Some Advantages of Pruning Trees & Shrubs in the Winter…
David L. Roberts, Ph.D.
Senior Academic Specialist
Michigan State University
Dormant pruning has many positive characteristics. Another benefit is that dormant pruning enables us to correct disease problems which would be more risky during the summer season. For example, it is very difficult to prune Fire blight from trees or shrubs during the highly infectious spring and early summer times without spreading the disease to other limbs or plants. During the winter, bacterial populations in cankered branches are lower and dormant, and hence, less likely to be transmitted via our pruning utensils. Oak Wilt has become a disease of major significance because of the pruning connection; open wounds during the warm season attract insects which carry the deadly fungal spores. The red oaks and members of like family are often killed within a month or two after coming in contact with the oak wilt fungus while members of the white oak family may succumb in several years.
What is the best time for pruning oaks?
From my perspective the safest and most appropriate time to prune oaks is dormancy, from late October to early March. Obviously, this time will vary from year to year depending on weather conditions. During early March in 2000, experts reported picnic beetle activity in March. During most years, we would not expect much if any picnic beetle activity during the first week of March or perhaps even through the entire month of March. It is wise to keep in mind that during freezing or near freezing conditions, transmission of oak wilt is almost impossible. Unusually warm climatic periods during dormancy may be conducive for oak wilt spread.
I have often been asked why my recommendations are different from some others’ recommendations which suggest only the cessation of pruning during April, May and June. Many of the oak trees being pruned are large, at least 100 years old and are in very prominent locations. They are simply irreplaceable. Although I am certain that the likelihood of oak wilt transmission declines sharply after the three primary months, most scientists admit that we do not understand all of the mechanisms or vectors of oak wilt. No one will admit that the transmission of oak wilt is impossible, for example, during the month of August. In a recently new USDA bulletin, the authors suggested that oak bark beetles may also transmit oak wilt. I believe it is smart to avoid any possibility of oak wilt by limiting our pruning period.
The Benefits of Pruning Oaks During the Dormant Season:
The occurrence of dead wood is a common reason to prune oaks. Unfortunately, I think in most instances it is not possible to prune dead wood without the exposure of live tissue – the reason why dead wood should be done dormantly and not during the spring and summer months on oaks. Proper pruning of trees can make them more storm resistant as well, whether the storm threat is from ice damage or from high winds. Pruning is often regarded as a proactive, health maintenance insurance plan to guard against potentially serious problems, analogous to the ever more popular and wise preventative care in human health plans.