In regions where frosts occur and the ground freezes in winter, there's a typical "last frost date"—the date in spring by which you can somewhat safely assume no more frosts or freezes will occur until fall. You can find out this date in your area by checking with your local co-operative extension office or asking experienced gardeners in your neighborhood. Frosts can and do occur after the last frost date, however. Here are strategies for coping with late frosts:
Protect Plants from Cold Snaps
Avoid planting tender plants in low-lying areas. These may be "frost pockets" into which cold air drains on nights with radiational cooling—clear, calm nights when heat rises from the earth's surface and cold air, which is denser, sinks down.
Plant marginally hardy trees, shrubs and perennials near a wall or building, where they may receive extra warmth.
Don't worry too much about wind, aside from its drying effects. Winds can damage plants by breaking their stems or branches, but plants are not affected by "the windchill factor" that forecasters discuss, according to Dr. Ed Brotak, a meteorologist who covers weather and climate for Horticulture. "The windchill deals with the wind's ability to remove heat from warm-blooded animals, which generate their own heat. Plants don't.")
Watch weather reports and weather.gov, the website of the National Weather Service, for frost warnings, watches and advisories. Clear skies, a lack of wind and a chilly afternoon generally precede a night with a drastic drop in air temperature. "On a clear night, the dew point can be a good indicator of how low the temperature will get by morning," notes Brotak. Keep in mind that official frost/freeze warnings—which mean the event is imminent or already occurring—are issued by the National Weather Service only when frosts threaten after the average last freeze date. So if April 15 is the average last frost/freeze date in your area, warnings will be posted only after that date—even if it has been an unusually warm spring and you've been able to start planting before April 15.
If a light frost is predicted, drape tender plants, newly planted perennials and anything with sensitive new growth or flower buds with old bed sheets. If a killing frost is predicted, prop the sheets up on stakes so that the material doesn't tough the foliage, and pin down the edges with stones or bricks. The sheets will trap warm air, preventing it from radiating into the atmosphere.
Move container plantings to a sheltered but unheated location, such as a porch, garage or shed.
If you wake to find tender plants covered in frost, try spraying them with water to wash the ice crystals away. If the sun melts the crystals before you have a chance to do this, irreversible tissue damage will occur. The plant may survive, but it will have scars. Annuals will likely die, except for the hardiest annuals.
If potted plants have been left out and are affected by frost, don't try to warm them by bringing them indoors; the sudden change in temperature would damage them further.
What If It Snows?
"Snow collecting on a plant surface will not directly harm the plant structure," says Brotak. "In fact, the snow may help to insulate the plant from even colder air temperatures, keeping the temperature on and around the plant close to 32˚F."