Weve had just about every kind of weather imaginable in recent weeks, from unseasonably warm, sunny days to bone-chilling lows, hailstorms, snow showers, thunder and drenching rains. Last weeks arctic blast, in particular, has many gardeners wondering just how much damage occurred to their prized ornamental plants.
In general, the further along flowers are in their development, the more susceptible they are to freeze injury. Many flower buds are at risk of injury when temperatures drop down to 30° F or below. Theres no question that early blooming rhododendron cultivars like rosamundi, which were already showing flower color, took a real hit. The amount of injury depends on just how cold it gets, how long it stays cold and how open the flower buds are.
In addition, the combination of bright sunshine, wind and cold temperatures could be worse than the cold itself. The problem of cold temperatures is exacerbated when the leaves of some broadleaf evergreen plants can heat up to 50° to 60° F during bright, sunny days. This causes a type of rapid de-acclimation. When the sun sets, the de-acclimated leaf tissue freezes rapidly. Ice crystals form in the individual cells within the leaf, rupturing and killing them. The effect is death of leaf tissue, called sun scald. The leaves on the outside of the plant, and especially those on the south and southwest side, will be most affected. Damage is often most severe on leaves that are perpendicular to the suns rays. Plants vary in their susceptibility to sun scald.
Wind and sun alone, or in combination, can damage evergreens because they cause the plants to transpire or lose water through their leaves. The water is not replaced because the roots cannot pick up water in cold or frozen soil. The leaves turn brown, starting with brown edges or needle tips and progressing between the veins or down the needles.
The most severe damage will be on tissue farthest way from the veins, such as leaf edges, leaf tips and tops of plants. Damage is usually most severe on the side of the plant exposed to sun and airflow. The most acute damage happens on the south and west side of the plant.
For now, the best advice is to take a wait-and-see approach to plant damage. Dont be in a hurry to charge into the garden with pruning shears and start whacking at dead looking wood. Wait until spring growth begins. Winter damage often wont show up until the plant starts to move sap through its system. Although some plants may look dead and have leaf damage, they may revive and begin growth. When you do prune, take out only whats necessary. Plants will recover faster with plenty of leaf surface left to help rebuild.
On the other hand, branches broken from the weight of snow and ice can be pruned now. Make pruning cuts nearly flush against another branch, being sure to leave the branch collar intact. If possible, do not leave small stubs at the ends of branches after pruning. If a branch has completely been broken off a tree, removing the damaged stub will help keep the tree healthier. Improperly pruned branches with stubs left intact are more susceptible to disease entering the decaying end of the branch.
Likewise, dont be in a hurry to prune plants where damage is limited to simply bent branches. Some of these will resume their natural position within a few weeks. Arborvitae and other plants which have copious amounts of foliage and therefore enormous weight on individual branches may need to be tied together and temporarily staked in order to regain their natural growth habit.