Understanding Co-Dominant Unions

Great Western tree Care – An Arborist’s Guide for Homeowners

When homeowners look at a tree, they usually notice the canopy, the shade, or the overall shape—but what matters most to a tree’s structural safety is how the branches and stems are attached. One of the most common structural defects in urban trees is something called co-dominant unions, and it plays a major role in why trees split or fail during storms, heavy snow, or high winds.

As a certified arborist, I assess hundreds of trees each year with co-dominant unions—and most homeowners don’t realize there’s a problem until a failure happens.

Let’s break down what co-dominant unions are, why they form, and what you can do to protect your trees.

What Is a Co-Dominant Union?

A co-dominant union (also called a co-dominant stem) is when a tree has two or more main stems growing from the same point, often at the same age, size, and height. Instead of one strong central trunk, the tree develops multiple “leaders” competing to be the dominant stem.

If you look at the trunk and see a “Y” shape with two trunks the same size—that is a co-dominant union.

Examples:

• Two equal trunks splitting five feet off the ground
• A main trunk that divides into two large, equal branches instead of one central leader
• Two major limbs emerging opposite each other from the same location

This growth pattern is common—and it is one of the top reasons trees split apart.

Why Co-Dominant Unions Are a Problem

To understand the risk, think about how branches attach to a trunk. A normal, strong branch attachment has:

• wood grown around and over the branch base, forming a solid weld
• branch bark ridge visible where the branch meets the trunk

With co-dominant stems, this doesn’t happen. Instead of a strong bond, the union often contains:

1. Included Bark

When two equal stems push against each other as they grow, the bark from each stem gets trapped inside the union. This is called included bark.

Included bark:

• prevents solid wood formation between stems
• creates a weak point with very little structural support
• increases risk of a split during wind, snow, or ice

2. No Strong Wood Connection

Where bark is included, wood cannot fuse the stems together. Instead of a thick, solid connection, you get a hollow or weak union. The more those stems grow outward and upward, the more leverage they apply against each other.

3. Poor Load Distribution

Co-dominant stems normally form a narrow, tight “V” shape instead of a wide, strong “U” shape.

A tight “V” shape:

• concentrates stress at the union
• increases pressure during storms
• acts like a wedge that forces the stems apart

Wide, open “U-shaped” unions are much stronger and safer.

How Co-Dominant Stems Form

Trees develop co-dominant stems for several reasons:

Early Growth Habit

Some species naturally like to form multiple leaders if not trained young:

• Linden
• Elm
• Ash
• Poplar & cottonwood
• Maple
• Ornamental pear

A lot of common Colorado landscape trees fall into this group.

Incorrect Pruning

If a young tree is not pruned to establish a central leader in the first 3–5 years, branches of similar size compete—and the tree never develops strong structure.

Storm Damage or Breakage

If the original leader is damaged or broken, two new stems sometimes form from the same point, creating a co-dominant structure as the tree grows.

Why Co-Dominant Unions Fail

When a tree with co-dominant stems grows large, it becomes a leverage problem:

• Each stem gets heavier
• Each stem catches more wind
• Each stem grows further away from the union
• Each stem pushes harder against the other

Eventually, under enough force—the union splits. Failure often happens in:

• windstorms,
• heavy wet snow,
• ice events,
• or even on hot days when wood is stressed.

These failures are usually sudden, without warning cracks visible to a homeowner.

Signs of a Risky Co-Dominant Union

Homeowners can spot risk indicators:

• A “V-shaped” fork in the trunk
• Cracks forming in the union area
• Bark pressed inward in the crotch (included bark)
• Swelling or bulging at the union
• Canopy leaning heavier to one side
• Dead or decayed wood near the union
• Branches twisting during wind

If you see multiple signs, it’s worth having a tree risk assessment.

How Arborists Fix or Reduce the Risk

We have several tools to improve tree structure depending on the tree’s age and size.

1. Early-Stage Structure Training (Best Option)

If the tree is young (1–5 years):

• We remove or reduce competing leaders.
• We encourage one central trunk.
• We shape the canopy for long-term strength.

This is simple, affordable, and prevents decades of problems.

2. Reduction Pruning

On mature trees, we may:

• Reduce the length/weight of one or both stems
• Thin the canopy to reduce wind resistance
• Create better balance and lower leverage

This doesn’t eliminate the union, but lowers the force during storms.

3. Cabling & Bracing

For large, valuable trees that already have co-dominant stems, we can install:

• Steel support cables high in the canopy
• Through-bolts (braces) across the union

These systems:

• stabilize the stems
• prevent spreading and tearing
• reduce risk of catastrophic failure

Cabling is not a DIY job—it requires engineering knowledge and the right equipment.

4. Removal (If Risk Is Too High)

If the tree:

• leans over a target (house, driveway, play area)
• shows significant cracks or decay
• is already splitting

…removal may be recommended.

We always weigh:

• safety,
• stability,
• value,
• and homeowner goals.

Why Homeowners Don’t Notice the Risk

Co-dominant stems often look beautiful. They give trees a full, symmetrical shape. To the untrained eye, there’s no visible danger—until the day the tree splits.

This is why routine inspections matter. A quick visit from an arborist can identify structural defects years before they become a problem.

Good News: Co-Dominant Stems Are Preventable

If you plant a new tree, remember:

The first 5 years are the most important.

Simple structural pruning early builds a tree that:

• lives longer,
• needs fewer interventions,
• and is safer around homes and people.

This is one of the best investments you can make in your landscape.

Final Thoughts

Co-dominant unions are extremely common in urban trees, and many failures we respond to after storms could have been prevented with:

• early training,
• selective pruning,
• or support systems.

You don’t have to diagnose this alone. A certified arborist can evaluate the union, the species, the site conditions, and the risks to your property.

If you suspect your tree may have co-dominant stems—or you simply want to know if your tree has a stable structure—a professional inspection is the safest approach.