Codominant Leader Stem Decision Tree

Why this matters

Codominant stems are the single most common structural defect in landscape trees, and the decisions an arborist makes about them - subordinate one to make it a lateral, cable both for life, or remove the tree - determine the next 20-50 years of that tree's structural integrity. A young codominant union with no included bark intervenes easily with subordination pruning. A mature codominant union with deep included bark over a high target is a failure waiting for the next ice storm. The wrong call at either life stage produces an undesirable outcome: over-intervention on a sound young tree, or under-intervention on a high-risk mature one. This tree separates codominant stems by age, included bark presence, target, and species and routes each case to the structurally correct action.

Codominant stems defined

A codominant stem is one of two or more roughly equal-diameter leaders rising from a single junction in the lower trunk. Tree biology favors apical dominance - one leader takes the lead and others become laterals. When two or more leaders compete and neither subordinates, the union point bears stress that no single trunk would carry.

Strong vs weak union: a strong union has wood fibers from both leaders interweaving across the junction, like a chain link. A weak union has bark trapped inside the seam (included bark) instead of interwoven wood. The included bark grows outward with the tree, and the union has no structural connection in the seam - it is a glued joint, not a welded joint. Under load, it splits.

Visual identification: U-shaped union from outside, often with the bark ridge between stems running vertical or slightly outward. V-shaped union with bark turning inward into the seam, often with visible swelling on the outside surface of the union from the included bark mass. The V is the warning sign.

Species predisposition: silver maple, Bradford pear, willow, mulberry, ash, and many ornamental species form codominant stems readily and develop included bark. Oak, beech, and most hickories tend to form stronger unions. Species choice in landscape design influences how often you encounter the problem.

Quick checks

Step back 20 feet and look at the tree silhouette. Two or more equal leaders is the codominant signal. Note where the union sits relative to the canopy - low-trunk codominant unions are higher-risk than high-canopy ones because more weight is leveraged below the union.

Look at the union itself. U-shape vs V-shape. Bark ridge present and pointing outward (good) or seam closed and bark folding inward (included bark, bad).

Look for cracks at the union. A vertical crack at or just below the union, often visible only in cold weather when the wood is dry, is structural failure in progress. Movement during wind is also confirming.

Measure stem diameters above and below the union. Stems roughly equal in diameter at the union zone are competing for dominance. One stem clearly larger has already subordinated and is the dominant leader - no intervention needed.

Look at the target. Codominant tree over a brush pile is a low-stakes problem. Codominant tree over a master bedroom is a high-stakes problem. The decision matrix shifts with target consequence.

Path 1: young tree (under 15 feet, trunk diameter under 4 inches at union) with codominant stems

Best action: subordination pruning. Cut back the less-dominant of the two competing leaders to a strong lateral, reducing its growth rate so the other can become dominant. Sometimes called "structural pruning" or "leader selection."

Process: identify which stem will be the central leader of the future tree (often the more vertical and centered one). Cut back the competing leader by 30-50% to a lateral that is at least 1/3 the diameter of the cut. Do not remove the entire competing leader - it provides foliage and energy. The reduction shifts dominance over the next 2-3 years.

Re-prune at 3 and 5 years to maintain the dominance. By year 10, the tree should have a single central leader and the historical codominant stem is now a subordinate lateral.

This is the cheapest possible long-term solution and the answer that protects the tree's structural future. Worth selling proactively to customers with young landscape trees.

Path 2: mature tree, codominant stems with U-shaped sound union, no included bark, low target

Best action: monitor.

Process: photo-document the union, measure spacing of the stems, note any cracks. Re-inspect on a 3-5 year schedule. Healthy U-shaped unions can hold for the tree's full life.

Customer education: the union is sound now and likely to remain sound. Annual monitoring is not necessary; long-cycle inspection is sufficient.

Path 3: mature tree, codominant stems with V-shaped union, included bark, high-value target

Best action: cable or remove depending on additional factors.

Cable when: tree is otherwise healthy, no decay at the union, species is sound (oak, maple), customer values the tree and accepts long-term inspection costs. Install a static steel cable per ANSI A300 Part 3 or a dynamic synthetic system (Cobra) between the two leaders above the union, in the upper third of each stem. Document with serial-numbered hardware and report.

Cabling reduces the union load excursion under storm conditions and gives the tree 10-20 more years of viability if otherwise healthy. Cables must be re-inspected every 3-5 years and replaced as wear or corrosion warrants.

Remove when: union shows active cracking, decay at the seam, tree is over the house or other catastrophic target, species is brittle (Bradford pear, silver maple), or customer cannot commit to ongoing cable maintenance. Honest recommendation. The tree is a hazard now and increasing.

Path 4: mature tree, V-shaped included bark union, no high target

Best action: monitor with explicit risk disclosure.

The tree may stand for decades or split tomorrow. Without a high-value target, the consequence of failure is low. Customer informed of the structural defect, the failure mode (split through the union under wind or ice load), and the future of the tree.

Some customers will choose to remove preemptively. Some will accept the risk. Either decision is defensible if documented.

Path 5: triple or quad codominant stems

A tree with three or more competing leaders is structurally worse than a two-stem codominant. Loads concentrate at a single low-trunk junction.

Young trees: aggressive subordination of two of the three leaders to make one dominant.

References

• ANSI A300 Part 1 Pruning - subordination and structural pruning
• ANSI A300 Part 3 Support Systems - cabling and bracing
• ANSI Z133 Arboricultural Operations - Safety Requirements
• ISA Best Management Practices: Pruning, current edition - structural pruning of young trees
• ISA Best Management Practices: Tree Support Systems, current edition
• ISA Best Management Practices: Tree Risk Assessment, second edition
• ISA Tree Risk Assessment Qualification (TRAQ) methodology
• Shigo, A.L. and others - included bark literature in arboriculture research