Urban and Community Forestry
What does that refer to?
Why are such stems important to recognize?
- 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.
How can you tell if there is a serious problem?
- 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
- Classifying codominant stems into 3 risk stages can aid in their
- Risk Stage 1: does the union between the two stems form 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?
What can be done about them?
White oak with "ears" at union of codominant stems
Where can I get more information?
- Risk Stage 1
- If the tree is young enough, prune out one of the stems; the tree
will fill in the missing canopy
- For codominant stems greater than about 4" in diameter, pruning
out one stem can cause more problems than it solves
- It leaves an unbalanced crown susceptible to mechanical failure
- It creates a large open wound susceptible to decay fungi
- Risk Stage 2
- Carry out an aerial inspection, probing the union itself to estimate
- Reduce the end weight of the stems through proper crown reduction
- For specimen trees, cabling and pruning can help in some situations
- You need a balanced crown and sound wood in the upper leaders for
- Make sure any such work follows the ANSI A300 standards
- When the stem is large and you can not cable, consider removing
the tree--especially when there is a significant target such as a busy road
or inhabited building
- Use a drill or other tool (such as a Resistograph®) to determine
the thickness of sound wood
- There are no firm published criteria, but look for at least 1" of
sound wood for each 6" of attached stem diameter
- The longer the stem above the union, the greater the breaking force,
so give yourself an extra margin of safety for long stems, particularly
those with lots of foliage
- Risk Stage 3
- If there is a crack or other indication of incipient failure, remove
the tree as soon as possible--especially if there is any kind of target
Gilman, Edward. 2002. An illustrated guide to pruning.
2nd edition. Albany, NY: Delmar. For other information,
advice and help on this topic, call offices of your State Urban Forestry
Coordinator or University Extension service, or visit urban forestry web