Crown Reduction
Crown reduction reduces the height or spread of a tree by cutting stems back to living lateral branches, which take over as the new leaders. The USDA Forest Service guide How to Prune Trees says it is mostly used on trees that have outgrown their space, and that each lateral kept should measure at least a third of the removed stem’s diameter where the two join, to prevent branch dieback.
Sources: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry ; Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
Crown reduction vs topping
Topping cuts large upright branches between nodes, without regard for the tree’s structure. The Forest Service guide says topping and tipping (cutting side branches between nodes) produce epicormic sprouts that are weakly attached and end up supported by a decaying branch. Purdue Extension adds that topping leaves woody stubs that decay and raises the likelihood of failure.
Crown reduction, also known as drop-crotch pruning, beats topping on three counts: it looks more natural, it lasts longer before the next pruning, and it is less stressful for the tree.
Sources: How to Prune Trees (NA-FR-01-95, revised 2012) ; What’s Wrong with Topping? (FNR-FAQ-14)
The one-third rule
A reduction cut takes a bigger branch or trunk back to a smaller side branch. Colorado State University Extension treats the cut as a heading cut, which pruning standards generally do not accept, when the side branch left measures under a third of the removed stem’s diameter.
A method of last resort
The Forest Service guide calls crown reduction rarely needed and a method of last resort: it often leaves large wounds on stems that may lead to decay, and it is never appropriate for a tree with a pyramidal growth form. When a tree keeps outgrowing its space, the better long-term answer is to replace it with one that fits.
Crown reduction among the three pruning methods
The Forest Service guide describes three ways to prune a crown: thinning (selective branch removal for light and air), raising (removing lower branches for clearance, keeping live crown at least two-thirds of total tree height), and reduction (shortening to laterals).
How the ISA arborist exam tests it
Expect questions on the one-third rule, why reduction cuts carry more decay risk than removal cuts, and why topping is not an accepted way to reduce size. Try 3 reviewed practice questions:
Each option describes shortening a 4-inch-diameter leader back to a lateral branch. Using the one-third guideline described by Colorado State University Extension, which cut would be classed as a heading cut rather than a reduction cut?
Show answer and explanation
Correct answer: D. Cutting back to a lateral 1 inch in diameter
Colorado State University Extension classes a cut as a heading cut, which pruning standards generally treat as unacceptable, when the side branch that remains has a diameter under one-third that of the stem being removed. One-third of 4 inches is about 1.3 inches, so only the 1-inch lateral falls below the threshold. The 1.5-, 2- and 3-inch laterals are large enough for a reduction cut.
- A: A 2-inch lateral is half the leader's diameter, well above one-third.
- B: At 1.5 inches the lateral is just over one-third of 4 inches, so this is a reduction cut.
- C: A 3-inch lateral is three-quarters of the leader's diameter.
Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
Report a problem with this question · ID prun-015
Why does a reduction cut generally carry a higher risk of decay than a properly made removal cut at a branch that has a collar?
Show answer and explanation
Correct answer: C. A reduction cut is made where no branch collar or branch protection zone exists
A removal cut made just outside a branch collar preserves the branch protection (defense) zone, which strongly resists the spread of decay. When a stem is cut back to a lateral in a reduction cut, there is no collar or protection zone at the cut, so wound closure depends on the cut size and the vigor of the remaining lateral. A proper reduction cut leaves no stub, is not limited to the growing season, and keeps a lateral that is meant to keep growing.
- A: A correct reduction cut is made at the lateral and should not leave a stub.
- B: Reduction cuts are not restricted to the growing season.
- D: The retained lateral is chosen because it can keep growing and take over as the new end of the stem.
Sources: Making the Cut: It Matters When Pruning Trees, Penn State Extension ; Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
Report a problem with this question · ID prun-014
A homeowner asks to have a large, healthy silver maple topped so it will be “safer” near the house. Which explanation is accurate?
Show answer and explanation
Correct answer: B. Topping leaves decaying stubs and weakly attached regrowth, raising failure risk
Purdue Extension notes that topping manages height and spread but leaves woody stubs that decay, weak branch attachments, and an increased likelihood of failure. The sprouts from topping cuts grow fast and are weakly attached, and topped trees often regain their original height within a year or two, so the tree becomes more hazardous, not safer. Topping harms trees of every species, and sealing the cuts does not change these outcomes.
- A: Topped trees often regrow to their former height within a year or two, with weaker structure.
- C: Topping is harmful regardless of species or growth rate.
- D: Sealing the cuts does not prevent the decay and weak regrowth that topping causes.
Sources: What’s Wrong with Topping? (FNR-FAQ-14), Purdue University Extension, Forestry and Natural Resources ; What’s Wrong with Topping? (FNR-FAQ-14), Purdue University Extension, Forestry and Natural Resources
Report a problem with this question · ID prun-025
0 of 3 correct
Common mistakes in the field
Topping instead of reducing
Sawing leaders off between branches to lower a tree leaves stubs that decay and weakly attached sprouts. It makes a tree less safe, not more.
Cutting back to laterals that are too small
A side branch thinner than a third of the removed stem turns the cut into a heading cut. Choose a larger lateral or a different cut point.
Reducing conifers with a pyramidal form
The Forest Service guide says crown reduction should never be used on trees with a pyramidal growth form.
Sources
- How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
- Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
- What’s Wrong with Topping? (FNR-FAQ-14), Purdue Extension, Forestry and Natural Resources
Written in our own words from the public sources above, checked October 19, 2026. Arborist Practice Test is not affiliated with ISA.