I’m Glad You Asked: Knots and Knotholes

Dr. Lew Feldman, Garden Director

What are knots and knotholes and how do they form?

Knots are visible imperfections in wood. They are typically circular and darker than the surrounding wood area and when the knot separates from the surrounding wood a knothole form. Let’s consider how knots and knotholes originate.

The explanation begins by observing the trunks of trees, as can be viewed in the UC Botanical Garden’s Redwood Grove. You’ll notice that the lower portions of the tree trunk often show dead, branches, which as a consequence are no longer elongating (blue arrow).

As time passes and the tree increases in width/diameter the dead branch is engulfed in and covered by the expanding, newly produced wood (illustrated in the diagram below by the movement apart of the red lines).

Sawing a tree trunk parallel to its long axis (indicated above by the dashed line in the far right panel, and below) reveals the knot.

 

A log sawed parallel to its long axis (left) reveals knots (right).So we now know how knots arise; they are essentially a dead branch that has become embedded in newly produced wood. But how does the knothole form? Because the branch is dead prior to the time it is engulfed by the expanding, living tree trunk tissue, it is often only weakly connected to the surrounding tissues. As a result, when a board containing a knot is cut in the lumber mill, the weakly attached knot can fall out, creating a hole (a knothole). In certain trees, however, such as pine, the knot is usually more securely connected to the surrounding tissue and hence does not easily fall out, resulting in so-called, knotty-pine wood paneling.

So next time you are in a knotty pine-paneled room, have a close look at the individual knots and ask yourself, what feature reveals that they are embedded branches.