Getting ready for frost and colder weather

 

Eat-All Greens rows with frost in December. October and part of November are still productive growing weeks in central Virginia. Photo Bridget Aleshire

Gardening does not end with the first frost! We will work our way from clearing the least hardy crops to those we can leave outside all winter. See our table Winter Kill Temperatures of Cold-Hardy Vegetables. We will harvest or cover all the frost-tender crops, make a last harvest of rhubarb (the stems are hardy to 22°F (-6°C)) and hoop and rowcover the last outdoor lettuce. Our June-planted potatoes will be our last big harvest for the year.

We pull up the biggest Purple Top turnips and Cylindra beets, leaving the others a bit more room to size up before their killing temperature of 12°F (-11°C). Any day now, we’ll start harvesting fall leeks (King Richard and Lincoln), keeping the winter-hardy ones (Tadorna) for the winter.

DIY weather-forecasting

I recommend learning your 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,
  • and recurrent waves of cold Canadian air in winter.
Beds after rain, Photo Wren Vile

Rain (statistically 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.

Blackberry leaf with frost.
Photo by Ezra Freeman

Our average first frost date is October 14. Actually from our own records it has averaged 10/20 (13 years of our own records). So it’s time to start thinking about frost. It’s good to be prepared.

According to Dave’s Garden in Louisa County where I live, the threshold of 36°F (2°C) has a 50% likelihood on Oct 3; the 32°F (0°C) threshold has a 50% likelihood by Oct 13 and the 28°F (-2°C) threshold is as likely as unlikely by Oct 27. The 90% chances occur by Oct 14, Oct 28 and Nov 12 respectively.

Another great website for local weather info is WeatherSpark.

Now that climate change is here, it pays to be ready for weather different from what we have experienced previously. Keeping records helps, as does having good thermometers for air and soil.

Four Ranges of Cold-Hardy Crops for Harvest at Various Stages of Winter

This simple model helps reduce confusion and set priorities

1. Crops to harvest before cold fall weather (32°-25°F) and store indoors:

Michihili Chinese Cabbage. Photo Southern Exposure Seed Exchange

Chicory for chicons or heads; crosnes/Chinese artichokes, dry beans,  Chinese cabbage, peanuts, “White” Peruvian potatoes at 32°F (0°C) approximately, pumpkins, seed crops, sweet potatoes at 50°F (10°C), winter squash.

2. Crops to keep alive in the ground into winter to 22°-15°F (-6°C to -9°C), then harvest.

Bucket of freshly harvested Detroit Dark Red beets for storage.
Photo Pam Dawling

Store: Beets before 15-20°F (-9.5 to -7°C), cabbage, carrots before 12° F (-11°C), celeriac before 20°F (-7°C), kohlrabi before 15°F (-9.5°C), winter radish including daikon before 20°F (-7°C), rutabagas, turnips before 20°F (-7°C).

Use soon: Asian greens, broccoli, cabbage, chard, lettuce, radishes

3. Hardy crops to store in the ground and harvest during the winter.

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

In zone 7, such crops need to be hardy to 0°-10°F (-17.8°C to -12.3°C): Collards, horseradish, Jerusalem artichokes, kale, leeks, parsnips, scallions, spinach.

4. Overwinter crops for spring harvests before the main season.

A stormy winter day, garlic, rowcovered spinach beds and our hoophouse.
Photo Wren Vile

In zone 7, they need to be hardy to 0°-10°F (-17.8°C to -12.3°C): Cabbage, carrots, chard, collards, garlic and garlic scallions, kale, multiplier onions (potato onions), scallions, spinach.

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.
Ice on the pond.
Credit Ezra Freeman

Frost Alert Card

For just this time of year, we keep a Frost Alert Card reminding us which crops to pay attention to if a frost threatens. We check the forecast online at 3.30 pm (we find that’s late enough to be fairly accurate about night temperatures and early enough to give us time to get vulnerable crops covered).

The big decision is the triage of harvest/cover/let go. Our list is not just crops that will die with the first frost but also ones that will soon need covering as temperatures decrease.

  • Cover lettuce, zucchini, summer squash, cucumbers, beans, Chinese cabbage, pak choy, lettuce and celery.
  • Harvest crops listed above that can’t or won’t be covered.
  • Harvest all ripe tomatoes, eggplant, corn, limas, cowpeas, okra, melons.
  • Harvest peppers facing the open sky, regardless of color. (Often only the top of the plant will get frosted).
  • Check winter squash and harvest any very exposed squash.
  • Set up sprinklers for the night, on tomatoes, peppers and a cluster of beds with high value crops.
Peppers that are protected by leaves can survive a light frost. Photo Pam Dawling

We really like this pepper strategy we have developed: by picking just the peppers exposed to the sky, we reduce the immediate workload (and the immediate pile up of peppers in the cooler!) and we often get a couple of milder weeks after the first frost before the next. By then the top layer of leaves that got frosted the first time will have died and a whole new layer of peppers will be exposed and need harvesting. This way we get fewer peppers at once, and a higher percentage of ripe peppers, which have so much more flavor.

Our overhead sprinkler strategy is useful if a frost is coming early when we still have many tomatoes we’d like to vine-ripen. Keep the sprinkler running until the sun is shining on the plant sin the morning, or the sir temperature is above freezing again. The constant supply of water during the night does two things. First water gives off heat as it freezes. Yes, really. It’s easier to understand ice taking in heat to melt, but the flip side is that water gives off heat as it freezes. This latent heat of freezing helps warm the crops. And if ice does form, the shell of ice around the plants stops more cold damage happening.

Frost Alert List

Task

Crop

Notes

 

Harvest all edible

Asparagus beans

 

Harvest all edible

Eggplant

 

Harvest all edible

Okra

 

Harvest all edible

Tomatoes

Including green ones

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, 6, then cover

Uncover once mild again

 

 

 

Thick row cover

Squash

Spring hoops or none. Ditto

Thick row cover

Slicing cucumbers

Spring hoops or none. Ditto

Thick row cover

Celery

Double hoops – leave covered

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

Roma paste tomatoes and peppers

Ditto

Set sprinklers

Other vulnerable raised bed crops

Ditto

Sun Gold cherry tomatoes. Pick the green and the ripe ones before a frost.
Photo Pan Dawling

Cold Weather Crop Protection

  1. Rowcover – thick 1.25 oz rowcover gives about 6F (3.3C) degrees of frost protection. Use hoops.
  2. Low tunnels and Quick Hoops are wider version of using rowcover. They need the edges weighting down. Best for climates where the crops are being stored in the ground until spring, when they start growing again. Less useful in climates like ours which have very variable winter temperatures, and are warm enough that we realistically expect to harvest during the winter, not just before and after.
  3. Caterpillar tunnels – 2 beds plus 1 path, tall enough to walk in. Rope holds cover in place, no sandbags.
  4. High tunnels (= hoophouses), single or double layer. Double layer gives 8F (4.5C) degrees of protection, plus plants can survive 14F/8C colder than they can outside, without extra rowcover; at least 21F/12C colder than outside with thick rowcover. Leafy crops are not weather-beaten. We strongly believe in two layers of plastic and no inner tunnels (rowcovers) unless the night will be 8°F (-13°C) or colder outdoors.

Hoophouse Notes

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. Brite Lights chard got frozen leaf stems.

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

Other Posts on These Topics

10/18/16 Getting Ready for Frost

10/21/19 Preparing for Frost and Cold Weather

10/19/20 Harvest and Maturity Indicators in Vegetable Harvests.

10/16/18 Season Extension and Frost Preparations