As trees grow and mature, they often develop knots and other imperfections. The presence of knots is most noticeable when the tree is cut down for timber. Knots sometimes weaken timber, but can also be manipulated for their artistic quality in finished products. Usually knots are a normal part of tree growth; however, they can be the result of disease or improper pruning.
Knots occur in wood when a branch gets embedded in the tree trunk as it increases in diameter. When the branch is living, it creates what is known as a tight knot or intergrown knot. The embedded branch continues to grow with the rest of the tree. Tight knots are firmly attached to the tree. They do not fall out easily when the tree is cut down and used for lumber.
A loose knot, also known as a black knot, occurs when a dead branch becomes embedded in the growing tree trunk. Loose knots are not physically attached to the rest of the tree. They easily fall out, leaving a hole when the tree is cut into lumber. Loose knots reduce wood quality and strength much more than tight knots.
Whereas tight and loose knots are a natural part of tree growth, epicormic sprouts occur after excessive pruning. After the forest has been thinned, trees are exposed to more light and higher temperatures. This results in numerous epicormic branches sprouting on the tree. If these are not properly removed, they will cause knots to develop.
Black Knot Disease
Black knot is a disease that commonly affects cherry and plum trees. Although the term "black knot" describes both a tree disease and a loose knot, the two conditions are not related. Black knot tree disease causes knots to develop on the trunk, branches and twigs of trees. The knots are usually elongated, black and rough. During the spring, black knot can be transmitted to other trees during windy periods. Pruning and fungicides are used to control black knot outbreaks.
- "Trees: Their Natural History"; Peter Thomas; 2000
- Purdue University Cooperative Extension Service; Fertilizing, Pruning, and Thinning Hardwood Plantations; James McKenna, et al.; May 2006
- North Dakota State University; Insect and Disease Management Guide for Woody Plants in North Dakota; September 2005