Q: I have loose knots in oak and pine. What causes them to be loose and even fall out sometimes?

A: I suppose you know that knots are the result of branches growing in the stem of the tree. When the branch dies but doesn't fall off the tree immediately, the growth of the stem will result in wood growing around this dead branch. The new growth in the stem that grows around the branch will not be attached intimately to the branch.

Lumber cut from this wood will have knots that are attached only by the geometry of the knot. In fact, if there is bark on the branch, the bark will be included between the knot and the stem wood, giving the knot a dark ring around the outside. The knot wood is also commonly dark colored due to staining in the branch; the result is a dark (or so-called "black") knot, rather than the "red" knot when the branch is alive. When the lumber dries, the dead-branch knot will shrink more than the surrounding wood and any sap around the outside will evaporate. The result is a loose knot. There is no way to avoid this problem, other than to eliminate black knots.