Developing Strong Branch Unions
Note: In this publication the term “trunk” refers to the trunk or parent branch and “side branch” refers to the adjacent side branch arising from the trunk (parent branch). The same relationship exists between a side branch and secondary side branch.
Most storm damage in Colorado landscape trees results from failures at the branch union (crotch) primarily with codominant trunks (adjacent trunks of similar size). Primary objectives in training young trees are to develop strong branch unions and eliminate structurally weak codominant trunks. Figure 1]
Figure 1. codominant trunks (adjacent trunks of similar size) account for the majority of storm damage in Colorado landscapes.
Structural strength of a branch union is based on the development of a branch collar. The branch collar is where the annual growth rings of the trunk overlap the annual growth rings of the side branch, like shuffling a deck of cards. In lumber the branch collar is called the knot. Figures 2 and 3]
Figure 2. Structural strength of the branch union (crotch) is based on development of a branch collar.
Figure 3. The branch collar is where annual growth rings of the trunk overlap the annual growth rings of the side branch like shuffling a deck of cards. This creates a very solid section of wood, known as the “knot” in lumber.
For the branch collar to develop, the diameter of the side branch must be less than half the diameters of the adjacent trunk/parent branch. Less than one-third is preferred. If the side branch is too large in diameter, prune back the side branch by 25% to slow growth ore remove the branch entirely. As the branch collar develops, side branch tissues connect into the trunk in a wedge shape, making a structurally strong branch union. [Figure 4]
Figure 4. As the branch collar develops, side branch tissues connect into the trunk in a wedge shape making a structurally strong collar. For the branch collar to develop the diameter of the side branch must be less than half the diameter of the adjacent trunk/parent branch. Less than one-third is preferred.
A branch collar will not develop on codominant trunks (adjoining trunks of similar size) making this branch union structurally weak. [Figure 5]
Multiple branches arising at the same location also compromise the branch collar’s structural strength. Some tree species (like elm, maple, and crabapple) naturally develop multiple branches at one location. This predisposes the tree to storm damage if the situation is not corrected by structural training when the tree is young. [Figure 5]
Figure 5. (Left) A branch collar does not develop on codominant trunks, making the branch union structurally weak. (Right) Multiple branches arising at the same location are also structurally weak as the branch collars can’t knit together into a strong union.
The development of a branch collar significantly reduces the potential spread of decay. Also branch unions with a right angle of attachment are more effective in preventing the spread of decay. To reduce the potential for decay, 1) prune to develop branch collars (the side branch must be less than half the diameter of the adjacent trunk) and 2) select branch unions with a wide angle of attachment. In pruning, remove codominant trunks and narrow branch unions while young (smaller than two inches). [Figure 6]
Figure 6. Branch unions that form a right angle are more resistant to decay. A branch union with codominant trunks and a narrow angle of attachment is highly prone to the spread of decay.
In training trees for structural integrity, the arborist has zero tolerance for codominant trunks. Codominant trunks should be corrected immediately when they start to develop or at least before the trunks grow to a two-inch diameter.
In training trees for structural integrity, give attention to developing strong branch unions with branch collars. For the collar to develop, the side branch must be less than half the diameter of the adjacent trunk. Less than 1/3 is preferred. If the side branch is too large, significantly prune back the side branch to slow its growth rate.
In pruning, select branch unions with a wide angle of attachment. This makes the branch more resistant to the spread of decay and avoids problems with included bark creating weak branch attachments.
Ideally, all pruning cuts are two inches or smaller in diameter. Any pruning cut four inches and larger must be justified by taking into account the high potential for decay. For additional information on pruning cuts, refer to CMG GardenNotes #613, Pruning Cuts.
CMG GardensNotes on pruning
- Developing Strong Branch Unions, #612
- Pruning Cuts, #613
- Pruning Evergreens, #618
- Pruning Flowering Shrubs, #619
- Pruning Maturing Shade Trees, #616
- Structural Pruning of Maturing Shade Trees, #617
- Structural Training of Young Shade Trees, #614
- Structural Training Summary, #615
- Tree Growth and Decay, #611
- Homework: Pruning, #620
- References and Study Questions: Pruning, #610
Books (available from the International Society of Arboriculture)
- An Illustrated Guide to Pruning, Third Edition. Edward F Gilman. Cengage Learning. 2011
- ANSI A300 Pruning Standards, Part 1. American National Standards Institute. 2008
- Best Management Practices (Revised 2008). Edward Gilman and Sharon Lilly. International Society of Arboriculture. 2008.
- Dr. Ed Gilman at University of Florida: http://hort.ifas.ufl.edu/woody/pruning.shtml
- CMG GardenNotes are available online at www.cmg.colostate.edu
- Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
- Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating
- Extension programs are available to all without discrimination.
- No endorsements of products mentioned is intended nor is criticism implies of products not mentioned.
- Copyright. 2010-11. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.
Revised July 2012