Gardening Articles: Flowers :: Annuals
A Gardener's Guide to Frost
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 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.
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.