Pruning is an important component of maintaining tree health and can be used to accomplish a variety of goals. A dense tree canopy can be thinned to allow increased light penetration to gardens below. Maintenance pruning encourages tree health by removing diseased and broken branches, and is also used to address safety concerns. Finally, pruning helps gardeners achieve an aesthetically pleasing tree form.
When to Prune
The appropriate time of year to prune depends on tree type. For most landscape trees, late winter to early spring, before new growth emerges, is the best time to prune. Wounds heal quickly this time of year and insect and disease activity is low.
Now for the exceptions to the rule. Maples, birches and walnuts tend to ooze sap heavily when pruned in late winter and early spring. Though not a significant plant health concern, the sap is unsightly and messy. Prune these species in late spring or early winter.
Most evergreens, including pines, require very little pruning. However, pines trees are sometimes pruned to increase density. This is done by pinching back the new growth, called candles, as it elongates in late spring.
Avoid pruning trees in late summer, as new growth does not have time to harden before the onset of freezing temperatures.
The 3 D’s of Pruning
Some type of pruning should not be limited to certain times of year. Damaged, diseased or dead plant material, the 3 D’s of pruning, can be removed any time of year. Damaged limbs should be removed as soon as possible as they may impact safety or provide opportunity for insect and disease problems. When pruning diseased trees, always remember to clean your tools between cuts and destroy diseased material to prevent further infection. Removing diseased, damaged and dead limbs with proper pruning cuts will encourage tree wounds to seal properly.
What to Cut
Aside from managing the 3 D’s, determining which branches or how much plant material to remove during pruning can be challenging. To guide the process, have a clear goal in mind. Do you need to thin the canopy to increase light filtration? Are you pruning to improve the shape and appearance of the canopy? Do you need to manage the size of the tree? Thinning and shaping utilize the same pruning techniques, while size management requires more aggressive pruning.
Thinning and Shaping: Look at the tree’s structure carefully to select branches for removal. Start with branches that are crossing or growing in toward the center of the tree. Cut branches back to the point of attachment. Remove water sprouts, which growth straight up toward the sky. Finally, look for branches that interfere with the overall shape or appearance. Do not remove too much material at one time, you can always return next season to make additional cuts.
Size Management: Trees sometimes grow too tall for an area. A technique called heading back or drop-crotch pruning can be used to reduce tree size and direct new growth. However, if trees are under power lines, call a professional to do the job.
Identify limbs for heading and cut back deeply to a strong lateral branch that is at least one-third the diameter of the branch being cut. This help maintains strong limbs and visual integrity of the tree. Avoid topping trees which means cutting limbs back to stubs, much as a hedge is sheared. This encourages quick regrowth of weak branches and necessitates repeated pruning.
If the situation allows, start pruning trees when they are young. Early structural pruning encourages strong branching, a well-balanced canopy and reduced long-term maintenance needs.