Frost- If, When, and How to Protect Your Plants


By Eliot Tozer

It's late fall. The sky is blue, and the sun is bright. Then your local weather forecaster ruins everything with these chilling words: "Possible frost warning tonight." Once the initial panic subsides, reason sets in. Frost is a local event, and it's possible to predict with considerable certainty whether it will hit the plants in your garden. So relax, walk outside, and pay attention to these six signs to predict the likelihood of frost. Then, if necessary, spring into action and find how to protect your plants from frost. 

1. Look Skyward

Clear, calm skies and falling afternoon temperatures are usually the perfect conditions for frost. Frost (also called white or hoarfrost) occurs when air temperatures dip below 32°F and ice crystals form on the plant leaves, injuring and sometimes killing tender plants. However, if temperatures are falling fast under clear, windy skies -- especially when the wind is out of the northwest -- it may indicate the approach of a mass of polar air and a hard freeze. A hard, or killing, frost is based on movements of large air masses. The result is below-freezing temperatures that generally kill all but the most cold-tolerant plants.

But if you see clouds in the sky -- especially if they are lowering and thickening -- you're in luck. Here's why. During the day, the sun's radiant heat warms the earth. After sunset, the heat radiates upward, lowering temperatures near the ground. However, if the night is overcast, the clouds act like a blanket, trapping heat and keeping air temperatures warm enough to prevent frost.

2. Feel the Breeze

Wind also influences the likelihood of frost. In the absence of wind, the coldest air settles to the ground. The temperature at plant level may be freezing, even though at eye level it is above freezing. A gentle breeze, however, will prevent this settling, keep temperatures higher, and save your plants. Of course, if the wind is below freezing, you'll probably have fried green tomatoes for tomorrow's supper.

3. Check the Moisture

Just as clouds and gentle winds are your friends, so are humidity and moisture. When moisture condenses out of humid air, it releases heat. Not much heat, true, but perhaps enough to save the cleomes. If the air is dry, though, the moisture in the soil will evaporate. Evaporation requires heat, so this process removes warmth that could save your peppers.

4. Check Your Garden's Location

This can have a tremendous influence on the likelihood that early frost could wipe out your garden while leaving your next-door neighbor's untouched. For example, as a general rule, temperature drops 3°F to 5°F with every 1,000-foot increase in altitude. The higher your garden, the colder the average air temperature and the more likely your plants will be hit by an early freeze. So gardening on a hilltop isn't a great idea, but neither is gardening at the lowest spot on your property. Since cold air is heavier than warm air, it tends to sink to the lowest area, causing frost damage. The best location for an annual garden is on a gentle south-facing slope that's well heated by late-afternoon sun but protected from blustery north winds. A garden surrounded by buildings or trees or one near a body of water is also less likely to be frosted.

5. Scrutinize the Soil

Your garden's soil type can affect the amount of moisture it holds and the plants' ability to withstand cold weather. Deep, loose, heavy, fertile soil releases more moisture into the surrounding air than thin, sandy, or nutrient-poor soil. The more humid the air, the higher the dew point and the less likely that frost will form on those plants. Heavily mulched plants are more likely to be frosted, since mulch prevents moisture and heat in the soil from escaping and warming the surrounding air. (Light-colored mulches such as hay or straw have the additional disadvantage of reflecting sunlight and heat during the day.)

6. Know Your Plants

The plant itself determines the likelihood of frost damage. Immature plants still sporting new growth into the fall are most susceptible -- especially the new growth. Frost tolerance tends to be higher in plants with maroon or bronze leaves, because such leaves absorb and retain heat. Downy- or hairy-leaved plants also retain heat and reduce wind-drying of the leaves. Compact plants expose a smaller proportion of their leaves to cold and drying winds. By the same token, closely spaced plants protect each other.

What's a Gardener To Do?

So you've checked the weather conditions and decide that, yes, Jack Frost is coming and protecting your plants is worthwhile. You'll want to do two things: First, cover your plants, both to retain as much soil heat and moisture as possible and to protect them against strong winds, which can hasten drying and cooling. Use almost anything to cover plants: newspapers, bushel baskets, plastic tarps, straw, or pine boughs. Spun-bonded fabric row covers, also called frost cloth or frost blankets, will only protect plants from frost down to 30°F, polyethylene row covers to 28°F. How you cover your plants to protect them is just as important as the material you use. Cover the whole plant before sunset to trap any remaining heat. Lightweight coverings such as row covers and newspaper should be anchored to prevent them from blowing away.

Second, keep the soil moist by watering your plants the day the frost is predicted. Commercial fruit and vegetable growers even leave sprinklers on all night to cover plants with water. As the water freezes, it releases heat, protecting the plants, even though they're covered in ice. To prevent damage, the sprinklers need to run continuously as long as temperatures remain below freezing.

And as you survey your garden's fading glory, you may take heart from the experience of John Loudon, a 19th-century British horticulturist. Loudon stuck four stakes into a plot of grass to support a cambric handkerchief 6 inches above the surface and found that the temperature beneath it remained 9°F warmer than the temperature of the surrounding air. Yes, you can offer protection from the frost and prevent damage by properly covering plants that aren't hardy. 

Do Consider Dew

The dew point is the temperature at which the air is totally saturated with moisture. Television and radio meteorologists may state the dew point temperature during routine forecasts.

The more moisture the air contains, the higher the temperature will be when the moisture starts to condense as dew, producing heat. And, obviously, the higher the temperature, the less chance of frost. For example, a dew point of 43°F almost certainly means no frost that night.

Interestingly enough, frost is more likely to form on a dry evening when the air temperature is a warmish 50°F and the dew point is a low 33°F than when the air temperature is a cooler 43°F and the dew point is 41°F.

Research your frost dates

You can lookup your frost dates by zipcode using our handy frost-dates report tool.

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