by Eric North, Research Fellow, University of Minnesota Department of Forest Resources
What is pruning?
One of the most frequent questions I am asked with regard to tree care is: “When should I prune my tree?” While I think people are actually asking what time of year they should prune their tree, it might be better to start with the question, “What is pruning?” Pruning is wounding!
Does that mean that pruning is bad for trees? It depends. When done well, pruning can improve the health and form of trees. When done poorly pruning can reduce tree health and create dangerous situations. When following proper pruning techniques developed with tree biology in mind, pruning can aid in the development of tree form and structure. Unlike people, trees do not heal; at least not in the same sense as people heal. When a healthy person cuts his or her finger, the human body responds by replacing the damaged cells with new cells that serve the same function. When a tree is cut, the damaged tissue is not replaced. The damaged tissue remains and is “walled off” from inside the tree in a process known as compartmentalization. Compartmentalization is the chemical and physical process by which trees form a protective barrier between the damaged tissues and existing or new tissues. Anyone who has seen a knot in a piece of wood has seen the results of compartmentalization. The knot is an old branch that new tissue (i.e. wood) has grown around before the tree was cut-down. Instead of healing, trees grow new wood around wounds, but the wound remains.
In the pruning guide book The ABCs Field Guide to Young and Small Tree Pruning by Andrew G. Pleninger and Christopher J. Luley, Ph.D., the authors outline a simple approach to tree pruning. Unlike the many great resources on how to prune, which often start with the proper tools and move directly into making your first cuts, Pleninger and Luley start with “Assess the Tree.” Assessing the tree helps to answer the question of why are you pruning. Sometimes pruning is done to raise the canopy by removing the lower branches, so that mowing or gardening is easier. Sometimes tree branches break in a storm and need to be removed for safety reasons, pruning to increase flower or fruit production, or to reduce disease. First ask why you are a pruning, what is the end objective? Knowing your objective is the first and very important step to consider before cutting branches. Create a plan of which branches to remove and remove the smallest amount to accomplish your objective.
When to prune?
What time of year is the best time to prune? There are several things to consider when deciding what time of year to prune your tree. Here are some general guidelines to consider for planning when to prune:
- Know the species and associated diseases or pest.
In Minnesota, oaks are negatively affected by the fungal disease known as oak wilt. The fungus is spread overland by oak bark beetles. The beetles are attracted to freshly cut oak trees. The timing of oak pruning should be carefully considered to avoid pruning when the beetles are actively flying. Knowing the diseases and insects associated with a tree species is an important step in selecting an appropriate time to prune.
- Flowers and fruit
Generally if the tree or shrub produces spring flowers (think lilacs) the buds differentiation into flower or vegetative growth on twigs that are one year old. This means that if you want flowers, you should prune immediately after flowering. If you wait until the new buds have formed you will likely prune off the buds for next year’s flowers. Another great reason to know your pruning objective and a little about the tree species before you start pruning.
- Safety pruning
If you are pruning to remove broken or dead branches then pruning should occur as soon as can be done safely. If you need to prune an oak for safety reasons during a high risk period for oak wilt, a clear coat of Shellac can be used to cover the wound immediately after making the cut to reduce the risk of attracting oak bark beetles. In the case of urban trees, safety is often a primary concern and should be addressed as quickly as possible.
- Watch out for stressful conditions
Yes, trees can be stressed. Remember even when properly done, pruning is wounding and a tree’s ability to respond to a wound is better if the tree is not stressed. Potentially stressful conditions are: drought, newly planted trees, recently injured trees (e.g. storms, lawnmowers, etc.), or root disturbance to name a few. Pruning a tree that is already stressed can make matters worse. If the trees needs to be pruned, consider managing the stress (e.g. in droughts, water your tree).
How to prune
The basics of how to prune can be found in several great books and online resources. Two excellent free resources are available online: How to Prune Trees – US Forest Service and Tree Owner’s Manual. The basics are to make the smallest cut possible, cut only the branch to be removed (not the trunk or other main branch), use the proper tools safely, and if you need to get off the ground it’s probably time to hire a certified arborist.