Codominant Stems
Codominant stems are two or more upright stems or leaders of about the same diameter that grow from the same point on the trunk and compete to be the main stem. Purdue Extension calls this a poor stem structure, and Colorado State University Extension calls it a major structural defect.
Sources: Corrective Pruning for Deciduous Trees (FNR-563-W), Purdue Extension, Forestry and Natural Resources ; Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension
Why codominant unions fail
Colorado State University Extension explains that the union between codominant leaders does not develop a branch collar and tends to trap included bark, so a tree with codominant trunks is more prone to damage or failure under snow and wind. The USDA Forest Service guide How to Prune Trees adds that included bark prevents a strong attachment and often causes a crack below the point where the stems meet.
UF/IFAS Extension notes that codominant stems make branch failure more likely in strong storms, and that the risk rises when the stems are of similar diameter.
Sources: Structural Pruning of Young Shade Trees (CMG GardenNotes #613) ; How to Prune Trees (NA-FR-01-95, revised 2012) ; Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure (ENH1246)
How to recognize them
Purdue Extension’s check is easy: a trunk that forks into two main stems shaped like a slingshot, with a V-shaped union in the canopy, is codominant. The stems usually sit tight against each other just above the trunk, and in more severe cases included bark shows in the middle of the union, with bark bulging out on each side of it. Colorado State adds that a bark inclusion rolls smoothly into the union instead of forming the rough ridge of a branch bark ridge.
Purdue warns that a codominant union showing decay, or oozing a slimy liquid and cracking, may be at imminent risk of failure or splitting.
Sources: Corrective Pruning for Deciduous Trees (FNR-563-W) ; Structural Pruning of Young Shade Trees (CMG GardenNotes #613)
Correcting codominant stems on young trees
Purdue Extension’s approach has three steps: choose the stem best suited to lead, find the stems competing with it, and remove or subordinate those competitors. In Colorado State’s example, the competing leader is shortened with a reduction cut; with fewer leaves it grows more slowly, and the unpruned leader takes over. It can also be removed completely, and Colorado State says a competitor approaching 2 inches in diameter might best be removed before it grows larger and develops heartwood, whereas one larger than 4 inches is probably better shortened than removed, to avoid exposing heartwood.
Alabama Extension says that, rather than cutting a codominant stem back to the trunk, it is typically better to subordinate it: reduction cuts suppress its growth without a large pruning cut at the trunk and reduce the mechanical stress on the union.
Sources: Corrective Pruning for Deciduous Trees (FNR-563-W) ; Structural Pruning of Young Shade Trees (CMG GardenNotes #613) ; Assessing the Structure of Trees in the Landscape
When the stems are already large
For a large competing stem, Purdue Extension suggests subordinating it over several growing seasons, until it is removed or the leader is twice its diameter. Each reduction still takes weight off the weak union in the meantime.
UF/IFAS Extension says the risk from codominant attachments can often be partly mitigated by corrective pruning or by a cable system installed by an experienced arborist. For how defects like these are rated, see tree risk assessment.
Sources: Corrective Pruning for Deciduous Trees (FNR-563-W) ; Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure (ENH1246)
How the ISA arborist exam tests it
Codominant stems come up in pruning, risk and climbing questions: why the union is weak, how to correct two competing leaders on a young tree, which kind of union is most likely to fail, and whether a union with bark pressed inside it can be trusted as a tie-in point. Try 4 reviewed practice questions:
Why is the union between two codominant stems of similar diameter structurally weaker than a union where the side branch is much smaller than the trunk?
Show answer and explanation
Correct answer: C. No branch collar forms between them, and included bark often develops
A branch collar, where the trunk's growth rings overlap the side branch's rings, forms only when the side branch is clearly smaller than the trunk. Colorado State University Extension notes that where codominant leaders join, no branch collar develops, and the union is prone to included bark, which keeps the wood of the two stems from knitting together. Purdue Extension pictures a codominant union as a V, like a slingshot, so its angle is narrow rather than wide.
- A: Growth rate and wood density are not what weakens codominant unions.
- B: Codominant stems typically form narrow, V-shaped unions, not wide ones.
- D: Shading is not the reason codominant unions are weak.
Sources: Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension ; Corrective Pruning for Deciduous Trees (FNR-563-W), Purdue Extension
Report a problem with this question · ID prun-018
A young shade tree has two upright codominant leaders of nearly equal size, and removing either one at the union now would leave a large wound. Which approach is generally better?
Show answer and explanation
Correct answer: D. Shorten one leader with a reduction cut so the other becomes dominant
Rather than removing a codominant stem at the union, extension guidance favors subordinating it: a reduction cut shortens and slows one leader so the other can take over as the dominant trunk, without a large wound at the union. Equal thinning keeps the two leaders competing, and a cable does not change which leader dominates. Waiting for a crack lets a known defect become a failure risk.
- A: Thinning both leaders equally leaves them in competition, so the weak union keeps developing.
- B: A cable does not correct the structure; subordination pruning is the tool for that.
- C: Waiting for a crack allows a known defect to become a failure risk.
Sources: Assessing the Structure of Trees in the Landscape, Alabama Cooperative Extension System ; Making the Cut: It Matters When Pruning Trees, Penn State Extension
Report a problem with this question · ID prun-019
Which of these branch or stem unions has the highest likelihood of failure?
Show answer and explanation
Correct answer: A. A V-shaped union with included bark and an open crack below it
Narrow V-shaped unions often contain included bark, which significantly weakens the junction. When an open crack, decay or conks are also present, the weakness has progressed toward failure: the USDA Forest Service field guide for Oregon and Washington assigns high failure potential to a V-shaped fork that has embedded bark plus an open crack, decay or conks, and medium failure potential to the same kind of fork without them. A U-shaped union, or a branch much smaller than the trunk, forms a stronger attachment.
- B: Included bark alone is a weakness, but adding an open crack raises the likelihood of failure.
- C: A U-shaped union without included bark is the stronger form.
- D: A branch much smaller than the trunk is more strongly attached than codominant stems.
Sources: Field Guide for Hazard-Tree Identification and Mitigation on Developed Sites in Oregon and Washington Forests (R6-NR-TP-021-2013), USDA Forest Service, Pacific Northwest Region (GovInfo) ; Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure (ENH1246), UF/IFAS Extension
Report a problem with this question · ID risk-022
While choosing a tie-in point, a climber notices two upright stems of similar size with bark pressed into the union between them. How should this union be judged?
Show answer and explanation
Correct answer: D. As weak, because included bark keeps the stems from being well attached
Bark trapped inside the union of two similar-sized upright stems is called included bark. It prevents a solid wood connection and can act as a wedge that forces the union apart when it is loaded, so the union is weak. The Forest Service climbing guide tells climbers to inspect all unions at or below the tie-in point before choosing one, and this union should not be trusted as an anchor.
- A: Included bark does not reinforce a union; it separates the stems and can wedge them apart under load.
- B: Live unions with included bark are a recognized weak point; failure is not limited to dead branches.
- C: Diameter alone does not determine union strength; how well the stems are joined matters, and included bark weakens the joint.
Sources: National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service ; National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service ; National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service
Report a problem with this question · ID safe-005
0 of 4 correct
Common mistakes in the field
Waiting until the stems are big
Purdue Extension notes that problems cost more to fix the longer pruning waits, and that pruning wounds over about 4 inches can be hard to close and may decay. Alabama Extension adds that younger trees respond better to pruning, and their defects are easier to correct or avoid altogether.
Taking off a large codominant stem in one session
Purdue Extension says reducing a larger stem in steps, rather than removing it completely in one session, may be better for limiting stress on the tree and helping it recover.
Tying in at a codominant union
The USDA Forest Service climbing guide describes a weak union as one where branches of similar size grow so close together that bark forms inside the union, and warns that this bark can act as a wedge and split the union under load. Inspect every union at or below the planned tie-in point first. The same guide says that, unless a species naturally forks, a climber should not go above a forked bole and should treat any fork with suspicion, since a fork is a likely weak point.
Sources
- Corrective Pruning for Deciduous Trees (FNR-563-W), Purdue Extension, Forestry and Natural Resources
- Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension
- How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
- Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure (ENH1246), UF/IFAS Extension
- Assessing the Structure of Trees in the Landscape, Alabama Cooperative Extension System
- National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service
Written in our own words from the public sources above, checked October 19, 2026. Arborist Practice Test is not affiliated with ISA.