When a freeze is predicted, what happens to your fall vegetables? Perhaps nothing, depending on the length and depth of the freeze. A light frost, during which the temperature falls below 32 degrees Fahrenheit and ice crystals begin to form, can actually improve the flavor of many cool weather greens, such as spinach, collards, and kale. A hard freeze, however—when the air temperature dips below 25 degrees Fahrenheit for at least four consecutive hours—can wreak havoc on your garden. Even these cold-hardy greens will need some extra protection if frequently exposed to temperatures in the low 20s and teens.
You can protect your cool season crops (including lettuce, which is more easily injured than other greens), by throwing a blanket or row cover over the plants. In zones 8 and 9, where tomatoes and peppers bearing fruit may still be in the ground, plants can easily be covered to protect the final harvest. Row covers are made of non-woven polyester; lengths of medium-weight interfacing from the fabric store are close to the same thing and may be easier to find. This insulating material traps the heat radiating from the ground and can keep plants up to five degrees warmer than the outer air. You don’t want to use plastic or any other covering that conducts heat. Also, be aware that cold air flows like water to lower areas, so gardens in low spots are sometimes the first to get frostbitten.
It is worth covering plants a time or two early in the season because these early freezes may be followed by many days of ideal growing weather. If a front blows through, be sure to weigh your cover down with bricks or other available weight as best you can to keep it from blowing around and exposing the plants.
If a freeze does result in damage to your garden, don’t give up on it. Sometimes only the upper or outer parts of a plant are injured, and the plant will continue to grow for a while.
In colder areas, snow can protect plants from extreme cold so that they stay in the garden longer. Spinach, kale, leeks, onions, and other fall garden plants have spent days under the cover of snow only to emerge in perfect condition when the snow melts. Be thankful when a snow precedes a bitter cold—it could preserve your bounty.