Weak Branch Unions: A Very Visible Hazard

Weak branch unions are easy to spot before they become a problem…

Weak branch unions are one of the most common hazards we see on clients’ properties. This hazard is easily identifiable and, thus, easily avoided. But what are the signs that tell us a weak branch union likely exists?

The most frequent weak branch union we see is called a co-dominant stem. This is where two stems of roughly the same size arise from the same point on the tree. (This is also known as a “schoolmarm” in forestry circles…but it’s up to you to learn the backstory on that.) The junctions between co-dominant stems are often considered the weakest portion of a healthy tree. They are widespread in forested areas; you’ll often see them on area trees, especially ponderosas. It will look like the tree has two or more tops. This creates a weak branch union because the two stems, or leaders, are growing so closely together that the bark on each leader interferes with the formation of a proper, strong union. This is called included bark, and it doesn’t have the structural strength of a normal branch crotch. Furthermore, included bark can act as a wedge, forcing the weak branch union to split apart.

weak branch unions
Removal of a ponderosa with co-dominant stems earlier this spring. Weak branch unions are especially troubling near structures.

During the windstorms of recent years, we’ve seen countless trees lose one of their co-dominant stems. Wind will point out any weak branch unions you have, if you’d rather not pay a professional to investigate. The problem doesn’t stop with the toppling top of the tree—and subsequent property damage—though. Once one of the co-dominant leaders is gone, a wound remains and leaves the tree structurally weaker and vulnerable to infection. The best approach is to be proactive about trees with co-dominant leaders. Either remove one of the leaders in a controlled environment, or cable the two stems together so they support one another and don’t sway past the trigger point in the wind. We are well versed in both approaches and can talk you through options.

Another example of a weak branch union is when epicormic branches (also known as watersprouts or suckers) are allowed to grow into sizable limbs. Epicormic branches grow quickly—often in response to poor pruning practices, injury or some kind of environmental stress—and they always display weak branch unions. It’s best to tend to suckers when they are newly formed, before they pose any real hazard to structures and people.

Finally, any branch that comes off the trunk at a very acute angle or doesn’t display a ridge of raised bark at the crotch is of potential concern. If you have trees on your property that exhibit any of the signs of a weak branch union, call us for a free consultation and estimate. Considering the widespread impacts of recent windstorms—and the fact that weak branch unions are especially susceptible to failure in wind—a call to us will at least provide peace of mind.