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: [email protected] 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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Getting ready for frost, harvesting sweet potatoes

Sweet potato harvest. Photo Nina Gentle
Sweet potato harvest.
Photo Nina Gentle

Our average first frost date is October 14. Actually from our own records it has averaged 10/22 over the last 11 years. So any time now, we could get a frost. We had a very light patchy frost 10/10 with a recorded overnight low of 36F, but now we are having a warm spell (very nice). It’s good to be prepared. We harvested our sweet potatoes (not a good yield, too many deer). We are getting ready to harvest our white potatoes.

DIY weather-forecasting

I recommend learning the local weather patterns by keeping records and watching what happens. Here’s what I’ve learned about ours:

  • Our mid-Atlantic climate is controlled by three weather systems,
    • mainly by moisture from the Gulf of Mexico,
    • the Bermuda High Pressure area in summer,
    • recurrent waves of cold Canadian air in winter.
  • Rain (fairly evenly distributed throughout the year in our county)
    • has slight peaks in January, February and March
    • and again in early June and August.
  • Some parts of our area can experience long periods of drought.
    • September-November is the drier season but it’s also the hurricane season, so the net result is very variable.
  • We use Wunderground.com but subtract 5F° from their forecast night lows for our nearest town, and mentally downgrade the chance of rain by 10%, as rain often passes us by as it scoots along the river valley north of us.

Local Information

According to  a Freeze / Frost Occurrence Data pdf for Virginia which I have, in Louisa County the threshold of 36F has a 50% likelihood on Oct 3; the 32 F threshold has a 50% likelihood by Oct 13 and the 28F threshold is as likely as unlikely by Oct 27. The 90% chances occur by Oct 14, Oct 28 and Nov 13 respectively. The data for spring are available here. probably the site has the fall info too, but I’m out of time to look, due to three hours without internet this afternoon.

Frosty daikon leaves. Photo Bridget Aleshire
Frosty daikon leaves.
Photo Bridget Aleshire

Help predicting frost

We keep this reminder list handy at this time of year, when trying to make our own frost forecasts:

Frost is more likely at Twin Oaks if:

  • The date is after 10/14 or before 4/30.
  • The daytime high temperature was less than 70°F (21°C).
  • The sky is clear.
  • The temperature at sunset is less than 50°F (10°C).
  • The dew point forecast is low, close to freezing. Frost is unlikely if the dew point is 43°F or more.
  • The Wunderground 3.30pm forecast low for Louisa Northside is 37°F (3°C) or less.
  • The soil is dry and cool.
  • The moon is full or new.
  • There is little or no breeze, although if temperatures are falling fast, the wind is from NW and the sky is clear, then polar air may be moving in, and we’ll get a hard freeze.

We also make for ourselves a Frost Alert Card, so we can quickly know which beds we need to attend to if we decide a frost is likely. (RBs means raised beds)

Task Crop Notes
Harvest all edible Asparagus beans
Harvest all edible Eggplant
Harvest all edible Okra
Harvest all edible Tomatoes Incl green
Harvest all edible Peppers exposed to the sky
Harvest all edible West Indian gherkins
Harvest all edible Pickling cucumbers
Harvest all edible Corn
Harvest all edible Beans #4, 5 Uncover once mild again
Thick row cover Beans #5,6 Ditto
Thick row cover Squash Spring hoops or none. Ditto
Thick row cover Slicing Cucumbers Spring hoops or none. Ditto
Thick row cover Celery
Thick row cover Last Lettuce bed Double hoops – leave covered
Set sprinklers Slicer tomatoes Overnight from before 32F till after sun shines on plants
Set sprinklers Romas and peppers Ditto
Set sprinklers Other vulnerable RBs Ditto

FrostProtectionFundamentalsPracticeandEconomicsFAO.pdf

is a 126 page book which includes explanations of freezes and frosts (both radiation frosts and advection frosts), and a chapter on recommended methods of frost protection for food crops. probably none of us will need the information on wind machines and helicopters (maybe little drones work for small scale growers?). The publication includes frost forecasting and frost damage physiology (for growers not in a panic about immediate frosts, who want to understand what the plants are going through).

Potato harvest November 2015 Photo Lori Katz
Potato harvest November 2015
Photo Lori Katz