know the limits
To understand what steps to take when freeze warnings threaten, you need to know the point at which precious vegetation fades to a frost-burnt brown. The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours.
Of course, there are exceptions to this rule. Seedlings, with their tender new leaves, often give up when temperatures drop to 32-33°F. tropical plants have different low temperature thresholds. some crash when temperatures drop to 40°F; others crumble at 35°f. Other plants are naturally hardy and can withstand temperatures as low as 18-20°F. To find the threshold for your plants, look for gardening books and online resources.
quick fixes for frost warnings
Pick It Up: The easiest cold protection scheme is to move plants out of harm’s way. this works with seedlings in flats and potted plants. moving plants under a deck, into a garage or shed, or onto a covered porch often offers ample protection.
Count on water: Irrigate the soil just before sunset to increase the nighttime air temperature around the plants as the water evaporates. Fill gallon jugs or buckets with water and place them in the sun during the day. at night, move them near endangered plants. water will moderate the temperature of the air; if it freezes, it will release heat. For the greatest effect, paint some containers holding water black to maximize heating during the day.
keep air moving: cold, still air does more damage to plants. Stir up a breeze all night with an electric fan to prevent frost from forming on the plants. remember to protect the electrical connections from moisture.
Cover plants: Protect plants from all but the deepest frosts (28°F for five hours) by covering them with sheets, towels, blankets, cardboard, or a tarp. You can also invert baskets, coolers, or any container with a solid bottom over the plants. cover plants before dark to trap warmer air. ideally, the covers should not touch the foliage. anchor fabric coverings in windy conditions.
in the morning, remove covers when temperatures rise and frost dissipates. heat from the sun can build up under solid covers, and plants can die from the high temperatures.
spreading blankets – keep gardening blankets, often called row covers, on hand. These covers are made of synthetic fibers or plastic in different thicknesses. place row covers directly over plants, or create a tunnel by suspending them over a bed with stakes.
Turn on the lights: An incandescent bulb generates enough heat to raise the temperature of nearby air enough to protect a plant from freezing. the bulbs must be close to the plants (within 2 to 3 feet) for this technique to work. (Fluorescent bulbs don’t generate enough heat for this task.)
Protect individual plants: Install heated lids (rigid plastic containers with ventilation holes) over individual seedlings at planting time. hot lids act like cloches (mini greenhouses), but the vent holes eliminate the daily chore of putting on and taking off the cover. Create the equivalent of a hot lid using two-liter plastic bottles or gallon jugs with the bottoms cut off and the lids removed (but saved). replace caps at night when cold temperatures take hold.
A variation on the hot top idea is a water wall teepee, which surrounds individual plants with a sleeve of water-filled tubes. the water absorbs the sun’s heat during the day. at night, as the water slowly freezes, it releases stored radiant heat from the sun, keeping the air inside the tipi free of frost.