August storms off the lake do not usually take whole trees down. What they take is half a tree, split straight down the middle, leaving a raw vertical wound where the trunk divides. Almost every one of those failures traces back to the same structure: codominant stems, two trunks of roughly equal size rising from one point.
The union between them looks solid from the driveway. Inside, there is often no wood holding the two halves together at all. That matters because codominant stems are visible from the ground and correctable long before a storm finds them.
How a Normal Branch Union Is Built
To see why codominant stems fail, look first at a union that works. When a branch is clearly smaller than the trunk it comes from, the trunk lays down wood around the base of that branch each year, wrapping it. That overlapping tissue forms the branch collar, the visible swelling at the base of a limb, and it is a mechanical joint with interlocking wood fibers running through it.
You can see the top of that joint from the ground. The branch bark ridge, a raised line of bark in the crotch, points up and outward when the union is sound. It is the surface evidence that wood is knitting the two parts together underneath, and University of Maryland Extension has clear illustrations of both features.
What Goes Wrong When Two Stems Are Equal
Codominant stems are defined by equality: when neither stem is dominant, neither one wraps the other. Both keep growing upward at the same speed, and the tissue between them has no reason to form a collar. In a narrow, V-shaped crotch the two stems press bark against bark, and that bark gets trapped in the union as they expand. Arborists call it included bark.
Bark is not structural. It cannot transmit load, and it physically blocks the wood fibers of the two stems from fusing. So the union is two separate cones of wood pressed together with a layer of dead tissue between them. It is held by friction and by the small amount of wood at the very back of the crotch.
Every year both stems add diameter, and that added wood wedges the union apart from the inside. The crack usually starts at the base of the seam, invisible, and travels down into the trunk.
Why August Is When They Fail
Three loads arrive together in late summer. The canopy is carrying a full season of leaves, which acts like a sail. Thunderstorm downbursts push a sudden horizontal load through that sail. And on fruiting trees, the weight of a crop hangs off the ends of the same limbs.
A union with included bark has almost no resistance to a stem being levered outward. The failure is sudden and it is usually complete, because once the seam opens, there is nothing behind it to stop the split from running down the trunk. That is the difference between losing a limb and losing a tree.
Spotting Codominant Stems From Your Driveway
You do not need to climb anything to find these.
- Two stems of similar diameter forking from the same point, usually within the lower half of the tree.
- A tight V-shaped crotch rather than a wide U.
- Bark turned inward in the seam instead of a raised ridge pointing outward.
- A vertical crack, a bulge, or a line of callus tissue below the union, which is wound wood the tree has laid down trying to stabilize itself.
- Movement. On a breezy day, if the two stems rock independently rather than together, the union is already working loose.
Silver maple, Bradford pear, ash, elm and many ornamental plums are the usual candidates in Orem yards.
An Orem Silver Maple After a Downburst
We get this call every August. A silver maple in an Orem back yard, planted decades ago, forks into two stems about head height. A storm comes through in the evening with a short, hard gust. By morning one stem is lying across the fence, and the other is standing with a vertical split running down into the trunk.
Looking at the failure surface tells the story. The wood on each face is clean and smooth over most of the area, with dark weathered bark in the middle. That dark tissue is included bark that had been sitting inside the union for years. The break did not happen in the storm. The storm only finished something that had been separating since the tree was young.
Your Options on a Tree With Codominant Stems
There are three real choices, and which one fits depends on the size of the tree and how far the union has already opened.
- Subordination pruning. The first option we reach for on a young or mid-sized tree. One stem is shortened so the other becomes dominant, which shifts growth into the leader and lets the union close properly over time. It is slow, takes little wood, and the result is permanent.
- Supplemental support. Cables and braces installed in the upper canopy limit how far the stems can move apart. Support hardware does not repair the union; it reduces the load on it and has to be inspected on a schedule. This is a tree preservation measure, not a one-time fix.
- Removal. When a crack is already running down the trunk, when decay has entered the seam, or when the target beneath is a house or a play area, taking the tree out is the honest answer.
Safety and When to Get It Assessed
A split union is not a do-it-yourself job. The remaining stem on a half-failed tree is under stored tension, and cutting into it releases that load unpredictably. People are injured by the part of the tree that was still standing, not the part on the ground.
Keep clear of anything hanging, do not put a ladder against a cracked trunk, and never approach a failure that has landed on or near a power line. Call the utility first and us second.
Have a tree assessed when you can see a crack below the union, when one stem leans over a structure, or when a union sits above a driveway, patio or bedroom. Our tree risk assessment looks at the union, the target underneath and the likelihood of failure together, and tree care in Orem is scheduled around that. The USDA Forest Service and Utah State University Forestry Extension both publish structural pruning guidance for young trees, which is where this problem is easiest to solve.
FAQs About Codominant Stems
Are codominant stems always a problem?
No. A fork with a wide, U-shaped angle and a visible bark ridge often has enough wood in the union to hold. It is the narrow forks with bark turned inward that concern us.
Can a split trunk be repaired?
Not restored to its original strength. Bolts and cables can hold a partially split stem in place where removal is not an option, but the wood fibers do not rejoin, and the tree needs monitoring from then on.
When should codominant stems be corrected?
As early as possible. Structural pruning on a young tree takes a few small cuts, while the same correction at maturity means removing a large stem and leaving a large wound.
Do cables need to be checked?
Yes, on a regular schedule. Hardware can loosen as the tree grows, and a cable that is no longer doing its job gives a false sense of security.
Your Next Step
If a tree on your property forks into two equal trunks above something you care about, get it looked at before the next storm rather than after. Call Rivendell Tree Experts to schedule a tree risk assessment in Orem, or describe the fork and what sits under it through our contact page.