In regions with cold winters where frosts occur and the ground freezes, 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. There are "frost pockets" into which cold air drains on radiational cooling nights—clear, calm nights when heat rises from the earth's surface and cold, denser air sinks down.
Plant marginally hardy trees, shrubs and perennials where they will be protected from winds, such as behind a row of evergreens. Plant them near a wall or building and they will receive extra warmth from it. (Winds can damage plants, but they are not affected by "the windchill factor" that forecasters discuss, according to Brotak. "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 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. (NB: 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. 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 they don't tough the foliage, and pin down their edges with stones or bricks. The sheets trap warm air, preventing it from radiating into the atmosphere.
As an alternative to sheets, some gardeners run a sprinkler over the plantings through the night when a light frost is predicted. This sometimes works, sometimes doesn't . . . but will certainly affect your water bill, and may be considered a waste of water.
Move container plantings to a sheltered but unheated location, such as on a porch or into a 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, irreversible tissue damage will be done. 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 near 32˚F."
Get to know your climate better with Weather Facts.
Extend your growing season with The Winter Harvest Handbook by Eliot Coleman.
Get illustrated how-to articles for under $5 with Horticulture's Smart Gardening Techniques downloads. Over 20 to choose from!