April: To Prune, or Not to Prune, That is the Question.

ShearsAs warmer weather has spread through much of Wisconsin, I have talked with more and more gardeners who are chomping at the bit to get into their yards and start the 2019 gardening season.  One of the activities these gardeners are contemplating is the pruning of their trees and shrubs.  They often ask me whether spring is a good time to prune from a plant disease perspective.

The answer is:  It depends.

Overall, I am a proponent of winter pruning.  Typically there is a slight warming at the end of January or at the beginning of February where it’s warm enough to prune and not freeze to death, but not warm enough that disease-causing organisms are likely to be active.  In my mind, pruning during this window reduces the risk of pathogens infecting through pruning wounds.  There can be exceptions to this rule of thumb however.  There is some research that indicates that pruning honey-locust trees in the summer (during hotter, drier periods) can reduce the risk of Nectria canker compared to pruning in the winter.

When pruning in the spring, the trees that I have the most concern about are oak trees.  In particular, I worry about transmission of the oak wilt fungus, Bretziella fagacearum.  This fungus can be moved from tree to tree by sap beetles that become active as temperatures warm.  These insects are attracted to wounds, including those caused by pruning.  Some municipalities restrict pruning of oaks after April 1 in anticipation that temperatures will shortly be warm enough that sap beetles will be active.  Use of a calendar date as a cut off for pruning oaks can be problematic however if spring arrives early.  For that reason, I really ONLY recommend winter pruning for oaks.  If for some reason, you really need to prune an oak at another time of the year, paint over any pruning wounds.  You can use a commercial pruning paint if you like, but left over latex paint (from painting a room in your home) will work just as well.  The paint provides a physical barrier that makes a wound less attractive to sap beetles.  Be sure to paint wounds on oak trees IMMEDIATELY.  There is research that indicates that sap beetles can visit wounds in as little as 10 minutes.

In general, when pruning tress other than oaks in the spring (or any time other than the winter), always be sure to prune when it’s dry and when there is dry weather predicted for several days post pruning.  Dry weather is less favorable for fungal spores (which might land on a pruning cut) to germinate and infect.  Also be sure to prune properly based on the type of tree or shrub.  Laura Jull of the UW-Madison Department of Horticulture has authored several excellent fact sheets on how to prune evergreens, deciduous trees and deciduous shrubs.  Check these out!!  When pruning out diseased branches, prune four to six inches below obviously diseased areas if you suspect a fungal disease and 12 inches below obviously diseased areas if you suspect a bacterial disease.  In the best of all possible worlds, you should decontaminate your pruning tools between every cut to limit possible movement of pathogens via your tools from branch to branch or from tree to tree.  You can use a 30 second dip in 70% alcohol (e.g., rubbing alcohol) or in a commercial disinfectant for this.  Alternatively, you can use a spray disinfectant, spraying your tools until they drip and then allowing them to air dry.  Except in situations where oaks are pruned during the growing season, I do not recommend using paint on pruning cuts.  Paints tend to slow down the formation of callus tissue, the tissue plants produce to naturally cover over wounds.

Finally, avoid what I tend to try to do when I prune, which is to prune off my fingers.  OUCH!!

For addition information on plant diseases, the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.