Oaks and Elms are some of the most beautiful trees that we have in our landscapes here in the Midwest and throughout many parts of the world. Protecting and managing them for optimum health and longevity is a benefit in a multitude of ways. That is why you may have heard of dormant season pruning and why Oaks and Elms fall into this discussion.
There are two fungal diseases that are lethal to Oaks and Elms, oak wilt and Dutch Elm Disease (DED), respectively. These fungal diseases are vectored by an insect which is ultimately attracted to fresh wounds on a tree. When that insect lands on the tree, the insect itself does minimal damage, but the fungal spores that it may carry will infect the tree and ultimately lead to its demise.
Hence, pruning your Oaks and Elms in the dormant season significantly reduces that risk because the insects are also dormant and the tree has the time to naturally heal the pruning wounds before the insect is moving around again. Now, certainly, during the growing season, a tree can sustain wounds from a variety of occurrences such as animal damage, storm damage, natural failures, etc. Depending on the severity of damage will determine if it will be necessary or not to prune the wounded area out of the tree. Pruning that area out will allow for faster wound healing and a protective fungicidal sealant can be applied to the pruning cut(s) to mitigate risk of attraction of the insect that spreads the disease. These determinations are best to be made by an ISA Certified Arborist that can assess the risks involved and make the best recommendation of treatment for the tree.
As you know, there are a variety of Oak and Elm species in our local area. All are susceptible to these diseases to varying degrees. Native Elms such as American, Red (Slippery) or Rock are highly recommended to be pruned during the dormant season. Whereas Siberian Elm, is found to be highly resistant to DED and can be pruned throughout the year. In addition to native and non-native Elms, there are also a variety of Elm cultivars that have been developed over the years specifically to be resistant to DED and those can typically be pruned throughout the year as well. As far as Oaks, they also have varying degrees of susceptibility, with the White Oak family (White, English, Swamp White, Chinkapin, etc.) being more resistant than the Red Oak family (Red, Pin, Black, Scarlett, etc.). Those within the Red Oak family can die within weeks of contracting the disease, whereas the White Oak family may take months with the possibility of treatment and control of the disease.
In the end, our recommendation is to conduct all of our Oak and Elm pruning during the dormant season, excluding Siberian and various hybrid Elms. We will occasionally do Oak and Elm pruning during the growing season, but only if it is an emergency situation assessed and determined by an ISA Certified Arborist. There are many factors when it comes to managing trees and sustaining their benefits to the environment and our society, and that is why it is always important to seek safe and professional arboriculture services for the care of your trees in your landscape.