Why do we prune trees? Pruning is an invigorating process; it spurs growth. Fruit and nut trees are pruned annually to promote flowering and open up leaf canopies so fruits and nuts ripen uniformly. Ornamental trees should be pruned when young to select main branches that will support their growing canopies. Mature trees should only be pruned when they pose a hazard, when there is something wrong with their basic structure. Perhaps there are too many branches in the center of the tree making the canopy so heavy that strong winds could blow the tree over, or a heavy branch is hanging over a roof. Don’t assume that all trees need to be pruned every year, they don’t.
When is the best time to prune landscape trees? The time to prune depends on the kind of tree and the desired results. Light pruning can be done any time of year. Deciduous trees are usually pruned when dormant and will exhibit rapid, vigorous growth in the spring. The goal of pruning is to direct the growth. Conifers and other evergreens respond best to pruning in late winter just before the spring growth spurt.
Consider the natural tree shape. Avoid pruning trees into unnatural shapes. Some trees, like redwood, pine, and liquidambar have central leaders with a tall straight trunk. A central leader is the extension of the trunk that grows straight up through the tree. All of the lateral branches are connected and radiate from this central leader.
In contrast, most trees, like ash, elm, oak, maple and pistache start with a central leader but eventually develop a wide-spreading crown on a short trunk. The lateral branches grow as fast as or faster than the central leader and with time there is no main leader. This trunk is connected to three to five large limbs (scaffolds) which make up the tree canopy.
Use thinning cuts
The best way to prune a mature tree is with thinning cuts. Thinning removes a lateral branch at its point of origin or cuts it back to another lateral branch of decent size. Thinning opens the tree canopy, reduces weight on heavy limbs, and can reduce a tree’s height while retaining the natural shape. A tree that is properly pruned with thinning cuts requires less pruning over time.
Do not head or top trees
A poor method of pruning a mature tree is with heading cuts. Heading (or stubbing) is cutting large limbs back to lateral branches that are too small; or cutting large limbs with no regard to the lateral branches on the tree. Unfortunately, we see this wrong kind of pruning all over in our cities, neighborhoods, and shopping malls. This method is incorrectly used on medium and large trees (even by commercial tree trimmers) because it is thought to be faster and cheaper than thinning. The results in most cases are unattractive trees that will soon need to be pruned again, if they live. This is the wrong way to prune trees.
When branches are headed (stubbed or topped), regrowth is vigorous with many small weak shoots originating at the cut instead of just one sturdy branch. The tree quickly loses its natural beauty and health and can become a greater liability. Sometimes the tree will die within a few years and the homeowner won’t associate the death of the tree with the poor pruning job performed a few years earlier.
Use thinning cuts to remove branches that cross, are diseased, or are dead. Aim for a natural shape and try to balance the branches around the tree. On rare occasions heading cuts are the only legitimate solution to a tree problem or hazardous limb. Please avoid making random cuts along the limb.
Find pruning tips on the Master Gardeners’ web site. Download our “Ornamental Tree Selection and Guide.” Lots of step-by-step pruning books are available in stores, nurseries, and libraries. For large complicated jobs hire a licensed arborist or a reputable professional. Tell them not to head, top, or stub the tree. Tell them to thin your tree branches. You will be much happier in the long run, and so will your neighbors!
Due to the shelter-at-home guidelines, the Master Gardeners have canceled all public events for the time being, but their phone lines are still open at 559-684-3325. Or visit their web site at ucanr.edu/sites/UC_Master_Gardeners.