Pruning

March 5, 2005

By Sandy Woods, Master Gardener

When a tree has grown so large that it has overtaken our yard, we may start searching for the pruning sheers. But a tree may need pruning for a variety of reasons: to remove dead, diseased, or dying branches, to control shape and size, to influence flowering and fruiting, to invigorate stagnant growth, or simply to shape a tree for design purposes. Unfortunately, done improperly, pruning can be very destructive, destroying the shape and structure of a tree and predisposing it to severe future problems.

There are two main types of pruning cuts: heading cuts and thinning cuts, and a woody plant responds differently to each. A heading cut removes a portion of a shoot or branch, leaving only buds or a tiny twig on the remaining portion. Heading cuts generally result in a flush of vigorous, upright and dense new growth from just below the cut. Unless managed, heading cuts can result in an increased number of branches. Thinning cuts, on the other hand, remove lateral branches at their point of origin or to a lateral whose diameter is at least one-half larger than the diameter of the removed branch. Thinning results in a reduced number of branches. A woody plant responds to thinning by becoming more open and does not usually produce a flush of new vigorous growth from the cut. Foliage grows more deeply into the tree because more light can penetrate the canopy.

The dormant season (late fall or winter) is the best time to prune, although dead or diseased branches can and should be removed at any time. Pruning during the dormant period minimizes sap loss and subsequent stress to the tree and minimizes risk of fungus or insect infestation. Finally, in the case of deciduous trees, pruning when the leaves are off can give you a better idea of how your pruning will affect the shape of the tree.

When deciding how much to prune a tree, as little as possible is often the best rule of thumb. All pruning places stress on a tree and increases its vulnerability to disease and insects. Prune no more than 25% of the crown and ensure that living branches compose at least two thirds of the height of the tree. Heavier pruning risks fatally damaging your tree.

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM. You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM. The San Luis Obispo Master Gardeners website is at http://groups.ucanr.org/slomg/. Questions can be e-mailed to: mgsanluisobispo@ucdavis.edu.