During the spring, from the first signs of bud swell through post-bloom, fruit buds, flowers, developing fruit and emerging shoots are the most sensitive to low temperature (frost) injury.
"Plants vary in their tolerance to below-freezing temperatures, but for the most part hardy plants can tolerate temperatures slightly below freezing without serious injury," says Elizabeth Wahle, fruits and vegetables specialist with University of Illinois Extension. "For example, apple trees in full bloom will usually tolerate temperatures down to 28 degrees F. before injury is measurable; as temperatures drop lower, injury increases. For an apple tree in full bloom, significant injury would be expected if the tree were exposed to temperatures below 25 degrees."
A tender non-hardy plant like tomato left outside without protection will not tolerate freezing temperatures and can actually suffer chilling injury anywhere below 55 degrees. Protecting plants in the home garden from frost damage usually is accomplished by trapping the heat that is radiating from the ground with clear plastic or similar material like straw, bed linens, old tablecloths and lightweight blankets.
Old pillowcases are great for slipping over taller upright plants and cloches made out of various materials work well for smaller plants that cannot be brought inside. A cloche is traditionally a blown glass bell jar that acts like a mini-greenhouse around plants in less than desirable conditions. If you don't have a traditional cloche, large glass jars, milk jugs or buckets can provide similar frost protection.
"To get the full benefit of the frost protection materials, plants need to be covered before temperatures have dropped below freezing," advises Wahle. "Be careful not to crush the plants. Secure containers and edges of row and plant covers to keep them from blowing away in windy conditions."
"Don't make the mistake of thinking that plants can remain covered for a few days when the weather calls for more than one night of frost," adds Wahle. "The protection needs to be taken off every morning when the sun comes out. Plants can easily overheat under a heavy container, a layer of straw, plastic or fabric."
One final issue pertains to rhubarb that has suffered freeze injury. There is an increased chance of toxins normally found in the leaf blade to migrate into the petiole, the portion normally eaten, after exposure to below-freezing temperatures. Do not eat rhubarb that has been exposed to freezing temperatures, especially those petioles (stalks) that are wilted or withered due to freeze injury. Remove these stalks and add them to the compost pile. In addition, do not feed damaged rhubarb to livestock.
Wahle says that other early harvested crops like asparagus do not have this poisoning concern and are safe to consume following a freeze. Asparagus, though safe to eat after a freeze, can still be injured. Freeze injury results in spears turning brown and soft, or they may dry and wither.
Source: Elizabeth Wahle, Extension Educator, Horticulture, email@example.com
Pull date: April 11, 2009