Frost Facts and Landscape Plants
Some plants tolerate frost and cold temperatures better than others. Plants always killed by frost include summer annual flowers and summer vegetables like impatiens, marigolds, coleus, tomatoes and peppers.
Subtropical and Tropical plants are the most sensitive to frost (citrus, hibiscus and bougainvillea). Many herbaceous perennial plants will die back, but their roots and storage organs will survive the winter and growth will resume in spring (salvia, begonia). Tolerant vegetable plants include lettuce, spinach, chives, and the cole crops such as broccoli, cauliflower, Brussels sprouts, cabbage, and kohlrabi. Carrots and parsnips can be left in the ground all winter if you mulch them heavily. We can’t change winter weather, but we can protect our favorite garden plants from winter frost damage.
Understanding cold weather
The sun warms the earth’s surface during the day and the heat is radiated back into the atmosphere during the night. The coldest temperatures occur about daybreak.
Clouds at night absorb and reflect heat back to the earth.
Calm, clear nights pose the greatest danger of frost, since there’s no wind to mix the ascending warm air with the descending cold air and no clouds to radiate heat back to the soil.
Humidity slows temperature change in the air.
Cold air settles downward, flowing like water, to the lowest point. Hot air rises.
Cold wind compounds temperature loss
How does frost occur? Frosts occur on still, clear nights. Heat from the ground radiates to the sky, and the surface gets colder and colder. When the surface reaches the freezing point, then the layer of water vapor in contact with the surface freezes into ice.
How plants respond to cold weather
The effects of cold temperatures vary with plant species, growth stage, age, general health, and water content. Young, actively growing, flowering, and dehydrated plants are more vulnerable.
Tender plants have a low tolerance of freezing temperatures. Tropical and subtropical plants fall into this category.
Hardy plants resist or tolerate freezing temperatures (as in “hardy to -20° F).
Cold temperatures and short day length slow plant growth and cause dormancy, making plants less susceptible to frost damage.
Frost injury occurs when ice crystals form on the leaf surface and draw moisture from (dehydrate) the leaf tissue.
The lower the temperature, the longer the exposure, and the faster the temperature drops, the greater the damage to the plant. Prevent frost damage Start by selecting frost tolerant plants. Choose the best sites in your garden for sensitive plants. Plants on the south or the west have the greatest opportunity to absorb heat and light during the day and reradiate it at night, however south and east exposures warm up earliest in the morning. Plants in the open are more exposed and susceptible to damage than if they were planted next to a warm wall. Full sun is warmer than shaded locations. Loose mulches and cultivated soils are less insulating than bare, firm moist soil because there’s more air space that loses heat more quickly.
How to protect plants from frost
Protecting plants from frost damage usually is accomplished by trapping the heat that’s radiating from the ground with clear plastic, by moving the plant to a sheltered location, or by directly providing heat via light bulbs. Each measure gives about 2-3 degrees of protection. Keep plants well watered to thwart the desiccation of frost.
Get ready
If an unexpected light frost catches you unprepared, simply drape a plastic tarp, blanket, or newspapers over plants before nightfall to trap the heat. It’s better to drape the cover over a frame or wire cages so it doesn’t touch the plant leaves. Leaves touching the covers are likely to be frost damaged. People tend to leave coverings on for weeks, but the coverings should be removed during the day to allow heat in and then re-covered at night.
You can use the larger size electric holiday lights (not the minis) to generate a little warmth especially if plants are covered, or even single light bulbs in outdoor approved fixtures. Be sure to hang the bulbs near the base of the plant, allowing the heat they generate to rise. Take care that the bulb isn’t so close to the trunk, a branch, or the cover so that it could burn.
You can protect plants from a light frost where the temperatures get no colder than 28 degrees. Lower temperatures will probably kill plants, even with protection. Mimic citrus growers Citrus growers keep their orchard floors weed-free and well-watered in winter because it gives them a couple of degrees of warmth each night. During the day, the bare soil absorbs more heat than one covered with plants or weeds, and in the evening the radiated heat keeps the air around the trees warmer. This protects the trees from frost for a few more hours each night. In addition when frost becomes a real concern, citrus growers turn on the sprinklers. As the water freezes on the plant it gives off heat and this heat keeps the plants from freezing. They continue sprinkling until the ice melts off the plants.
Lucky for us, some plants love winter. Pansies, violas, snapdragons, Iceland poppies, cyclamen, stock, and flowering kale provide color in our cold, gray months and can be planted all winter long.
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