How to prune a tree: the cuts that close and the ones that don’t

How to trim tree branches: where to cut

The USDA Forest Service’s pruning guide starts from one rule: remove only branch tissue and leave the trunk’s tissue intact. Two landmarks show where one ends. The branch collar is the bulge on the underside of the branch where it joins the trunk. The branch bark ridge is the raised line of bark in the crotch on the upper side, running roughly parallel to the branch.

A proper cut starts just outside the branch bark ridge and angles down and away from the trunk, missing the collar. Cut that way, the trunk’s own tissue usually doesn’t decay and the wound seals fastest. A flush cut inside the ridge or collar injures the trunk and can lead to decay; a stub left too far out dies back, delays closure and can let canker fungi in.

You can grade your own work a season later. The Forest Service says a proper cut grows an even ring of woundwood all the way around; a flush cut grows it mostly on the sides, with little at the top or bottom.

Tip: Skip the wound paint. ISA says research shows dressings don’t reduce decay or speed closure, and most experts recommend against them. The Forest Service’s only exceptions are elms and oaks wounded during the months when Dutch elm disease and oak wilt spread.

The three-cut method for heavy limbs

If you can’t hold a branch with one hand while you saw it, its weight can tear a strip of bark down the trunk as it falls. The three-cut method stops the tear at a cut you chose, then lets you make the final cut with the weight gone. Clear the drop zone first: in one case OSHA describes, a piece of limb a foot thick and 20 inches long killed a worker standing under it.

1. UndercutAbout 12–18 inches out from where the limb attaches (ISA), saw a shallow cut up from the underside, outside the collar. It keeps the falling limb from peeling bark off the trunk.
2. Top cutDirectly above or a few inches farther out, saw down from the top all the way through. The limb drops, and any tear stops at the undercut.
3. Final cutTake the short stub off just outside the branch bark ridge and collar, angling down and away from the trunk.

The same method works on dead branches. The Forest Service notes that their collar is easy to find, because the collar keeps growing after the branch dies; cut just outside that ring of live tissue.

Thinning, heading and reduction cuts: which to use

Every pruning job is built from two kinds of cut: removing a branch where it starts, and shortening a branch back to a smaller one. What changes is which branches you choose.

Source: USDA Forest Service, How to Prune Trees (NA-FR-01-95, revised 2012); ISA TreesAreGood, Pruning Young Trees and Pruning Mature Trees (2021); checked 2026-10-02.

Heading is where pruning goes wrong. ISA’s young-tree guidance is to shorten a long branch back to a secondary branch or bud, because cuts between buds or branches can lead to decay, sprouting and misdirected growth. On a big tree, the Forest Service calls cutting large upright branches between nodes topping, and says it reliably produces weakly attached sprouts or dieback; the alternatives are on tree topping. Fruit trees are pruned by their own rules; see fruit tree pruning.

Reduction follows the one-third rule: the side branch you cut back to should be at least a third the diameter of the stem you remove, or the stem tends to sprout or die back. If a branch would lose more than half its foliage, the Forest Service says to remove the whole branch instead. Two trunks of equal size from one point are a structural problem to fix while the tree is young (ISA); a union that is already cracking is covered in split tree trunk, and how crews do thinning and reduction on mature trees is on crown thinning and reduction.

How much can you prune a tree at once?

The Forest Service caps one session at a quarter of the living crown, with anything more spread over the following years, to avoid stress and a flush of sprouts. ISA’s guidance is the same 25 percent and less for mature trees, and Portland Urban Forestry tells residents 20 to 25 percent of the canopy per pruning cycle.

In Portland the number is also in the code. The tree code says no tree may be excessively pruned or topped, and defines excessive pruning as removing 25 percent or more of a tree’s functioning leaf, stem or root area in one growing season (PCC 11.60.040; 11.80.020); the city says topping can bring fines or required replanting. Seattle treats removing live branches 4 inches or larger, or 25 percent or more of a tree’s foliage-bearing area, as reportable work: if you hire it out, the company must be on SDCI’s tree service provider registry, and the work must be posted online at least 3 full business days ahead. Fruit-tree pruning and hedge maintenance are exempt.

Branch size matters as much as the total. The Forest Service’s rule of thumb:

When to prune trees in Washington and Oregon

For most broadleaf trees the answer is the dormant season. The Forest Service gives four reasons: you can see the structure, wounds close fastest in the growing season that follows, disease spreads less, and sap flows less. ISA puts it the same way: growth and wound closure peak when pruning comes before the spring flush.

Species exceptions, the month-by-month calendar and the nesting-season check are in the best time to trim trees in the Pacific Northwest. Japanese maples have their own routine: how to prune a Japanese maple.

Pruning tools, and how to clean them

Sources: USDA Forest Service, How to Prune Trees (sizes converted from 2.5, 7, 10 and 4.4 cm); City of Portland, How to prune properly; ISA, Pruning Young Trees; checked 2026-10-02.

Keep tools sharp and clean. When you are cutting diseased wood, the Forest Service recommends sanitizing before each cut in 70 percent denatured alcohol or a bleach solution of 1 part bleach to 9 parts water, soaking 1 to 2 minutes, wiping off wood bits, and washing the tools with soap and water afterward because bleach corrodes metal. Pruning in the dormant season cuts the need for it.

Tree trimming to leave to a crew

Pruning a small tree from the ground is a homeowner job. Falls and falling objects account for a high share of the tree care deaths OSHA investigates, and the jobs below put you in the path of one or both. OSHA’s tree care page lists a NIOSH investigation of a boom-lift fall during tree trimming in Washington; ISA says pruning large trees “can be dangerous.”

Sources: USDA Forest Service; Cal/OSHA Tree Work Safety Guide (2025); WAC 296-45-035; OAR 437-002-0306; Seattle SDCI; City of Portland; checked 2026-10-02.

Power lines are the firm line. The Forest Service’s own guidance is never to prune trees touching or near utility lines and to call the utility instead, and in both states tree work within 10 feet of a line is line-clearance work. Utility programs by company are on utility tree trimming.

Modeled estimate, not quotes (method): crew-hours at about $225 per crew-hour in Oregon and $210 in Washington. Details on tree trimming cost and tree trimming.