Each year, gardeners keep a “weather-ear” out for predictions of that first hard frost. An early frost can stop the clock on plants before their time, long before you’re ready to say “goodbye” to your annuals or even “sleep well” to your perennials. On the flip side, a late killing frost in the spring can nip your hopes for emerging plants in the bud.
During the day, plants and soil absorb and store heat from the sun. As day turns into night, plants quickly begin to lose all their stored heat. Then, if temperatures drop enough, moisture freezes on plant leaves and buds. Clouds can help insulate and slow the loss of heat, but a clear, wind-free night will afford no protection from frost. The temperature within the soil and in the plant’s cells may even drop to a few degrees colder than the air.
Softwoods, actively blooming, and potted plants are the most susceptible to frost damage. The telltale signs are usually visible within two to three days. Browned, mushy leaves and buds sadly greet the unprepared gardener. The best way to cope with the effects of a sudden freeze is to plan ahead and have plant protection at the ready. Portable, potted plants can be brought into sheltered areas. Plants in large, heavy pots, and those growing directly in the ground, however, need to be covered.
Your first inclination may be to grab a vinyl tarp or plastic trash bags. Not the best idea. Plastic or vinyl materials are normally too thin to provide adequate insulation. Since they do not breathe, moisture can get trapped inside. If temperatures drop low enough, this moisture will freeze on your plants, causing more harm than good.
Instead of plastic, use natural fabrics like cotton or linen, an opened burlap bag, or newspaper. These materials are thick enough to provide insulation, but allow enough ventilation for moisture to escape. Commercial coverings may be purchased, but you probably already have materials around the house you can use. Bed sheets, for example, work well for covering large plants and shrubs. Newspaper can be used on low-growing foliage, but won’t stay on top of larger plants well. The important thing is to cover the plants before sunset and be sure the covering reaches the ground beneath the plant. This way, warmth absorbed into the soil during the day is trapped inside the insulating protection.
And remember, frost can even occur in normally frost-free areas, so always pay particular attention to fall and spring weather forecasts. Although he was referring to 18th century fire safety, the words of Ben Franklin hold true for plants, as well. “An ounce of prevention is worth a pound of cure,” so have your plant covering strategies ready to go.