What are codominant stems?

The term “codominant stems” is used to describe 2 or more main stems (or “leaders”) that are about the same diameter and emerge from the same location on the main trunk.  As the tree grows older, the stems remain similar in size without any single one becoming dominant. Codominant stems are one form of poor architecture and weak branch unions.

Why are such stems important to recognize?

  • Codominant stems tend to fail much more often than others, especially in storms.
  • Though such stems may look fine to the casual observer, they may actually be dangerous.
  • Early recognition of such stems allows remedial action when it does the most good.
  • Many of our most common street, highway, and park trees commonly form codominant stems.
    • Maples and oaks
    • Conifers that have lost the terminal during development

How can you tell if there is a serious problem?

  • Classifying codominant stems into 3 risk stages can aid in their management:
    • Risk Stage 1: does the union between the two stems from a “V” but there are no other symptoms?
      • A “V” union is much more likely to fail than a “U”
      • Stems with a “V” union compress bark between them as they grow, leaving little physical connection
    • Risk Stage 2: are there symptoms of decay in the union?
      • Can you see rotted matter between the stems?
      • Is there any fluid flowing from the union?
      • Are there woody plants growing in the union?
      • Do you see wide “ears” (swelling) on either side of the union?
    • Risk Stage 3: is there any sign of failure?
      • Can you see any cracks in the union itself?
      • Is reaction wood being formed rapidly at the base of the stems? (Reaction wood need to be defined)