ISA Proper Pruning Practices 2 — Questions and Answers

Question 1: What is the recommended maximum percentage of live crown that should be removed in a single pruning session for a mature tree?

10%
25% (Correct answer)
40%
50%

Correct answer: 25%

The general guideline is to remove no more than 25% of the live crown in a single pruning session for mature trees. Removing more can stress the tree and trigger excessive epicormic sprouting.

The 25% guideline for mature trees is based on the relationship between leaf area and the tree's ability to produce energy through photosynthesis. Removing too much foliage reduces the tree's energy reserves, weakens its defense systems, and can stimulate a stress response of dense epicormic (water) sprout growth. Young, vigorous trees may tolerate slightly more removal, while stressed, declining, or recently transplanted trees should have less removed. The arborist should always consider the tree's overall health, species tolerance, and site conditions when determining how much to prune.

Question 2: What is a 'reduction cut' in arboricultural pruning?

Removing an entire branch back to the trunk
Shortening a branch by cutting back to a lateral branch at least one-third the diameter of the removed portion (Correct answer)
Cutting a branch to a uniform hedge shape
Removing all branches below a certain height

Correct answer: Shortening a branch by cutting back to a lateral branch at least one-third the diameter of the removed portion

A reduction cut shortens a branch by cutting back to a lateral branch that is large enough (at least one-third the diameter) to assume the terminal role and maintain the branch's vascular connection.

A reduction cut (also called drop-crotch pruning) is used to decrease the height or spread of a tree or branch. The cut is made back to a lateral branch that is at least one-third the diameter of the branch being removed. This size ratio ensures the remaining lateral is large enough to produce sufficient auxin hormone to maintain vascular function and assume apical dominance. If the lateral is too small, the cut functions like a heading cut, stimulating excessive sprouting. Reduction cuts are preferred over topping because they maintain the tree's natural form while reducing size.

Question 3: Why should pruning cuts avoid leaving a 'stub' extending beyond the branch collar?

Stubs are unsightly and reduce property values
Stubs prevent the tree's natural wound closure process from sealing the wound efficiently (Correct answer)
Stubs attract woodpeckers that damage the trunk
Stubs cause the tree to lean in the opposite direction

Correct answer: Stubs prevent the tree's natural wound closure process from sealing the wound efficiently

Stubs prevent efficient wound closure because the branch collar cannot grow over the extended dead wood. The exposed stub becomes an entry point for decay organisms.

When a proper pruning cut is made just outside the branch collar, the collar tissue — which contains a chemical protection zone — begins to grow woundwood (callus) that gradually closes over the wound. A stub left beyond the collar has no living tissue capable of generating callus, so the dead wood remains exposed. This exposed wood provides an entry point for decay fungi and other organisms. The decay can extend into the branch collar and potentially into the trunk. Proper pruning technique involves a three-cut method: an undercut, a top cut to remove the branch weight, and a final cut just outside the branch collar.

Question 4: During which season is pruning generally most detrimental to oak trees in regions where oak wilt is present?

Late fall and winter
Spring through early summer (Correct answer)
Late summer only
Pruning timing has no effect on oak wilt risk

Correct answer: Spring through early summer

Pruning oaks during spring through early summer (April-July) creates fresh wounds that attract sap beetles carrying oak wilt spores. This is the highest-risk period for disease transmission.

In regions where oak wilt (Bretziella fagacearum) is present, pruning oaks from approximately April through July poses the greatest risk. During this period, nitidulid (sap) beetles — the primary overland vectors of oak wilt — are most active. These beetles are attracted to both the fungal mats on infected trees and to fresh wounds on healthy trees. Pruning creates wounds that release volatile compounds attracting the beetles, which may carry fungal spores. The recommended practice is to prune oaks during the dormant season (November-March) when beetle activity is minimal. If emergency pruning is necessary during the high-risk period, wound sealant should be applied immediately.

Question 5: What is 'lion-tailing' and why is it considered an unacceptable pruning practice?

Removing the entire top of the tree, which destroys apical dominance
Stripping interior branches and foliage, leaving growth only at branch ends (Correct answer)
Pruning branches into animal shapes for decorative purposes
Cutting all branches on one side of the tree for clearance

Correct answer: Stripping interior branches and foliage, leaving growth only at branch ends

Lion-tailing removes interior lateral branches and foliage while leaving tufts of growth at branch ends. This shifts weight outward, increases wind resistance at endpoints, and greatly increases the risk of branch failure.

Lion-tailing (also called over-thinning or stripping) involves removing most of the interior lateral branches and foliage from a limb, leaving only a cluster of growth at the tip — resembling a lion's tail. This practice is harmful for several reasons: it moves the center of gravity of each branch toward its tip, dramatically increasing the lever force on the branch attachment; it removes interior foliage that dampens wind energy, creating a 'sail' effect at the branch end; it reduces the branch's taper, weakening its structural integrity; and it removes photosynthetic tissue that produces food. Properly thinned trees retain evenly distributed foliage throughout the crown.

Question 6: What is the three-cut method used for when pruning large branches?

Making three small cuts around the branch collar for better healing
Preventing bark tearing by using an undercut, a relief cut, and a final collar cut (Correct answer)
Cutting the branch at three different heights to reduce its length gradually
Creating three flat surfaces for grafting purposes

Correct answer: Preventing bark tearing by using an undercut, a relief cut, and a final collar cut

The three-cut method prevents bark tearing on heavy branches: first an undercut, then a top cut further out to remove the branch weight, and finally a clean finish cut just outside the branch collar.

The three-cut method is essential for removing branches too heavy to support with one hand while cutting. Cut 1 is an undercut made 12-18 inches from the trunk, cutting about one-third through the branch from below. Cut 2 is made from the top, slightly further out from the trunk than Cut 1, cutting down until the branch falls — the undercut prevents bark from tearing down the trunk as the branch drops. Cut 3 is the final precision cut made just outside the branch collar, removing the remaining stub. This final cut should be perpendicular to the branch axis (not flush with the trunk) to preserve the branch collar and its chemical protection zone.

ISA Proper Pruning Practices 2

What is the recommended maximum percentage of live crown that should be removed in a single pruning session for a mature tree?