Predicting Limb Breakage
What does that mean?
- Predicting limb breakage means learning to see and interpret
the external signs that a limb is likely to fail.
- Some failures can not be predicted, and some predictions will not
be right. And even in predictable cases it is impossible to say exactly
when failure will occur. But a sound policy follows "best guesses"
based on years of careful field observations.
- As used here, a "limb" is bigger than 3-4" in diameter (=branch),
at least somewhat horizontal, and attached to a larger leader or trunk.
What are some of the symptoms
to look for?
- Symptom: limb without bark, missing
or dead leaves in crown
- Symptom: small or misshapen leaves,
or early color in one spot of the crown
Problem: limb decline
wounds with holes or cavities, esp. on the stress-bearing side or when lined
up one above the other
Problem: internal decay column, hollow
- Symptom: "dogleg" (sharp turn) in
a limb at the point where there is an old wound
Problem: decayed stress point
- Symptom: tuft of leaves at end ("lion’s
tail"), or great limb length (e.g., in silver maple)
Problem: unusually high mechanical load
- Symptom: long cracks along the length
of a limb, esp. serious when on both sides
Problem: delamination of internal layers,
separation of the beams
- Symptom: diameter over 15" in a hazard
species, esp. if limb is larger than central leader
Problem: limb weight beyond the normal
wood strength of the species
- Symptom: woody plants in a union (crotch),
swelling around or seepage from the union
Problem: included bark, internal decay,
weak attachment of limbs
What should I do if I see these symptoms?
Decay and dogleg on Japanese pagoda tree
Where can I get more information?
- Investigate the fundamental problem. Remember that these
symptoms may have causes elsewhere in the tree (e.g., in the butt or roots)
or earlier in its history (e.g., lightning strike).
- Evaluate the target. What will the limb hit if it falls,
and how serious would that be? If a large limb threatens people or property,
you should probably remove it.
- Identify the species. A symptomatic limb on a weak-wooded
species (such as silver maple, black willow, boxelder or cottonwood) is
more dangerous than on a strong-wooded species (such as white oak, sugar
maple, or shagbark hickory).
- Get an aerial inspection when useful. It is often
difficult to tell the extent of a defect from the ground, even with binoculars.
- Make sure a correct cut is done at the correct time, usually
taking it back to the collar on the leader. Limbs get pretty
big, so the wound will close slowly if at all, and you want to minimize infection
in any way possible.
- Don’t remove more than 1/4 of the crown. Besides, if that
many limbs are suspect, you should probably consider removal as a more cost-effective
Minnesota Department of Natural Resources, and USDA Forest Service. 1996.
How to Recognize Hazardous Defects in Trees. USDA Forest Service NA-FR-01-96.
20 pp. Can be downloaded or ordered off the Hazard Tree Web Page.
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 sites.