Knots in wood are the remnant of a tree branch that has become fully embedded in the trunk as the tree increases in girth. These features are a natural part of a tree’s growth, representing a record of its history. When a tree is harvested and processed into lumber, these embedded branch sections are revealed as characteristic circular or oval patterns that contrast with the surrounding grain.
How a Branch Becomes a Knot
The formation of a knot begins at the base of a branch, which is structurally integrated into the trunk wood. Every year, a tree adds a new layer of wood, known as an annual growth ring, deposited by the vascular cambium layer just beneath the bark. As the trunk expands in diameter, these new layers grow around the existing branches, progressively incorporating the branch base into the main stem. This process is known as branch inclusion.
The wood grain within the knot runs perpendicular to the grain of the main trunk, making the knot structurally and visually distinct. This sharp change in wood fiber direction causes the trunk’s grain to “flow” around the knot, creating localized distortion. When a log is sawn lengthwise into a board, the cross-section of the embedded branch base is exposed, resulting in the circular pattern seen on the surface. The contrasting appearance is due to this difference in grain orientation and the typically darker, denser wood of the branch.
Distinguishing Live Knots from Dead Knots
Knots are classified based on the state of the branch when it was overgrown by the trunk wood. This distinction determines whether the resulting knot will be firmly attached or loose within the lumber. A live knot, often called an intergrown or tight knot, forms when the branch was still alive as the trunk expanded around it. In this scenario, the cambial layers of the branch and the trunk are continuous, meaning the new trunk wood is fully fused with the branch wood.
The wood fibers of a live knot are structurally integrated with the surrounding wood, making it a solid part of the board unlikely to separate or fall out. Conversely, a dead knot, also called an encapsulated or loose knot, occurs after a branch has died but remains attached to the trunk. When the trunk’s annual rings grow over the dead branch stub, the new wood does not fuse with the dead branch tissue.
This lack of fusion means the dead knot is held in place by the surrounding trunk wood, often with a layer of bark or resin separating it from the main wood body. Because it is not fully bonded, a dead knot can shrink and loosen during the drying process, potentially separating from the board and falling out. Dead knots are typically darker and may have a ring of darker material around their edge, signaling the separation plane.
Why Knots Affect Wood Stability and Strength
The presence of knots significantly influences the mechanical properties and stability of processed lumber. The most substantial effect stems from the disruption of straight grain continuity in the surrounding wood, a phenomenon known as grain run-out. Wood fibers are strongest when tension is applied parallel to the grain, but fibers near a knot are forced to curve sharply around the embedded branch section.
This severe deviation in fiber direction concentrates stress, leading to a reduction in the wood’s tensile and bending strength. The knot itself is composed of wood that dries and shrinks at a different rate and direction than the surrounding trunk wood. This differential shrinkage creates internal stresses as the lumber seasons, often resulting in surface cracks, known as checks, or overall warping. For structural applications, these interruptions classify knots as a defect that must be accounted for in lumber grading, as they lower the load the material can safely bear.