Pruning Landscape Trees: An Overview

What Is Pruning?

Pruning is the purposeful removal of plant parts. Its use is preferred to less precise terms like trimming. This fact sheet deals mainly with pruning of branches on ornamental trees. It does not cover pruning of fruit trees for fruit production, shearing or other intensive crown shaping practices, or root pruning.

Why Prune?

Reasons for pruning landscape trees generally fit into three closely-related categories: health, hazard, and form. Step back and reconsider if you find yourself pruning off healthy, non-hazardous branches on a tree with good form.

Health

Pruning for tree health includes removal of insect or disease infested branches, broken or dead branches, crowded branches, and branches or portions of trunks with included bark. Pruning to “open-up” a tree’s crown to encourage light penetration or air movement generally is not needed.

Hazard

Pruning to reduce hazard to people or property includes removal of dead and decaying branches; branches that interfere with sight lines or travel along streets, driveways, or sidewalks; branches that rub against structures; thorny or spiny branches that might cause injury, and pruning for utility line clearance. Healthy, strong, properly attached branches that overhang a building do not usually pose an unreasonable safety risk and do not necessarily need to be removed.

Form

Pruning for tree form or shape involves removal of certain branches and leaving others to direct growth toward and away from certain areas. Pollarding, crown raising, and topiary all are examples of pruning for form

When to Prune

Time of Year

Pruning can be done at any time of year with special care and knowledge, but certain times are better than others. Pruning is best done in winter or early spring before buds swell when tree energy reserves are high and the tree is dormant. Though certain trees, such as birches, maples, and walnuts, may exude sap or “bleed” when pruned at this time of year, this is not a problem and stops within a few days. During the spring growth period bark is tender and easily damaged and pruning must be done very carefully. In fall pruning wounds may be more easily infected with decay microorganisms. Though summer pruning can be done, considerable dieback may occur at the edges of summer pruning wounds, possibly due to drying of the living tissues at wound edges. Quickly wrapping such wounds with white plastic sheeting can reduce this dieback.

Time of Life

At planting time only prune branches that are dead, broken, or that have insect or disease problems. Otherwise, prune early in a branch or tree’s life to avoid pruning large branches or letting serious problems develop. For maintenance pruning on established trees try to prune branches before they exceed 2 inches in diameter.

Pruning Severity

Avoid pruning off more than 20 to 25% of a tree’s leaf area in any year. Pruning stresses trees because pruning wound repair requires energy from food while pruning removes leaves that make food and wood that stores food. Younger and more vigorous trees can stand heavier pruning than mature or stressed trees. If heavy pruning is unavoidable, remove branches over two or more years to reduce stress

Pruning Tools

Sharp, well-maintained tools make cleaner cuts and are safer than dull tools. Use shear-type hand pruners for small twigs (sometimes called bypass pruners; anvil-type pruners cause slightly more injury than shear-type), loppers for small branches, and pruning saws for branches up to 3 or 4 inches in diameter. Good pruning saws have thin curved blades and teeth that are angled back to cut mainly as you pull rather than push. Chainsaws should only be used for large branch removal. Bow saws are not suitable for pruning since their blade wanders and the handle gets in the way. Sterilization of tool blades between cuts (with alcohol or bleach) often is recommended to avoid spreading disease, but researchers have shown that most quick sterilization techniques are not effective.

Pruning Techniques

Safety

Removing large branches or cutting down entire trees is dangerous. Call in an insured, well-trained professional arborist (preferably certified by the International Society of Arboriculture) if you are unsure of your abilities or if climbing is involved. Pruning near utility lines should only be done by certified line clearance professionals — call your electric company.

Where to Cut: Natural Target Pruning

Natural target pruning of dead and live branches; broadleaves (top) and conifers (bottom). Final cuts should be from A to B.

Good pruning involves removing as much of the branch as possible without leaving a stub or flush cutting. Good pruning cuts are called natural target cuts by arborists, who use two targets on the tree to show them where to make the cut. These targets are the branch bark ridge (BBR) and the branch collar. The BBR is an area of excess bark that accumulates where two branches meet. It extends down the branch or trunk on either side of the branch crotch. The branch collar is (typically) a swollen, wrinkled area at the branch base where branch and trunk (or branch and branch) tissues come together.

A natural target cut leaves the BBR and branch collar on the tree without leaving a stub. Such a cut passes just outside the BBR on top and usually slants out and down, leaving a bump but no stub (from A to B on diagram). Sometimes the swollen branch collar extends all the way around the branch base and the resulting cut is more vertical. Though the “targets” usually are easy to see on most broadleaved trees, some trees like sycamore constantly lose bark and don’t accumulate a BBR. Conifers also may not accumulate a typical BBR. In both cases, just cut outside any swollen or wrinkled branch collar.

Acknowledgments

The Utah Division of Forestry, Fire & State Lands (FF&SL) provided partial support for the production of this fact sheet. I also thank the Utah Community Forest Council and USDA Forest Service State and Private Forestry for their support. Several of the drawings are adapted from the brochure Homeowner’s Guide for Beautiful, Safe, and Healthy Trees from the USDA-Forest Service Northeastern Forest Experiment Station. Finally, I thank Tony Dietz and Brook Lee of FF&SL, Randy Miller of Utah Power, and Marita Tewes of Red Butte Garden and Arboretum for their careful review of this fact sheet.