least medium size and fairly speedy maturity (90 days or less). We’re having a red-cabbageless year. We’re still open to recommendations (OP or hybrid) – please leave a comment. 

What’s still alive at 14F?

Winter garden scene. Credit Ezra Freeman

Winter garden scene.
Credit Ezra Freeman

One of my ongoing topics of interest in the garden is how cold-tolerant various vegetables are. We’ve now had two nights at 14F (-10C) and several others in the teens. I took a walk round the frozen garden this morning to see what is still alive. We have Tyee spinach under rowcover, and Vates kale. The senposai is still alive, but some of the midribs have brown streaks. Sadly we don’t have any leeks this winter, as we lacked enough workers to tend them in late summer. We have a nice bed of Deadon cabbage, and I notice that some small heads of Melissa savoy that missed the bulk harvest are also alive. The Gunma cabbage stumps have some leaves and tiny heads still alive, but the Tendersweet are done in.

Our ongoing quest for a reliable red cabbage of at least medium size and fairly speedy maturity (90 days or less) yielded no success story this year. We grew Super Red 80 happily for many years, but then it stopped working for us – variable heads, slower maturity. If you have any recommendations (OP or hybrid) please leave a comment. We are working on our seed orders now, and this would be a great time to have some suggestions.

Back to today – our chard had all the leaves cut off in November, and seems to be dead. Some winters it hangs on later, if we leave some foliage to help it regenerate. We have also some years deliberately kept it alive for spring by using rowcover on it. We do that if we go into winter short of spinach beds.

The oats cover crop we sowed in August and early September look pretty much dead. All the broccoli looks dead. That’s as expected for the temperatures. Often we don’t get nights this cold till January – the cold came early this winter.

Our hardneck garlic tops look to be in good shape. The Polish White softneck tops are considerably smaller and look like they are suffering. They will grow back if they have died. Some of our Chandler strawberry plants look dead. Either that or they are extremely dormant! The deer were killing them off by eating the leaves. Too many deer!

Garlic shoots emerging through the mulch in November

Garlic shoots emerging through the mulch in November

The hoophouse is still bursting with great food. Plenty of salad greens: lettuce; various kinds of mizuna and ferny mustards like Ruby Streaks and Golden Frills and Bulls Blood beet leaves. And for salads or cooking we have spinach, chard, tatsoi, radishes, scallions, baby Hakurei turnips and their tasty greens, Red and White Russion kales, and more senposai. Soon we’ll start on the heading Asian greens: pak choy, Chinese cabbage, Tokyo bekana and Yukina Savoy. The first sowing of tatsoi (9/7) is starting to bolt, so we’re clearing that. The second sowing (11/15) needs thinning to an inch. The first round of baby lettuce mix (10/24) is ready for its second cut. In a few days we’ll make a second sowing of that. I love working in the hoophouse on sunny winter days. This afternoon I plan to complete the transplanting of an 11/9 sowing of spinach. We just love the sweet nuttiness of winter spinach!

The hoophouse winter crops are an important part of feeding ourselves year-round

The hoophouse winter crops are an important part of feeding ourselves year-round