Question: I’ve heard that pruning oaks in spring and summer is not recommended. Why?
Pruning oak trees during the growing season exposes them to a high probability of contracting oak wilt. Caused by a fungus (Bretziella fagacearum), oak wilt disease can kill a wide variety of oak species. The spores of this fungus adhere to the bodies of beetles and spread the disease as the beetles feed on the sap from freshly pruned or wounded oak trees. Once infected, an oak tree can transmit the disease to other oaks nearby through its root system. The red oak group (black, pin, northern and southern red and others with pointed leaf edges) are easily infected and can die within a few weeks. Oaks in the white oak group (white, bur, chinquapin, post and others with rounded leaf edges) are less susceptible. Oak wilt causes rapid loss of leaves during summer with wilting starting at the top of the tree. The disease may produce oval black fungal spore mats and blisters on the tree trunk and branches, a clear indication of infection.
The bottom line: Pruning an oak tree at the wrong time increases the risk of insect pest damage and diseases, like oak wilt. Prune during the dormant season to protect oak trees. According to the Virginia Cooperative Extension, November and December are the best months to prune oaks in Virginia. Avoid pruning from March through August. If oak trees must be pruned in spring or summer due to storm damage or for safety reasons, treat the trees with tree wound dressing or latex-based paint to seal the wound. If hiring a tree pruning service, be sure to contact a certified arborist to do the job.
Interested in learning more? Virginia Cooperative Extension provides a guide to pruning deciduous trees and detailed calendars for deciduous and evergreen tree pruning. In addition to adding beauty to the landscape, oaks serve a number of essential ecological functions, described by Doug Tallamy, well known entomologist and author, in his book The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees. This YouTube video describes why it is important to protect oaks.
References:
“A Guide to Successful Pruning, Pruning Deciduous Trees”, French, Susan C., Extension Technician and Appleton, Bonnie Lee, Extension Horticulturalist, Virginia Tech, Virginia Cooperative Extension, Publication No. 430-456, 2022.
“A Guide to Successful Pruning, Deciduous Tree Pruning Calendar”, Appleton, Bonnie Lee and French, Susan C., Virginia Tech, Virginia Cooperative Extension, Publication No. 430-460, 1998
“A Guide to Successful Pruning, Evergreen Tree Pruning Calendar”, French, Susan C., Extension Technician and Appleton, Bonnie Lee, Extension Horticulturalist, Virginia Tech, Virginia Cooperative Extension, Publication No. 430-461, 1998.
“Avoiding Pruning Oak Trees during Spring and Summer”, Iowa State University Extension and Outreach, Salamanca, Lina Rodriguez, Clemons, Zach, Apr 2020.
“Oak Wilt”, Department of Agriculture, Forest Service, Forest Health Protection, Rocky Mountain Region, 2011.
“Ok to Prune OAKS. Now’s the Time to Schedule”, deHaas, Thomas, and Stone, Amy, Ohio State University, Ohio State University Extension, Dec. 2022.
“Stop Pruning Oak Trees Now to Avoid Oak Wilt”, Wilson, Mary, Michigan State University, MSU Extension, Apr 2017.
Image source: J. O’Brien, USFS, Bugwood.org
