What Are Codominant Trees — and Why Are They Dangerous?

You’ve probably seen us talk about tree risk assessments and mention codominant spruce in previous posts. But what exactly makes codominant trees so hazardous?

A codominant tree is one that has two or more main stems (or “leaders”) of similar size growing from the same point on the trunk. Instead of one dominant central trunk, these stems grow vertically, side by side, often forming a V- or U-shaped union.

While this may look natural or even symmetrical, it’s a red flag for potential tree failure, especially in Calgary’s strong windstorms or heavy snow loads.

How to Identify Codominant Stems in Your Trees

Look for these key features:

• Two upright stems growing close together from the trunk
  
  
• A V- or U-shaped split where the trunk divides
  
  
• No visible branch collar, unlike normal limbs
  
  
• Both stems are similar in diameter and height
  
  

These characteristics are very different from normal branches, which typically grow horizontally and are much smaller than the trunk.

Why Codominant Stems Increase Tree Risk

Codominant stems compete for space as they grow. Over time, they press against one another, creating compression zones and reducing the structural integrity of the union.

Here’s why they’re risky:

1. Weak Unions and Bark Inclusions

• V-shaped unions don’t allow for natural growth separation
  
  
• Compressed bark — or bark inclusion — forms between stems
  
  
• These inclusions mean little to no wood connection, leaving the stems structurally weak
  
  
• Water and debris can collect in the union, accelerating decay
  
  

2. Hidden Rot and Decay

A tree might look perfectly healthy on the outside — green canopy, new growth, no visible damage — but codominant stems can hide interior rot, often right at the trunk’s core.

3. Imminent Structural Failure

If the stems begin to split, show cracks, or if reaction wood (swollen growth around stress areas) begins forming, it’s a sign that the tree is trying to hold itself together — but might not succeed.

Real Case: Codominant Spruce Tree in Redwood Meadows

One tree we removed in the community recently was a textbook case of codominant stem failure. From the outside, it looked normal, but once removed, we discovered:

• Compressed bark between the stems
  
  
• Fluid retention and decay at the base
  
  
• One stem was visibly compromised while the other was starting to rot internally
  
  

The swollen “ears” on either side of the union indicated structural stress — a sign we often spot during assessments.

Another tree we examined had a noticeable concave scar where one stem had begun pulling away from the other, showing clear signs of a failing union. That tree was a critical hazard due to its proximity to a walking path and play area.

When Is It Safe to Retain a Codominant Tree?

Early detection makes all the difference.

Good candidates for structural pruning:

• Young trees
  
  
• Early-stage codominant stems
  
  
• Trees with wide U-shaped unions
  
  

In these cases, we can select a dominant leader and prune the others, allowing the canopy to fill in naturally over time.

High-risk removals:

• Mature trees with tight V-shaped unions
  
  
• Visible decay or fungal growth
  
  
• Cracks or sudden reaction wood growth
  
  
• Targets nearby (homes, pathways, fences)
  
  

What Should You Look for in Your Yard?

As a Calgary homeowner, here’s what to watch for:

• Does your spruce or other tree split into two vertical stems?
  
  
• Is there any visible swelling, bark separation, or dark streaking?
  
  
• Have you noticed cracks or leaning after a recent windstorm?
  
  
• Are both stems the same size, starting low on the trunk?
  
  

If so, your tree could be a codominant hazard — and it’s time for a free tree risk assessment from Red Mountain Rigging.

FAQs About Codominant Trees and Tree Risk in Calgary

What is a codominant tree?
A tree with two or more main trunks (leaders) growing from the same point on the trunk, usually forming a V- or U-shaped union.

Are codominant trees dangerous?
Yes. They are structurally weaker and more prone to splitting or breaking — especially in high winds or storms.

Can codominant trees be saved?
If caught early, yes. Structural pruning can encourage a single leader to grow. Mature codominant trees often require full removal for safety.

What is a bark inclusion?
A compressed section of bark between codominant stems. It weakens the structural bond and often traps moisture, leading to decay.

How do I know if my spruce tree is codominant?
Look for two large vertical stems coming from the same trunk base with a tight V-shape between them. Our arborists can assess this for free.

Final Thoughts: Don’t Let Hidden Tree Hazards Linger

Codominant trees can seem harmless — even beautiful — until they fail. These types of structural defects are among the top causes of tree failure in residential areas.

If you’re unsure about the health of your trees, or just want a second opinion, reach out to us. Red Mountain Rigging offers free tree risk assessments across Calgary, Redwood Meadows, and surrounding communities.

We’ll inspect, assess, and give you honest, expert advice — no pressure, just peace of mind.

Contact us today to schedule your free quote or assessment.