Winter-Kill Temperatures

Dealing with winter weather in your hoophouse

December lettuce in our hoophouse after snowfall. Photo Pam Dawling

Dealing with winter weather in your hoophouse

We’ve just had Winter Storm Frida bring us a foot of heavy wet snow and, along with most of Virginia, several days without electricity. Six days in our case. Much longer, colder and snowier than I expected. I know, climate chaos makes predictions and expectations chancy! We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C) until recently.

In November 2018, I wrote a post for Mother Earth News entitled Dealing with Snow on your Hoophouse.

When the power goes out, close the air intake

Hoophouse air intake with a very useful flap to reduce or close the opening. Photo Kathryn Simmons

The first thing an operator of a double layer hoophouse should do when the power goes out (especially in winter) is to rush out and close the air intake. Without electricity to run the little blower, the air intake hole becomes an air outflow. Closing the flap on the blower stops the air escaping. In summer you might not mind, but in winter, the air between the layers of plastic maintains about 8F/5C warmer at night inside than outside, and plants can survive 14F/8C colder than they can outside, without extra rowcover; at least 21F/12C colder than outside with thick rowcover.

The “air bubble” also gives strength to the structure in withstanding loads caused by high winds or heavy snow. Soon after the power cut out on Monday morning, I set out towards the hoophouse. Then I remembered that when we changed the end wall plastic in the fall, we did not fit a cover flap. Worse yet, the air tubing projected out of the wall, making a hasty retrofitting impossible. I briefly considered “putting a sock in it” literally, but then pictured what would happen when the power came back on and sucked the sock into the motor, overheating it and starting a blaze. So, learn from our mistake – always fit a flap over the air intake!

Rolls of rowcover in our hoophouse ready to pull over the beds on very cold nights. Photo Wren Vile

We keep rolls of thick Typar rowcover at the ready and cover the individual beds if we expect the outdoor air temperature to be 8°F (-13°C) or lower. Otherwise we don’t use any inner rowcovers in our hoophouse. Without the insulating effect of the air bubble during the power outage, we decided to use the rowcovers when the outdoor temperature was going to be less than 16°F (-9°C). Several nights like that!

Perhaps this will be your first hoophouse winter. Some others will be remembering last winter, and hoping to find a better way of dealing with snow. Snow can be heavy stuff, so removing it is worthwhile. This can be done from the outside, pulling the snow down to ground level, and from the inside, bouncing the snow off the plastic. It isn’t necessary to remove all the snow. Once you have removed what you can, the daytime temperature inside the hoophouse will rise and help melt the rest of the snow. We have never needed to get out of bed in the night to tackle snow, but you might.

The Snobrum tool we use on the outside of our hoophouse.
Photo Pam Dawling

First we tackle the outside, or if snow is still falling or it’s frozen onto the plastic, we start with the inside. Either way, start with the south side, to get as much advantage from solar gain as possible. The important rule of snow removal is “First, do no harm.” Don’t make holes in the plastic in your efforts to remove snow! A hoophouse is most stable when the snow is evenly distributed. Since houses are stronger at the ends where they have end walls for support, it makes sense to start at the weaker middle and work in both directions if removing heavy snow.

Hoophouse snow scraping tool on a telescoping painter’s pole. (No snow in sight!) Photo Pam Dawling

For outside we use a tool (SnoBrūm™) which is sold for scraping snow off cars. It’s a foam board about 6″ × 18″ (15 × 45 cm) with a threaded insert that takes a telescoping pole. It is sold with a short pole, but a painter’s pole will give a much longer reach. If you haven’t yet got one of these handy tools, carefully use the back of a rake (wrapped in thick cloth). Pull the snow down between the bows, avoiding pulling the tool over the framework as much as possible, as that can easily make holes. Alternate with shoveling the snow away from the base of the hoophouse if the snow is deep (piles of snow pressing against the lower walls can do damage). Be careful not to hit the plastic with any metal tools. Don’t attempt to use the relatively fragile foam-board tool for pushing snow along the ground; keep it free of grit and use it only on the plastic.

Removing snow from a hoophouse at Dripping Springs Garden. Photo Mark Cain

Inside, don a visor (to keep blobs of icy water from landing on your face) and grab an old broom with its head covered in a cloth or bubble plastic, taped on. Make sure there are no sharp protrusions from the broom head. First walk along the paths nearest the walls banging the broom against the plastic. We find a double bouncing action works best, and we can get in a rhythm. This can be a long and tiring job! Start at waist height because if the lower snow falls down, sometimes the higher stuff will follow it down, saving you work. When you need to reach higher, tie or tape the broom to a longer stick. Or find something that screws onto your painter’s pole, such as a paint roller (!) and cover that with bubble plastic. Balance applying your energy with being gentle with the plastic. Do not use the foam-board scraper for bouncing snow — it will crack.

When you’re tired and no more snow is easy to move, wait and try again later. The warming effect of incoming daylight will help melt the snow next to the plastic, so that it is easier to start it sliding off next time you do the rounds.

Sometimes ice accumulates on the plastic. If you get freezing rain that sticks on the plastic, it creates a rough surface that keeps any snow that settles on top from sliding off until the ice thaws. The best way to remove ice is to melt it using warmth from inside. If you have double plastic and it is not windy, switch off the inflation blower — the heat from inside will reach the ice sooner. I recommend against breaking the ice free mechanically, as there is a big risk of the sharp edges of the ice cutting the plastic. You also risk abrasion every time you use a device on the outside of the plastic to pull snow or ice off the tunnel. This makes the plastic rough, and snow won’t slide off as well in the future.

Prevent Hoophouse Frame Collapse

Snow comes in various consistencies and weights. A foot (30 cm) of light fluffy snow may only contain as much water as 1″ (2.5 cm) of rain. But heavy wet snow can equal 1″ (2.5 cm) of rain in only 3″–4″ (8–10 cm) of snow. Each inch (2.5 cm) of rainwater-equivalent will load a structure with 5.2 lbs/ft² (25 kg/m²). This is about 6.5 tons (5.9 metric tons) on a 25′ × 96′ (7.6 × 29.2 m) hoophouse! Uneven snow loads make a frame more likely to collapse because there are points of higher pressure. This happens if snow is thicker on one side of the hoophouse or if adjacent hoophouses are too close and snow piles up between them when it slides off the roof. The weight of snow can buckle the side of the frame.

Some growers have wood posts ready to wedge under the trusses for extra strength. It sounds like a good plan. Our hoophouse doesn’t have any crosswise trusses, just lengthwise purlins. It’s still standing after 18 years.

When heavy snow is predicted, turn a heater on a couple hours before the storm begins, with the thermostat at 70°F (21°C) or higher. The cost of the fuel is less than a new hoophouse. A portable propane heater (non-electric!) is a good thing to have on hand for unheated tunnels or if a furnace fails or if the power goes out. If you have a very heavy snowfall and it is not possible to remove the snow, then cut the plastic and let the snow fall into the hoophouse to relieve the pressure and save the frame.

“If this, do that”

We have an “If this, do that” card at the front of our hoophouse log book, to help with decisions. It divides snow events into three types.

If the night temperature will be higher than 25°F (−4°C) and you expect less than 6″ (15 cm) of wet snow, leave the inflation blower on, go to bed and hope the snow will slide off.
If the night temperature will be lower than 25°F (−4°C), or there will be more than 6″ (15 cm) of snow and no wind, turn off the blower until the morning, letting the interior heat do its best to melt the snow. If it’s going to be extremely cold, you’ll have to balance this with your need to keep your crops alive.
If there is hard sleet or freezing rain, cycle the inflation off for three hours, on for three hours, with the goal of letting the interior heat melt some of the accumulation, and then using the inflation to push the slush off (give it some help in the morning). Repeat this cycle as needed.

Two great resources are Vern Grubinger, University of Vermont Extension, Prevent Greenhouse Collapse and the Rimol Greenhouses Blog How to Reduce Storm Damage to Your Greenhouse and High Tunnel.

In November 2020 I wrote Winter Preparations for Vegetable Gardens

My annual blogpost of Winter-Kill Temperatures for Cold-Hardy Vegetables is always very popular. Usually searches for this info increase in October and peak in early November, when growers are deciding when to harvest each crop as the cold increases. In mid-winter you might consult the table to determine which crops to prioritize for extra rowcover if a bad thing happens.

I have learned that there is more damage when the weather switches suddenly from warm to cold. And that the forecaster in Raleigh, NC says it needs 3 hours at the critical temperature to do damage. 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.

Winter Preparations for Vegetable Gardens

Frosty daikon radish
Photo by Bridget Aleshire

Winter-Kill Temperatures

My annual blogpost of Winter-Kill Temperatures for Cold-Hardy Vegetables is always very popular. In fact, it’s my most popular title! Usually searches for this info increase in October and peak in early November, so here are quick links for those of you who have been meaning to look something up.

Winter-Kill Temperatures of Cold-Hardy Vegetables 2020

Winter Kill Temperatures of Cold-Hardy Vegetables 2019

Winter-Kill Temperatures of Cold-Hardy Vegetables 2018

Winter Kill Temperatures of Winter-Hardy Vegetables 2016

Trimming roots from a leek in December.
Photo Pam Dawling

For several years, starting in 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 leave me a comment!) Each year I update the list, based on new things I learned during the recent winter.

We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C).

The winter 2019-2020 was mild, with our lowest temperature being a single night at 12°F (-11°C). The Koji greens became completely unmarketable but did not completely die. Yukina Savoy is indeed hardier (as I expected), 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). That 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. I also added some cover crop hardiness temperatures.

Rhubarb in spring.
Photo Kathryn Simmons

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. 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. All greens do a lot better with row cover to protect them against cold drying winds.

Winter hoophouse lettuce
Photo Kathryn Simmons

It’s worth noting that in a double-layer hoophouse (8F/5C warmer at night than outside) plants can survive 14F/8C colder than they can outside, without extra rowcover; at least 21F/12C colder than outside with thick rowcover

Salad greens in a hoophouse in zone 7 can survive nights with outdoor lows of 14°F (-10°C). A test year: 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.

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Seeking Reader Participation

Your experience with your soils, microclimates and rain levels may lead you to use different temperatures. I’d love to hear from readers if they’ve found my numbers work for them, or if they have a different experience. You can leave a comment here, and it will appear on the website, for others to consider. Or you can fill out a Comment Page and only I will see it, although I’ll pass on the information without your name, if I think others would like to know too.

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Preparing for winter

Rowcover rolling with crank handle.
Photo Rodaew Institute

I’ve also written posts about winter preparations

Getting ready for frost. This post includes info on DIY weather-forecasting, our Frost Prediction checklist, and our Frost Alert Card. Also a link to a 126 page book which includes explanations of freezes and frosts to help us make sense of advice we’ve not understood. FrostProtectionFundamentalsPracticeandEconomicsFAO.pdf

Preparing for Frost and Cold Weather. This post includes our Frost Alert Card, a Frost Predictions checklist of what to do when the first fall frost is expected; how to use sprinklers overnight to stop tomatoes from freezing; four ranges of cold-hardiness (some crops can wait in the garden till it gets colder); and different levels of crop protection, including rowcover, low tunnels, Quick Hoops, caterpillar tunnels and hoophouses (aka high tunnels).

Season Extension and Frost Preparations. This post includes my Season Extension slideshow; the Frost Alert Card and Frost Predictions checklist again; a diagram of our winter double hoop system to hold rowcover in place during the worst weather;

Double hoop system for winter rowcover.
Pam Dawling

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Changing Winter Temperatures

Here’s an article from the Virginia Mercury by Sarah Vogelsong, giving info about changing winter temperatures, particularly later fall frosts in Virginia:

The frozen Potomac River. (Sarah Vogelsong/Virginia Mercury)

Autumn’s first frost is falling later. For farmers, the consequences are wide-ranging

by Sarah Vogelsong, November 3, 2020

Halloween has come and gone. The clocks have been set back. Every evening darkness falls just a little bit earlier.

But for much of Virginia, the first frost still remains elusive.

Over the past century, the average date of the first frost has been moving progressively backward throughout the commonwealth, today landing a week or more later than it did at the turn of the 20th century.

“This is one of the clearest signs of not only the changing climate but … its impact on our systems,” said Jeremy Hoffman, who as chief scientist at the Science Museum of Virginia conducts extensive research on climate change in Virginia. “It’s not just here, it’s everywhere.”

As global temperatures have warmed, largely due to the release of greenhouse gases into the atmosphere, frost seasons have shrunk. The Fourth National Climate Assessment released by the Trump administration in 2018 reported that “the length of the frost-free season, from the last freeze in spring to the first freeze of autumn, has increased for all regions since the early 1900s.”

How the shifts have played out in different states with different geographic, ecological and topographic features varies. Data from the U.S. Environmental Protection Agency show that between 1895 and 2016, the average date of the first fall frost moved back by 7.1 days in Virginia.

On the local level, the changes may be even starker. Estimates of how much the average date has changed vary depending on the time range used and how scientists fit a line to their data points, but in most Virginia cities, they show unmistakable upward trends. Looking at first frost dates between 1970 and 2016, Climate Central, a nonprofit staffed by scientists and journalists, calculated that on average, the first frost today is 5.9 days later in Lynchburg, 8.9 days later in Harrisonburg, 12.8 days later in Roanoke, 15 days later in Charlottesville and 18.5 days later in Richmond. While their data show Norfolk’s first frost occurring about six days earlier on average, Hoffman said that longer-range data going back to 1940 show the first frost moving back by about five days. Still, he cautioned, variation does occur: “Localized things like weather” can “work against that dominant signal in datasets like these.”

First freeze dates for Richmond, 1930-2019. (Jeremy Hoffman, Science Museum of Virginia)

The implications of the shifts in the freezing season go beyond a few more days to enjoy warm weather, say scientists and policymakers. Perhaps most affected are farmers, whose livelihood is intimately tied to fluctuations in both short-term weather and long-term climate.

“Some things you can sort of manage around and some things you can’t,” said Wade Thomason, a professor of crop and soil environmental science at Virginia Tech and the state’s grain crops extension specialist.

For most farmers, the last frost of the year in the spring is the riskier of the season’s two endpoints, falling as it does when most plants are young and more vulnerable to temperature extremes. But ongoing changes in the first frost in the fall also have ripple effects.

“It can be a beneficial thing for some instances. We might get more grazing days for livestock operations in a year,” said Thomason. For some crops, like double-cropped soybeans that are planted following the harvest of another crop — typically a grain like wheat — “it can extend the season.”

Other effects are less immediately apparent. Many wheat farmers who typically plant in mid-October have begun to push back their planting dates to ensure plants don’t grow too quickly during the freezing months, making them susceptible to disease or falling over in the field. Specific types of forage rely on long periods of cool weather to thrive: in Northern Virginia and the Shenandoah Valley, farmers have noticed that orchard-grass stands are only living for four to five years instead of the once-standard 10.

“For years now, we’ve heard from farmers that the stands don’t persist like they used to,” said Thomason. Research has shown that one factor contributing to less persistence is warmer nighttime temperatures, he added, but because most operations rely on cultivars developed decades ago, “we haven’t adapted orchard grass that thrives in a warmer climate.”

Other crops affected by longer warm seasons? Tree fruits and wine grapes

“Virginia’s one of those places that we expect to get hotter and we also expect to get wetter,” said Benjamin Cook, a climate scientist with the National Aeronautics and Space Administration’s Goddard Institute for Space Studies whose research includes the effects of climate change on vineyards. Neither of those conditions are necessarily good for high-quality wine prospects, he said. Furthermore, farmers working in these areas face special risks because of the long time to maturity of their crops.

“Those are parts of the agricultural world that adaptation eventually becomes a lot more challenging, because you can’t switch crops from year to year,” he said. “You have to make a bet on something and wait four years to see if it pays off.”

Regardless of their specialty, all farmers face another consequence of shorter freeze seasons: more weeds and more pests.

“With longer growing seasons, with these warmer winters, the populations of insects are increasing, the mortality is lower, they can produce more generations a year, and that potentially presents a problem for agriculture and plants in general,” said Cook.

Those effects can be seen on the ground, said Thomason: “Maybe 30 years ago, we could stop worrying about them in early October, and now it may be a week or 10 days later.”

https://www.virginiamercury.com/ Virginia Mercury is part of States Newsroom, a network of news outlets supported by grants and a coalition of donors as a 501c(3) public charity. Virginia Mercury maintains editorial independence. Contact Editor Robert Zullo for questions: info@virginiamercury.com. Follow Virginia Mercury on https://facebook.com/thevirginiamercury and https://twitter.com/MercuryVirginia

Our stories may be republished online or in print under Creative Commons license CC BY-NC-ND 4.0. We ask that you edit only for style or to shorten, provide proper attribution and link to our web site.

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Winter-Kill Temperatures of Cold-Hardy Vegetables 2020

Overwintered Vates kale in central Virginia in March.
Photo Nina Gentle

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 vegetable production outdoors and in a hoophouse Pam Dawling from Pam Dawling

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.

Hoophouse beds in November.
Photo Ethan Hirsh

Hoophouse Notes

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.

Young Bright Lights chard.
Photo Pam Dawling

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.

Tokyo bekana in our hoophouse.
Photo Twin Oaks Community

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).

Garlic shoots in January.
Photo Pam Dawling

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).

Hoophouse video interview, plus 2019 Round up of favorite topics and posts you missed

No-Till Growers and Josh Sattin collaborated to post this Hoophouse Tips video interview of me talking about our hoophouse:

After a Best Ever day on November 7, 2019, when 876 people viewed my website (4,855 that week), December has been quiet. It’s not a big gardening month for most of us, and the month is full of holidays. And then there’s the urge to hibernate.

Nonetheless, I have been reliably posting every week, and you might have accidentally missed something, while entertaining the visiting aunts and uncles, or rushing to get the carrots harvested, or something involving food and drink. Here’s a chance to find the lost treasures!

December 24, I posted a Book Review: Grow Your Soil! by Diane Miessler

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Three potato forks to the left, four digging forks to the right.
Pam Dawling

On December 18, I posted Making Use of Greenhouse Space in Winter and Getting the Right Fork

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Purple ube grown in North Carolina by Yanna Fishman

December 10, did you miss Yanna Fisher’s splendid purple ube?

And Josh Sattin’s video interview with me? Legendary Farmer on a Leg endary Commune https://youtu.be/vLzFd4YP9dI

And Jesse Frost’s interview with me on his podcast No-Till Growers ? You can listen to it here and it’s also on You Tube: https://www.youtube.com/watch?v=p75gRIl0Hzs

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

On December 3, I posted Cooking Greens in December

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On November 26, I offered you a Book Review: Will Bonsall’s Essential Guide to Radical, Self-Reliant Gardening

My Top Six. no Seven, posts of all time:

Winter-kill Temperatures of Winter-hardy Vegetables 2016 with 23,776 views, peaking in November. Clearly lots of people want to know which crops will survive and which to hurry and harvest, or protect with rowcover.

Garlic scapes with 11,202 views. Garlic and sweet potatoes are the favorite crops on this site. Garlic scapes used to be under-appreciated and under-used. Not now!

Winter-kill Temperatures of Cold-Hardy Vegetables 2018 with 9,979, also peaking in November. My list gets updated each year as I learn new information. But the older ones come higher on internet searches.

Soil Tests and High Phosphorus Levels close behind at 9,503. High phosphorus is a worry for organic growers, especially those using lots of compost, as it can build up each year.

How to Deal with Green Potatoes at 8,613, with sustained interest through August, September and November. Obviously we are not the only growers with this problem, caused by light getting to the tubers.

Tokyo Bekana at 2,199 (who knew that was so popular?)

In 2019 other popular posts included Hunting Hornworms and the newer Winter-Kill Temperatures of Cold-Hardy Vegetables 2019. It takes a while for newer posts to gain on the older favorites.

Lettuce slideshow, Mother Earth News Fair, FaceBook Live, Top summer blogposts, upcoming events

We drove home seven hours from the Pennsylvania Mother Earth News Fair yesterday through the rain. The remnants of Hurricane Florence. We were among the lucky people. Earlier forecasts for Florence had the hurricane raging across central Virginia.

At the Fair, I gave two workshops: Fall and Winter Hoophouses and my new Lettuce Year Round, which you can view right here. Click the diagonal arrows icon to get a full screen view.

Lettuce year round Dawling from Pam Dawling

I had a bit too much material for a one-hour time-slot, so those of you who were there and felt disappointed at what I had to leave out, you can see it here.

While I as at the Fair I did a FaceBook Live Interview about gardening in hoophouses, with another author, Deborah Niemann. Look on Facebook for Deborah Niemann-Boehle or click the topic link above. She has several books: Raising Goats Naturally, Homegrown & Handmade, and Ecothrifty.

Shade cloth on a bed of lettuce in summer.
Photo Nina Gentle

Meanwhile, Mother Earth News tells me that my post 20 Tips for Success in Germinating Seeds in Hot Weather is in third place for most popular posts this summer.

The winner An Effective and Non-Toxic Solution for Getting Rid of Yellow Jackets’ Nests by Miriam Landman got 43,328 views in 3 months!

Weeding rowcovered spinach in winter.
Photo Wren Vile

Looking at my own website statistics, I find that for this week, the most popular posts are

For all-time, the bias is naturally on posts that have been around longest,

Garlic scapes! Three weeks to bulb harvest! Is most popular, followed closely by
Winter Kill Temperatures of Winter-Hardy Vegetables 2016.
How to deal with green potatoes is still #3.
The Complete Twin Oaks Garden Task List Month-by-Month,
Harvesting Melons
Book Review, Epic Tomatoes by Craig LeHoullier
Wnter Hardiness
Book Review: The Lean Farm by Ben Hartman and
Setting out biodegradable plastic mulch by hand

Rolling biodegradable plastic mulch by hand
Photo Wren Vile

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I’ve updated my Events page again, now that the September- April “Events Season” has hotted up. I’ve added in a couple of new ones and updated some others. Click the Events tab to find conferences and fairs near you, and be sure to come and introduce yourself!

Ira Wallace of Southern Exposure Seed Exchange, at the Heritage Harvest Festival Tomato Tasting.
Photo courtesy of Monticello

The Heritage Harvest Festival is September 21-22 Monticello, near Charlottesville, Virginia

I’m giving a Premium Workshop on Friday Sept 21, 3-4 pm Classroom 7. Click the link HERE to book for that.

Feeding the Soil

In this workshop I will introduce ways to grow and maintain healthy soils: how to develop a permanent crop rotation in seven steps, and why your soil will benefit from this; how to choose appropriate cover crops; how to make compost and how to benefit from using organic mulches to feed the soil. Handouts.

Book-signing Friday 4.15 – 4.45 pm.

On Saturday there are events all day from 10am to 5pm. $26 general admission.

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Saturday September 29, 2018 Allegheny Mountain Institute Farm at Augusta Health, Fishersville, VA 22939. 9 am – noon

I’m giving a two-hour Class on Season Extension, followed by one-hour Q&A teaching tour of the hoophouse and greenhouse.

Winter-Kill Temperatures of Cold-Hardy Vegetables 2018

Cold hardy winter vegetables 2017 dawling from Pam Dawling

2019 Note: I posted my 2019 list of Winter Kill Temperatures of Cold-Hardy Vegetables on April 30, 2019

March 2020 Update: Click the link

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.

-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

What’s growing in the hoophouse; reading; planning for winter.

Tokyo Bekana in the hoophouse.
Photo Pam Dawling

In the hoophouse we are perhaps half way through our bed preparations. The Tokyo Bekana was the first crop we transplanted from our outdoor nursery bed, and it’s looking very sturdy now. We’ve also transplanted some Yukina Savoy and the first of the lettuces.

Cherry Belle radishes in the hoophouse, early October.
Photo Pam Dawling.

The crops we direct sowed in early September are growing well, and we are harvesting the radishes and some of the tatsoi and Bulls Blood beet greens (thinning to 6″ apart). The spinach is big enough to start harvesting but we haven’t needed to yet.

Hoophouse tatsoi in early October.
Photo Pam Dawling

The newer sowings (the second radishes and the first brassica baby salad mix (mustards) have emerged and are ready to thin to 1″. Sometimes we use thinned seedlings as a salad garnish, but it takes more time than simply pulling them out, and it takes attention to keep them clean.

This summer we grew more cover crops rather than seed crops, which we have been growing in summer for several years, because we were short of workers. In the photo below you can see some healthy cowpeas I’m going to be pulling up later today, as well as some pulled up and dried buckwheat. We don’t dig our cover crops under, just let them die on the surface for as long as possible, shedding bits of dead leaf, then haul them to the compost pile. With the cowpeas, we hope to leave the nitrogen nodules from the roots, by ripping the plants up roughly!

Iron and Clay cowpeas as cover crop in the hoophouse.
Photo Pam Dawling

These cowpeas have been cut back two or three times over the summer, to keep them manageable. At one point, they were black with sooty mold growing on aphid honeydew. We wondered if it was going to be a bigger problem, but after we cut the plants back, most of the aphids seem to have died. We also got a healthy population of ladybugs.

December beds with row cover.
Photo Wren Vile

I gather readers are planning for winter, as many folks have been visiting my Winter-Kill Temperatures List of hardy crops. I update this list every spring, with the info from the previous winter. It’s useful for planning harvests based on forecast temperatures, and it’s useful for planning which winter crops will grow in your location, either inside or out.

On the same theme, I just discovered the WeatherSpark website which provides “The Typical Weather Anywhere on Earth”. Enter your nearest town or airport and you get clearly explained info with fascinating graphics of how the weather goes over the year in your locality. Note this is not a forecast site, it’s about average weather for each place. Useful to people who’ve recently moved and want to know what to expect this winter, or to new gardeners who haven’t paid so much attention previously. Or to those who want to check their assumptions (I really thought the wind was out of the west more of the time than records say). There are charts of high and low temperature, temperature by the hour each month, cloud cover, daily chance of precipitation (both rainfall and snowfall), hours of daylight, humidity, wind speed and direction and solar energy. A big help in making wise decisions. I know that climate change is going to cause havoc with averages, and we’ll need to learn to become better weather forecasters individually, and to use soil temperature and other metrics to decide when to plant. But this website explains things well.

Tomato seed strained in a sieve.
Photo Pam Dawling

I wrote a more concise description of saving tomato seed for the Mother Earth News Organic Gardening blog. For the full length version, see my two posts here and here.

The October Growing for Market is out. Flower farmer Erin Benzakein writes about getting to grips with the marketing side of running a farm. She encourages farmers to get good photos, step out from behind the camera, and dust off their website. I could use some of this advice! (I’ve been very busy writing a hoophouse book, and have necessarily paid less attention to giving presentations and to rejuvenating this website!

Kai Hoffman-Krull writes about on-farm trials of bio-char. I’m looking forward to reading that. Jesse Frost writes about winter CSAs and profiles some he visited. Chris Bodnar covers Italy’s thriving agricultural co-ops and asks if this could be a model for the next phase of the locally-grown movement. Lastly Zach Loeks offers the first of a two-part series on Transitioning to a permaculture market garden.

The September/October issue of Organic Broadcaster is also out. Articles include attending to soil health to improve production; the top reasons customers buy organic foods (accountability, environment, health); interseeding cover crops in cash crops; an interview with farmers in the MOSES Farmer-to-Farmer Mentoring Program; designing an efficient pack shed; and selecting the right meat processor.

Lastly, the campaign www.keepthesoilinorganic.org has posted a letter a letter recently sent out by farming mentor Eliot Coleman about the travesty of allowing hydroponics to be certified as Organic. Hydroponics is a system of growing plants anchored in holes in plastic tubes, or in blocks of inert material, and feeding them with a liquid solution of things that work to produce mature plants. The arrogance of imagining we know everything a plant needs is astounding! The idea that all the many complex ingredients of soil can be replaced with a synthetic concoction is staggering!

Eliot Coleman’s letter includes these quotes:

Organic farming is best defined by the benefits of growing crops on a biologically active fertile soil.

The importance of fertile soil as the cornerstone of organic farming is under threat. The USDA is allowing soil-less hydroponic vegetables to be sold as certified organic without saying a word about it.

The encouragement of “pseudo-organic” hydroponics is just the latest in a long line of USDA attempts to subvert the non-chemical promise that organic farming has always represented. Without soil, there is no organic farming.

Eliot Coleman will be a speaker, along with Fred Kirschenmann, Enid Wonnacott, Jim Riddle, Will Allen, Jeff Moyer, Dave Chapman, Anaise Beddard, Lisa Stokke, Tom Beddard and Linley Dixon at the Jacksonville Rally of the Keep the Soil in Organic movement. Oct 31, 2017 at 12:45 pm – 2:00pm EDT. Omni Jacksonville Hotel, 245 Water St, Jacksonville, FL 32202, USAThis Rally will be a gathering of organic farmers and eaters from all over the world. The march will begin at the Omni Jacksonville during the lunch break from 12:45 to 2 PM on Tuesday, the first day of the NOSB meeting. There will be a 5 minute march to The Landing from the Omni. Lunch will be available at the Rally. For more information, call Dave Chapman at 802-299-7737.