Multi-Stemmed Trees & Codominant Stems: Are They Dangerous?

Trees with V-shaped forks, codominant stems (when two main trunks develop on a tree), and multi-stemmed trees can add interest to your Connecticut landscape. But, while they probably look healthy, they’re often an accident waiting to happen. This tree shape is highly prone to breakage, usually splitting apart near the base or where the trunks separate.

If you have multi-stemmed trees or ones with two trunks growing in a V shape, you’ll have to take a few extra steps to ensure that they’re structurally sound and don’t pose a danger to people and structures around them.

In this article, we describe:

• how to identify codominant stems,
• why this tree shape is structurally unsound,
• how to prevent trees from developing codominant stems, and
• what to do if your tree has two or more main trunks (or large branches) growing upward at a narrow-angle.

About Codominant Stems on Trees

The term “codominant stems” refers to a tree that has grown two (or more) main stems. These tree trunks will:

• be about the same diameter,
• originate from the same point on the tree,
• grow upward at a very narrow v-shaped angle, and
• compete with each other to be the primary stem (or “leader”) for the tree.

Trees can also have branches that grow almost straight up from the main tree trunk. For example, Bradford pears are notorious throughout Connecticut for their narrow, v-shaped branch crotches.

Codominant stems and narrow branch crotches have many of the same problems and both can present some significant issues if not dealt with quickly.

What to Look For

Look for a “V” shaped angle where two or more stems of similar size come together. If you look up from there, you’ll see that the stems grow beside each other and are roughly the same height.

You may also see what is called “included bark”. Look for a line or seam in the bark where the stems come together, indicating that as the two stems have grown and pushed against each other, their bark has become trapped between the two stems. You may also see a crack in this area or bulges in the trunk. Both of these indicate that failure is more likely.

How Codominant Stems Are Formed

A tree with two trunks occurs when two apical buds at the top of a stem both grow at the same time. Instead of one becoming subordinate to the other, they both chemically act as the central leader on the tree. As a result, two stems form and become codominant.

Problems with Codominant Stems

The biggest problem with codominant stems is that they’re prone to failure. Often, trees will split apart where the two stems join, or where there are narrow branch crotches, multiple stems, or many branches growing from the same spot (this often happens when a tree has been topped).

When two branches or stems are roughly equal in diameter, they tend to be only weakly attached to each other and to the main tree trunk. Bark can also grow into the space between the branches (included bark), further weakening their attachment. The narrower the angle between the two stems, the greater the risk of failure.

Many people like the lush, full look of a multi-stemmed tree. Because each codominant stem is triggered to form a full canopy, these trees tend to have more branches overall. However, while the tree becomes overloaded on top, it doesn’t form twice as many roots to make up for the double stems. That can make the entire tree unstable.

Overall, the growth of a two-stemmed tree will make the entire tree much more vulnerable to high winds and stress from later growth.

Tree Species Prone to Growing Codominant Stems

Many types of trees can develop two or more main stems, but certain tree species are more prone to growing into a tree with two trunks. In Connecticut, we often see oaks and maples with codominant stems, as well as conifers that have been topped or hit by lightning.

Researchers think that these species are more apt than other trees to release chemicals that cue the development of the central leader versus branches, although they’re not sure why that happens.

How to Prevent Trees from Developing Codominant Stems

Prevention is the best way to help a tree when it comes to dealing with codominant stems. These stems usually develop when a tree is young; at that point, they can be easily pruned off. Most trees will respond quickly and will only need this kind of structural pruning a few times.

Structural pruning of a tree with codominant stems involves ‘subordinating’ all but the chosen primary leader and lateral limbs. They are cut back (“reduced”) to slow their growth, coaxing the main stem to become dominant again.

This kind of pruning is an acquired skill that’s based on in-depth knowledge of tree physiology and mechanics, as well as extensive experience. Do not do this yourself – you’ll likely only make the problem worse! Instead, call in a CT Licensed Arborist who can get your tree off to the right start.

The sooner you begin structural pruning, the easier it is to change the form of the tree. It will become exponentially more expensive to correct the longer you wait.

What to Do If Your Tree Has Codominant Stems

Once a tree hits maturity, structural pruning may no longer be a viable option. Cutting off one stem of a developed two-stemmed tree can cause a significant wound that may kill the tree. The tree may also be left unbalanced, putting it at risk of falling over.

Install Cabling

If you have a mature tree with codominant stems, have a Connecticut Licensed Arborist examine it. They may recommend installing cabling to act as a structural support system for the tree. Tree cables are usually placed between the codominant stems to reduce their movement and prevent them from splitting apart.

Once you have cables installed in the tree, you’ll need regular inspections to ensure that they haven’t shifted or weakened and that they’re still adequately supporting the tree.

Remove the Tree

If cabling isn’t possible, the arborist might recommend removing the tree. The most frequent reason is because there’s internal decay in the tree. Internal decay is more common in multi-stemmed trees because the weak attachment point between the tree stems and the included bark are easy entry points for fungal pathogens.

An arborist may also recommend tree removal if the tree is a hazard. Depending on the load, weight distribution within the tree, angle at which the stems are joined, number of stems involved, presence of nearby structures or people, as well as decay and other factors, a tree with codominant stems may be categorized as a hazard tree.

IT’S THE LAW – If your tree is hazardous, it’s your responsibility to have it removed or you may be liable for any damage the tree causes when it fails.

Concerned About Your Tree?

If you have a tree with codominant stems, give the Licensed Arborists at Rayzor’s Edge Tree Service a call at 203-258-5584 (or use our convenient online contact form) to schedule a tree inspection. We’ll recommend the safest option for your tree, whether that’s cabling, structural pruning, or removal. One of our experienced tree service crews can then take care of the job for you, from start to finish.