Tension wood

Q: Several times you have mentioned tension wood in this column. What exactly is tension wood? How is it identified?

A: Tension wood, which occurs only in hardwoods, is a special growth feature in the tree that involves the tree reacting to an outside force (such as the wind, another tree leaning against the tree or even sunlight competition). The tree reacts to these outside forces by creating a special type of wood the cell it creates has high cellulose content (absorptive, as cotton is 100 percent cellulose) and low lignin content (lignin is the stiffener for wood). As a result, the wood is harder to machine (weak) and also absorbs stain easily. However, tension wood is not distributed throughout the stem uniformly, but is found in streaks and spots.

In short, tension wood is not a drying defect. It is not considered a defect when grading lumber according to the NHLA rules.

Many hardwood species have tension wood. The more common species with tension wood include cottonwood, aspen and elm. Tension wood requires special care when processing (machining, sanding and finishing) compared to "normal" wood.

Tension wood is identified (without using a microscope) by the fuzzing that results when sawing, planing, sanding or conducting other machining processes. Tension wood will have much more fuzz, especially fine "peach fuzz," when compared to typical wood. This fuzz can be seen after sanding, especially when the sandpaper is worn. There are some excellent microscopic identification techniques as well.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.