Wait to see extent of freeze injury before deciding how to help damaged landscape plants

Chris Starbuck, MU Division of Plant Sciences

Damage to landscape plant from the Easter freeze of ‘07 has become increasingly evident with the return of more normal temperatures. Many trees, shrubs and herbaceous landscape plants were in full bloom, with extensive, succulent leaf and stem growth when the temperature dropped to the mid- or lower 20s over much of the state. Since flowers and new shoots have little cold tolerance, most plants exhibited brown petals and foliage. Fruit growers around the state have reported partial or total crop loss. Extension offices are fielding many questions from homeowners concerned about the health of plants in their landscapes. While some species, such as river birch and redbud, sent forth new shoots to replace those killed by the freeze fairly quickly, other species were only just beginning to show signs of recovery in the last week of April. For many plants, it may be months before the extent of the damage can be accurately assessed. In some cases, there may have been severe damage to the bark or to the vascular connections to buds and stems. This may result in a gradual dieback of stems or even death of entire plants of relatively cold sensitive species like Japanese maple and butterfly bush. In some cases, plants that appear to be recovering may collapse with the first heat because of vascular damage. Stem damage may also lead to increased incidence of canker causing diseases like fire blight and anthracnose.

Given that the Easter Freeze of 2007 is unprecedented, it is hard to predict the extent of damage it has inflicted on landscape plants. The best approach to dealing with freeze-injured plants will be to wait until at least mid-May before deciding what might be done to help them recover. By that time, it should be more evident which plants are likely to recover quickly and which ones may need some help. Avoid pruning until the extent of dieback becomes more apparent. Also, avoid fertilizing plants heavily, since this may force rank growth which will be difficult to manage and susceptible to drought injury. Keep in mind that developing fruits utilize much of the energy that plants harvest through photosynthesis. When fruit is eliminated by a freeze, a plant channels more energy into producing stems and leaves. Excessive pruning may create a monster that will take years to tame. Moderate pruning of soft growth in the summer may be the best approach, pruning off errant shoots as they manifest themselves. Mulching to maintain a uniform soil moisture level and irrigation during drought conditions are the two cultural practices that will provide the most benefit to recovering plants.

Many homeowners and gadeners have expressed concern about the extent of damage on Japanese maples. I have observed a wide variation in damage to this species with some cultivars showing little injury and others being killed to near the ground. Here are some images of a well established 'Garnet' maple on April 10 and after pruning on May 2. While most of the main branches survived the freeze, this plant will require extensive pruning over the next few years to get it back to an aesthetically pleasing shape.