Preparing for Frost and Cold Weather

The crew working on the sweet potato harvest.
Photo McCune Porter

We harvested our sweet potatoes in a three-day marathon between one heavy rain and the side-swipe from Tropical Storm Nestor as it blew by. It’s a very good sweet potato harvest for us this year!

We have been making terminal harvests of other crops, working our way from least hardy to those we can leave outside all winter. See our table Winter Kill Temperatures of Cold-Hardy Vegetables.  We have harvested or covered all the frost-tender crops, made a last harvest of rhubarb (the stems are hardy to 22°F (-6°C)) and hooped and rowcovered the last outdoor lettuce. Soon we’ll harvest our June-planted potatoes, our last big harvest for the year.

We have pulled 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. The winter radish are good down to 10°F (-12°C). We’ll harvest and store them before it gets that cold.

Savoy cabbage with frost.
Photo Lori Katz

Frost Alert Card

We’ve just had a couple of light frosts with no obvious crop damage yet. Our average first frost is 10/20 (13 years of our own records), so no big surprise there. 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.
When frost threatens, harvest all peppers exposed to the sky. Corona is one of our favorite orange peppers (still not ripe here). Photo  Kathryn Simmons

Our pepper strategy is worth noting: 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 plants in the morning, or the air 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.

These tomatoes had overhead irrigation all night when it got frosty. They thawed out and survived.
Photo Southern Exposure Seed Exchange

Four Ranges of Cold-Hardiness

– this simple model helps reduce confusion and set priorities.

  1. Crops to harvest before cold fall weather (32°-25°F) and store indoors: Chicory for chicons or heads; crosnes/Chinese artichokes, dry beans, Napa Chinese cabbage, peanuts, potatoes, pumpkins, seed crops, sweet potatoes, winter squash
  2. Crops to keep alive in the ground into winter to 22°-15°F (-6°C to -9°C), then harvest.
    1. Store: Beets, cabbage, carrots, celeriac, kohlrabi, winter radish (including daikon), rutabagas, turnips,
    2. Use: Asian greens, broccoli, cabbage, chard, lettuce, radishes
  3. Hardy crops to store in the ground and harvest during the winter. In zone 7, they 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. 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

Cold weather crop protection

Closing rowcovers after a winter spinach harvest.
Photo Wren Vile
  • Rowcover – thick 1.25 oz rowcover gives about 6F (3.3C) degrees of frost protection. Use hoops.
  • Low tunnels and Quick Hoops, wider version of using rowcover. Need 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.
  • Caterpillar tunnels – 2 beds plus 1 path, tall enough to walk in. Rope holds cover in place, no sandbags.
  • High tunnels (= hoophouses), single or double layer. Double layer gives 8F (4.5C) degrees of protection, plus plants tolerate colder conditions than they would outside. 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

View through the hoophouse doors in December.
Photo Kathleen Slattery
  • 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. The Typical Weather Anywhere on Earth. Enter your town and learn about the weather where you are. The weather for Louisa, Virginia (population 1.621) includes “Tourism” as a weather feature!

Frost Protection Practice and Economics FAO.pdf for all the technical info you could hope for.

PS – We solved our hoophouse sliding door problem – the track had got splayed too wide and the wheels jammed up.

2 thoughts on “Preparing for Frost and Cold Weather”

  1. Pam,
    Thanks for the depth of your info. Grants permission to head into fall and winter with joy, when insects and certain weeds recede, also less watering. And afternoons in the sun are sought after, not avoided.

    While gathering walnuts, have seen our naturalized chickweed (Stellaria media) taking off. Despite several hard freezes. I forget that certain edibles, such as winter annuals, know how to stay green most/all winter in Zone 6.
    Are there any ‘weeds’ you welcome in your high tunnels or under outdoor rowcover?

    And what of this dance: When to keep the covers off during those first few hard freezes, to kill unwanted weeds hoping to mature seed, and even to kill or chase away unwanted insects. Such as aphids. When is a modicum of stress to the main crop welcomed, or at least tolerated, with other goals in mind?

    Our “frost kissed kale” sells better at market…

    So on which crops do you welcome light to moderate freezes, and why?

    So thankful here for widespread rains in SW VA!

    1. Richard,
      Yes, the sunny afternoons are wonderful. We are up to our eyebrows transplanting into our hoophouse, Russian kales most recently.No, I don’t think there are any weeds I welcome in the high tunnel, as those winter annual weeds do set seed so quickly. (As does the summer annual galinsoga). Outdoors I don’t welcome the winter weeds either, except to anchor the soil over the winter where we don’t have actual cover crops. I know there are many edible weeds, but I love spinach, kale and lettuce more!
      We do try not to cover our spinach for the winter until after the galinsoga has been frozen off.I think frost improves the flavor of spinach too. Which leads right on to your next topic. We try not to harvest kale before frost, as it tastes so much better afterwards. The same is true for parsnips and Brussels sprouts too. Anyone else got crops to add to the “taste better after frost” list?

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