Pruning frost-damaged plants
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Many plants broke bud and started to grow early this year due to the unusually warm temperatures in April and early May. These warm temperatures were followed by freezing temperatures that caused damage to newly emerged shoots. The damage ranged from foliar malformations and necrosis to shoot die-back. In the cases where shoot die-back occurred, pruning out the dead and damaged portions of the plant are recommended to maintain the health and vigor of the plant.
First, let’s have a quick review of proper pruning techniques for small shoots and branches. Since large branches are usually not lost to frosts, we will not review how to prune them. Young shoots and small branches that have not formed branch collars should be either pruned back to the branch from which they emerge or to a lateral bud below the area of damage. When pruning to a lateral bud, the pruning cut should be made flat across the branch and just above the bud. A flat cut allows faster healing since there is less surface area exposed from the cut. Since the lateral bud will form the new shoot, cutting just above it reduces the amount of shoot material that will not grow and will eventually die back to the lateral bud.
Proper pruning when plants are damaged by frosts can be affected by severity of the frost and the type of horticultural operation due to the type of frost damage. Frost damage can often kill cambial tissue but not lateral buds above the cambial damage. Severe enough damage to cambial tissue causes girdling of the limb, which may not show up for several weeks after the damage has occurred. What generally happens is that a lateral bud begins to grow after the apical bud has been killed by frost, the bud continues to grow on reserves in the shoot above the cambial damage but eventually dies because it is no longer being supplied water and nutrients from the root system. This often perplexes people later in the season when unexpected branch die-back occurs.
In smaller operations and when aesthetics are important, it may be best to prune out the initial damage and then follow up with another pruning if cambial damage has occurred and further die-back is exhibited. For large operations where aesthetics are not the primary concern, postponing pruning for a couple of weeks will allow determination of whether cambial damage has occurred and then one pruning can be done to remove the original damage and the hidden cambial damage. Of course, there are some plants where it is more critical to prune the dead wood out immediately because of pathogen susceptibility but pruning can be delayed on the majority of plants.