Hollow or hazard?

Hollow or Hazard?

 Recently Hoppe Tree Service had a booth display at the 87th annual home and garden show, at the state fair park. At the booth one of our most attractive features was a large hollow oak stump. We had more people come up to the stump and ask, “Why do you have this giant hollow stump here?” This gave me a fantastic opportunity to explain to people how structural integrity is associated with a hollowed out tree. People were most surprised when I told them that just because a tree is hollow does not mean it is hazardous.

 The amazing fact is, that if a tree is perfectly circular, you can remove 70% of the diameter, which is 50% of the cross-sectional volume, and only reduce the trees strength by 25%.  This spectacular phenomenon is possible because the majority of the strength of a tree, or any cylinder, comes from the outer most rim that actively partakes in compressing and stretching when a force, such as wind, is applied. As it turns out we have known this for years. It is the reason we can make cars frames light and still strong or send people to the moon by using tubular metals instead of solid metal frames.

 What does this mean for your trees? Just because your tree has a hole in it or is hollow, doesn’t mean you necessarily need to have it removed. This is great news for people who are unsure about the stability of the tree. They love and want to keep the tree but are afraid of it falling; now they may not have to remove it. Or, as the example at the fair demonstrated, if you have a large tree you may not be able to see if it is hollow but would like to know if it poses a hazard to your property or more importantly your family. It has been my experience that many large trees show very few or no signs of being hollow, until they fail without warning.

 Unfortunately trees are not perfect circles like the first example above. This means that there are additional factors of failure that require close and careful scrutiny. Trees tend to be oblong and not perfect circles. The decay or hollow is not always in the center of the tree it often is off-center. The condition of the tree and direction and magnitude of external forces need to be considered. Different species of trees are more susceptible to decays then others. These are only some of the factors that need to be looked at more closely before determining if a tree needs be removed.

 I am often asked “how hollow is too hollow?” or “it doesn’t look hollow, how can we tell, short of cutting it down?” There is no simple answer. The best answer is having the tree tested. Here at Hoppe Tree Service we have the latest, scientifically advanced, tools and staff to make a determination about the tree. We use a tool called the Resistograph. It is a micro (1mm) drill, that is minimally invasive, that graphs the density of the tree. This allows us to draw an accurate picture of the inside of the tree with out causing damage or cutting it down. With this information we apply strength loss formulas, from the cutting edge latest scientific research, and incorporate other important factors, before providing our analysis. At Hoppe Tree Service we have specialized trained and certified staff that can assist you with your questionable trees.

 The take home message is that not all trees that are hollow are hazardous. Sometimes your eyes can’t tell if a tree is hollow. If you suspect your tree to be hollow or have a large decay susceptible tree* on your property, please have a trained technician come out and give you a piece of mind.


* Examples of decay susceptible species

silver maple


cotton wood


white birch

large white/red oak


John Wayne Farber

Certified Arborist WI-877A

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