A codominant stem is one of two (or more) trunks or major leaders that grow from roughly the same point and compete for the top spot, instead of one leader staying dominant with smaller branches below it. When those stems meet in a tight V instead of a wide U, bark gets pinched inside the joint rather than growing into solid wood. That trapped layer is called included bark, and it is one of the most common reasons a healthy-looking tree splits apart with no storm and no warning.
You will see this most on trees that were never trained when young: a tree that forks low into two or three “trunks” of about the same size. It looks full and symmetrical, which is exactly why homeowners rarely worry about it until a section peels away.
This is fixable early and manageable later, but it does not go away on its own. What you do about it depends on how old the tree is and whether the union has already started to crack.
What Makes a V-Shaped Union Different From a U-Shaped One
A strong branch union forms a U shape and grows a branch collar, a slightly raised ring of wood where the branch and trunk tissue overlap and lock together. A weak union forms a narrow V, and the two stems press bark against bark instead of wood against wood as they grow. According to Colorado State University Extension, codominant trunks do not develop a branch collar and are prone to included bark, because the tight angle leaves no room for that interlocking wood to form.
The result is a joint held together mostly by bark and a thin layer of live tissue, not the dense wood that carries a real load. It can hold for years, especially in calm weather, then fail all at once under wind, wet snow, or just the sheer weight of the canopy on a hot Boise afternoon.
A few signs point to a weak union rather than a normal fork:
- A tight, narrow angle between the two stems, closer to a “V” than an open “U”
- A visible seam or crack running down into the joint, sometimes with bark that looks folded rather than smooth
- Slightly swollen or uneven bark ridges on either side of the union
- Two or more leaders of similar diameter, rather than one clearly larger trunk
If a tree in your yard has one obviously dominant trunk and smaller side branches, that is normal structure. If it has two or three same-size “trunks” splitting off low, take a closer look.
Which Boise Trees Show Up With This Most Often
Some species are simply more prone to codominant stems and included bark because of how they naturally branch. Three come up often in the Treasure Valley:
Bradford and other Callery pear. University of Maryland Extension notes that branch connections on Callery pear can have narrow angles that predispose the wood to storm breakage, and that significant to catastrophic breakage is common because of the tree’s inherently weak branch attachment. Bradford pear in particular is known for growing a dense cluster of same-size upright limbs, which is close to a worst-case setup for included bark. Our worst trees to plant in Boise guide covers why this species keeps showing up on that list.
Silver maple. University of Illinois Extension describes silver maple as a fast grower whose wood stays weak, which leaves it prone to storm damage. Fast growth means the tree often throws up two or three competing leaders before anyone gets to it with a pruning saw.
Siberian elm. Utah State University Extension puts it plainly: Siberian elm has weak wood and is prone to branch breaks. Combine that with a habit of forking into multiple leaders and you get unions that are working against the tree from an early age.
None of this means you have to remove a tree just because it is one of these species. It means these are the trees worth a closer look at the trunk, not just the canopy.
Why Included Bark Is a Structural Problem, Not a Cosmetic One
It is tempting to treat a tight fork as normal, especially if the tree has looked that way for years. The issue is that included bark does not get stronger with age the way a healthy union does. Every year the two stems add more wood on the outside while the included bark stays trapped in the middle, doing nothing to hold the joint together. The tree keeps getting heavier while the weak point stays weak.
When that joint finally gives way, it rarely warns you first. A split can start high in the canopy and run down into the trunk, or it can happen at ground level and take most of the tree with it. Either way, by the time you see a visible crack, you are usually looking at a tree that needs a same-day inspection rather than a routine trim. If you already see a seam opening up, read our guide on a crack in a tree trunk for what to check before the next windstorm.
If a codominant union looks like it is already failing, or a limb has come down, call us at (208) 247-9766 or use the contact form for a same-day look. We offer free on-site estimates with no obligation.
Fixing It Young: Subordination Pruning
The cheapest and most effective fix is catching codominant stems while the tree is still small, usually within the first several years after planting. Instead of removing a whole leader, an arborist uses a reduction cut to shorten the weaker of the two competing stems. That technique is called subordination.
Colorado State University Extension explains that subordination reduces foliage on the competing stem, which slows its growth while the main leader gets more light and grows faster. Over a few seasons, one leader clearly takes over as dominant and the other settles into a normal side branch, letting a real branch collar form where a weak V used to be.
This is standard tree trimming work, priced like any other pruning job: light trimming on a young tree under 25 feet typically runs $150-$400, more if there are several stems to correct or the tree is larger. It is a small job now compared to what a mature codominant trunk can turn into later.
Managing It in a Mature Tree: Cabling and Bracing
Once a tree is fully grown, you cannot subordinate a 40-foot trunk the way you would a young sapling; there is too much wood, and removing that much canopy at once would stress the tree. For a mature tree with a codominant union that has not cracked yet, the usual fix is a cable or brace system: a steel cable installed high in the crown to limit how far the two stems can sway apart, sometimes paired with a rod through the union itself for extra support.
Cabling does not undo included bark. It manages the risk by taking some of the load off a weak joint so it is less likely to fail in wind. It works best installed before a crack shows up, not after. Cabling and bracing systems typically run $200-$900 per system depending on the tree’s size and how many unions need support; see our tree cabling and bracing guide for how the process works and when it makes sense.
When the Union Has Already Cracked
Cabling helps a sound but weak union stay together. It is not a fix for a joint that has already started to split. Once you can see daylight through a crack, or bark has visibly separated along the seam, the wood fibers holding that side of the tree together are already compromised. At that point an arborist needs to assess whether the tree can be saved by removing the failing stem, or whether the whole tree has become a hazardous tree that should come down.
Removal cost depends on the tree’s height, how much of it is already compromised, and what is around it to work around, so we quote it on site rather than over the phone. Our tree removal cost calculator will give you a realistic range to start from. After any storm that puts stress on a marginal union, it is also worth running through our windstorm tree damage checklist before deciding whether a tree is still safe.
A codominant union under obvious stress, one that is leaning, cracking, or has already dropped a limb, is not something to wait out. Call (208) 247-9766 for a same-day look, or reach us through the contact form if it is not an active emergency. We offer free on-site estimates with no obligation either way.
Codominant Stems vs. Normal Branching: A Quick Comparison
| Normal branch union | Codominant union with included bark | |
|---|---|---|
| Angle | Wide, open “U” | Tight, narrow “V” |
| Branch collar | Present, visible ridge | Absent or barely formed |
| Growth over time | Union gets stronger with age | Weak point stays weak, load keeps increasing |
| Typical fix | None needed | Subordination pruning (young) or cabling (mature) |
| Left unaddressed | Not a concern | Can split with no warning, especially in wind |
Frequently Asked Questions
Is a codominant stem always a problem?
Not always. Two leaders with a wide, open angle and no visible seam can be structurally fine. The concern is specifically a tight V-shaped union where bark has gotten pinched between the stems instead of growing together as solid wood. If you are not sure which you have, it is worth a quick look from an arborist rather than guessing.
Can I fix included bark myself?
Subordination pruning on a young tree is a real pruning technique that requires knowing which cuts to make and how much to remove without stressing the tree, so it is not a good DIY project past very small saplings. On a mature tree, cabling requires drilling and hardware installed at height and is not a homeowner task. An assessment first will tell you whether the tree even needs work.
How do I know if my tree’s fork is dangerous?
Look for a narrow angle between the stems, a visible crack or separated bark along the seam, or two trunks of nearly equal size rather than one clearly dominant leader. Any bark that looks folded into the joint instead of smooth is a sign of included bark. If you see an actual crack, treat it as urgent rather than something to monitor.
Does cabling stop a split from ever happening?
Cabling reduces how far a weak union can move in wind, which lowers the risk of a sudden failure, but it does not repair the included bark itself. It is a support system, not a cure, and it works best installed before any cracking starts. A union that has already split needs a different plan, usually removal of the failing stem or the whole tree.
Are Bradford pear, silver maple, and Siberian elm always going to split?
Not every tree of these species will fail, but all three have a documented tendency toward weak branch structure, and Bradford pear specifically toward narrow, storm-prone angles. If you have one of these in your yard, especially an older, larger specimen with multiple same-size trunks, it is worth having the structure checked rather than assuming it is fine because it has held up so far.
Why does this matter more in Boise than in wetter climates?
Included bark unions can fail anywhere, but Boise adds a few pressures of its own: hot, dry summers that stress trees, followed by wind events and occasional wet, heavy snow that load a weak union all at once. A joint that has been quietly weakening all summer is more likely to give way the first time a real gust or snow load hits it.