Evaluation of Trunk Cavities
What does that mean?
Why is it important?
- You need an explicit and reasonable policy when deciding what
to do about a tree that is hollow.
Cavity in main trunk of an old poplar
- Trees with cavities = trees with decay. Because the defect
is on the inside, it is harder to see and judge the danger from the outside.
- A reasonable policy can’t be based on emotion. Extreme opinions
such as "It’s a beautiful tree, leave it alone!" or "It’s a rotten tree, cut
it down!" just cause more trouble.
- The happy medium is a policy that has two goals:
- to protect trees from unnecessary removal
- to protect people and property from unnecessary harm
What do I need to know to make a reasonable decision?
- Does the tree have a likely target?
- Even a "bad" tree is not a hazard tree if it has no significant
target to hit.
- Remove a tree with a large cavity when it has a stationary target
that is constantly occupied like a house.
- What kind of tree is it?
- Different species have different mechanical strengths and decay
- Be more cautious with species of high hazard potential such as
silver maple, cottonwood and other poplars, willow, basswood, boxelder,
black locust, and tree-of-heaven.
- What is the tree’s condition?
- Look for good overall balance, a full and normal crown, and at least
4" annual shoot extension.
- A declining and leaning hollow tree is much riskier than a vigorous
and upright one.
How can I save a tree with internal rot?
- How extensive is the rot?
- Traditional methods include "sounding" (hitting the trunk) and probing
a hole. Both are good techniques for finding the largest cavity, and to
get a rough idea of its size.
- A more scientific but still low-cost method is to drill with a
3/16" long-shafted bit a few places around the trunk at the most likely
point of failure. Measure how far up the bit you are when resistance drops,
the shavings become discolored, or nothing more comes out.
- A precise record of resistance patterns can be had with new equipment
such as the Resistograph®, though they are quite expensive.
- Research shows that a cavity is unlikely to make a tree fail
at the spot measured if there is 1" or more of sound wood per 6" of diameter
-- i.e., average sound wood thickness ¸
trunk diameter = 0.15 or more. This guideline has good scientific support
for trees under 40" in diameter.
- Tall trees with a large canopy and on an exposed site require a
higher ratio of sound wood to diameter.
Where can I get more information?
- You can’t. You can try to slow its decline, but you can’t
stop the decay.
- The best--and cheapest--intervention is usually to help the tree
do its own work of containing the decay. Supply weekly water during dry
spells and provide 2-6" of organic mulch.
- Fertilization will usually not help much, although aerating the
soil to promote root growth often benefits older trees.
Nelda P. Matheny and James R. Clark, 1994. A Photographic Guide to
the Evaluation of Hazard Trees in Urban Areas, 2nd ed., Savoy, IL: ISA.
See also the Hazard Tree Web Page at http://www.na.fs.fed.us/spfo/hazard/index.htm.
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.