Codominant Stems and Included Bark: The Hidden Split Risk in Yard Trees
By Daniel Reyes, Tree Care & Risk. Last updated: July 22, 2026
That tidy-looking double trunk on the big maple, pear, or oak in your Rochester yard is one of the most common reasons large trees split apart in a storm. The warning sign is easy to miss: a seam of bark buried in the fork where the two stems meet. This guide explains what codominant stems and included bark are, how to read a strong union from a weak one, and when the fix is a few well-placed pruning cuts versus a support system or removal. It is a structural topic, and it is distinct from surface bark splitting and frost cracks on a trunk, which are a different problem entirely.
What are codominant stems, and why do they matter?
Codominant stems are two or more upright stems of nearly equal diameter that grow from the same point on a trunk, competing to lead the tree instead of one clearly dominant trunk. Arborists generally flag a stem as codominant once it reaches about one-third the diameter of the trunk it rivals. That side-by-side competition matters because it builds a structurally weak fork right where the tree carries the most leverage.
In an open-grown yard tree with plenty of light on all sides, this happens easily: nothing forces a single leader, so the tree keeps two. The junction of two or more codominant stems is one of the most common places the aboveground part of a tree fails, according to arboriculture research summarized by the International Society of Arboriculture and university extension programs. The bigger and heavier the two stems grow, the more load the weak fork has to hold.
Why is included bark so dangerous?
Included bark is bark that gets trapped inside a tight, V-shaped union as two codominant stems widen and press together, so the stems never fuse into solid, continuous wood. The result is a weak seam that behaves like a wedge, prying the stems apart under wind, ice load, heavy fruit, or wet foliage. Without wood-to-wood connection, the union has far less holding strength than it looks like it should.
The strength penalty is measurable. A Bartlett Tree Research Laboratories study by arborist E. Thomas Smiley, published in the ISA journal Arboriculture & Urban Forestry, found that included bark reduces a codominant union's strength by roughly 14% to 20% compared with unions that have no trapped bark, with the larger penalty on smaller-diameter stems. Smiley's central finding is blunt: codominant stems with bark trapped in the union are significantly weaker than those without included bark. In Upstate New York, that lost strength gets tested every lake-effect snow load and ice storm.
Field note · When to call a pro
Want a certified arborist to take a look?
Monster Tree Service of Rochester offers free estimates and a full plant health care program across the Rochester area.
Get a Free Estimate →How do you spot a high-risk union in your yard?
You spot a high-risk union by reading the shape and surface of the fork where the two trunks meet. A narrow V with a raised, pinched seam, or a visible line of bark tucked between the stems, points to included bark, while a rounded, U-shaped crotch with a raised branch-bark ridge is far stronger. Cracks, bulges, seams that ooze, or a prior partial tear all raise the stakes.
Location decides how much that defect matters. A weak V-shaped union on a tree in the back corner of a Greece lot is a lower priority than the same defect on a maple leaning over a Pittsford driveway or bedroom. If you find a fresh crack or a stem that has already started to peel away, treat the tree as a potential hazard that may need removal and keep people out from under it until it is assessed.
Can structural pruning prevent failure on a young tree?
Structural pruning can largely prevent codominant failure when it starts young, because a small tree is cheap, safe, and quick to correct. An arborist selects one stem to be the dominant central leader and then shortens or subordinates the competing stem so the chosen leader outgrows it over time. This follows ANSI A300 (Part 1) pruning standards, the national benchmark for how cuts should be made.
The International Society of Arboriculture recommends beginning this work a year or two after planting and repeating it as the tree grows, noting that structurally pruning a young tree several times can nearly eliminate the codominant problem before it starts. ANSI A300 also caps how aggressively a mature tree should be pruned, generally no more than 25% of live foliage in a single year, though young trees tolerate more. Getting the timing right is why the best window to prune trees in Upstate New York is worth planning around rather than doing on impulse.
What are the options for a mature codominant tree?
A mature codominant tree has three main paths. An arborist can install a cabling and bracing system that limits how far the stems can move at the union, reduce the crown to lower the weight and leverage pulling the fork apart, or remove the tree when the defect and its target make it too risky to keep. Which path fits depends on the union's condition, the tree's value, and what sits below it.
Support systems come with an honest caveat: cabling and bracing reduce the failure risk of a weak union but do not eliminate it, and the hardware needs periodic inspection. That is why the choice between saving and removing a tree is a judgment call, laid out in our comparison of cabling and bracing cost versus removal. A formal tree risk assessment weighs the likelihood of failure against the target below to settle which option is defensible.
Field note · When to call a pro
Want a certified arborist to take a look?
Monster Tree Service of Rochester offers free estimates and a full plant health care program across the Rochester area.
Get a Free Estimate →When should you call a certified arborist?
Call a certified arborist whenever a codominant tree overhangs a house, driveway, or play area, when you see a crack or fresh split at the union, or when you simply cannot tell a strong U-shaped crotch from a weak V. An ISA Certified Arborist can quantify the failure likelihood, weigh the target below, and recommend the right response: subordination pruning, a support system, crown reduction, or removal.
Do-it-yourself fixes backfire here. Bolting the stems together with hardware store parts or cinching a strap around the trunk can wound the wood, girdle the stems, and hide the problem while it gets worse. The cuts and the cabling both have to be sized and placed correctly to do any good, which is engineering work, not yard work.
FAQ
Does every tree with two trunks eventually split?
No. Plenty of double-trunk trees stand for decades. The risk climbs when the union is a tight V with included bark, the two stems are large and heavily weighted, and the tree overhangs something valuable. A rounded, U-shaped union with a clear branch-bark ridge is usually sound structure, not a defect.
Can I just bolt a codominant tree together myself?
No. Hardware store bolts or a ratchet strap can girdle the stems, wound the wood, and create a false sense of safety while the union keeps weakening. Cabling and bracing are engineered to ANSI A300 standards, installed at specific heights, and sized to the tree by a trained arborist for a reason.
Is cabling a codominant tree a permanent fix?
No. A support system reduces failure risk but does not eliminate it, and the hardware needs periodic inspection and eventual replacement. Cabling buys years of safer service from a valued tree; it does not turn a weak, bark-included union into a strong one.
Should I remove a codominant tree near my house?
Not automatically. Removal is one option, but structural pruning on a young tree, crown reduction, or cabling on a mature one may preserve it safely. The right call depends on the union's condition, the tree's size and species, and the target below, which is exactly what a risk assessment sorts out.
