I often talk about pruning as a method for disease management. Pruning branches with canker diseases (e.g., Nectria canker, Cytospora canker, golden canker) can be critical in keeping diseases in check and preventing pathogen spread. Pruning healthy trees and shrubs is also important to promote proper growth, reduce the risk of structural failure during extreme weather (e.g., high winds), and, in some instances, enhance flowering.
When pruning your trees and shrubs, consider the following:
Match your pruning technique to the specific tree or shrub you are pruning
Specific trees and shrubs have particular pruning needs, and you need to choose the right pruning technique for the trees and shrubs in your landscape. Check out the University of Wisconsin Garden Facts Pruning Deciduous Trees, Pruning Deciduous Shrubs, and Pruning Evergreens for details.
Prune in the winter, when possible
From a disease standpoint, pruning in the winter is typically optimal. Disease-causing organisms and insects that carry them are not active, meaning there is less of a chance that infections will occur through pruning wounds. Are there exceptions? Absolutely! Some research indicates that pruning honeylocust in the summer can help reduce the incidence of Nectria canker. Also, pruning spring-flowering shrubs in the winter reduces flowering, so pruning these plants right after bloom is a better option.
Prune when it’s dry
If you prune during the growing season, prune when there’s a stretch of several dry days. Wet weather is a better environment for disease-causing organisms to infect.
Prune to minimize wound size
When removing large tree branches, be sure prune just outside the branch collar (i.e., the slightly swollen area where the branch attaches to the trunk). This will produce a smaller wound than cutting the branch flush with the trunk. Similarly, cut smaller branches perpendicular to the long axis of the branch (rather than at an angle). A smaller wound provides a smaller “target” that plant pathogens have to land on and infect. A smaller wound also means less time for a tree to produce protective tissue that grows over a pruning cut.
Avoid painting pruning cuts
Paint wounds (and do it immediately) only when pruning oaks and elms during the growing season. This reduces the risk of transmission of the oak wilt and Dutch elm disease fungi. Painting pruning cuts on other types of trees can slow development of the protective tissue described above.
Decontaminate, decontaminate, decontaminate
Optimally, decontaminate tools between every cut. Check out my January Plant Disease Pointers for details on what to use and how long to treat. Consider using two sets of pruning tools, one that you decontaminate, while you prune with the other. If decontaminating between every cut is not feasible, decontaminate as often as possible and definitely between plants.
And with that. . . Happy pruning!
Need more information?
For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/), or contact PDDC staff at firstname.lastname@example.org or (608) 262-2863.