We are all aware of the unusually warm weather pattern in March that brought plants out of dormancy close to a month earlier than normal. Our April weather has been more seasonal, with quite a few nights dipping below freezing and causing frosts. We are still early in the spring season, and more frosty nights may still occur. Frost damage occurs when sensitive young plant tissue freezes. Particularly affected are early flowering trees such as magnolia, pears, and apples. Temperatures below freezing can cause the tender flower tissue to be killed and turn brown rather quickly. Magnolia and other early-flowering trees are always at risk for frost damage, but the unusually weather this year has made them more susceptible than ever before.
Generally flowers are affected much more than leaf tissue, though a hard frost when leaf tissue is emerging may also cause leaf damage, browning or leaf drop to occur. Tree sensitivity to frost damage varies from species to species. Partial leaf damage is usually exhibited as necrotic (black, dead) spots on leaves causing leaf distortion as the leaves unfurl. Most hardwood trees such as maple and oak are very resilient to frost damage. If they are reasonably healthy, their dormant buds will become active and eventually they will reform new leaves, and by midsummer if will be most difficult to see that they were damaged. Most likely, the greatest damage will be some loss of growth and a slight reduction in health and vigor because of that lost growth. If the tree was marginal in terms of health and vigor, the damage may be more significant, and in a very few cases, cause branch dieback and in the most severe cases even tree mortality. Proper watering, fertilization, and insect and disease protection measures are recommended for trees exhibiting stress factors such as frost.
Evergreen trees such as spruce and pine trees are generally more resilient to frost damage. But frost damage loss is more significant than the loss of growth with deciduous trees because conifers have fewer dormant buds. However, most conifers will recover and may in some cases put on secondary growth later in the spring. In some cases, with young trees in particular, the needle and twig loss may be sufficient to cause mortality. Many of the conifers are also able to survive the frost damage because different parts of the tree are at different stages of growth expansion; earlier stages are more tolerant of cold conditions than later stages.
What you can do to protect your plants
Irrigate before the frost – A moist soil can hold four times more heat than a dry soil. It will also conduct heat to the soil surface faster than a dry soil, aiding in frost prevention. In a study performed years ago, the air temperature above a wet soil was 5 degrees F higher than that above a dry soil and the difference was maintained until 6 a.m. the next morning. Thus, plants should be well watered the evening before a frost.
Cover your plants – Covering plants can give you 2 to 5 degrees F protection. The covers can be laid right over the crop, or can be supported on stakes. The difference being that protection is less wherever the cover touches the plant. Any material can be used to cover the plants, however woven fabrics are better insulators then plastics or paper. The best time to apply covers is in the late afternoon after the wind has died down. Remove covers the next morning before the sun hits them.