Included Bark
a Weak Point of Trees and Shrubs

Curtis E. Swift, Ph.D., Area Extension Agent (Horticulture)
Colorado State University Extension
Tri River Area

Embedded Bark on an ash (Fraxinus) and aspen (Populus tremuloides)

One of the most common points at which above ground portions of trees fail is at the "junction of two or more codominant stems". Such stems having included bark are weaker than those without included bark. Included bark is bark embedded between opposing branches, a branch and a main stem or two co-dominant stems creating a structurally weak point in the tree. These bark-included attachments are easy to detect even when the tree is quite young. It is best to eliminate these branches as soon as they are noted. As these branches continue to enlarge in diameter and weight, the tree is likely to split at this weak point.

Dr. Thomas Smiley determined the pressure differential of force necessary to pull apart codominant stems with and without included bark. He found codominant stems are more susceptible to breakage when they had included bark. He found:

All codominant stem junctions should be considered weak
4-inch diameter codominant stems are 20% weaker when included bark is involved
10-inch diameter codominant stems are 14% weaker than non-included bark unions.

Care must be taken to avoid damage to the neighboring stem when removing a branch to correct such a condition. Damage to the tissue of the stem or branch that is left, can result in an entry point for disease and insect pests as well as future `frost cracks'.

Included bark tends to be a genetic problem, and if one branch crotch has this condition, future branch attachments also are often affected. When choosing a tree in a nursery, this is one factor that needs to be considered prior to purchasing the tree.

Reference:

Smiley, E. T. 2003. Does included bark reduce the strength of codominant stems? Journal of Arboriculture 29(2): 104-106

Place on the Internet May 21, 1996
Updated June 18, 2009