A knot in a tree is the base of a branch embedded within the main trunk as the tree grows. These natural occurrences are cross-sections of branches, visible once the tree is cut into lumber. They appear as circular or oval shapes, often darker than the surrounding wood.
How Knots Form
Knots develop as a tree grows in diameter, with new layers of wood forming around existing branches. As a branch grows from the trunk, the main stem’s radial growth gradually incorporates its base into the expanding wood. The trunk’s wood grain grows around the branch, intertwining its fibers with the main stem.
Knots are categorized into two main types: live knots and dead knots. Live knots originate from branches alive and growing when the tree was harvested; their growth rings are intergrown with the surrounding wood, making them solid and firmly attached. Dead knots, conversely, form when a branch dies or breaks off, and the tree continues to grow, encasing the dead portion. These dead branches lose their fibrous connection to the trunk, making dead knots darker and more prone to loosening or falling out, sometimes leaving a hole.
Effects on Wood Strength and Tree Growth
Knots influence the structural integrity of wood by interrupting the continuous grain pattern. The wood fibers around a knot deviate from their straight path, creating an uneven grain direction. This interruption can reduce the wood’s strength, particularly its tensile strength (resistance to pulling forces) and compressive strength (resistance to loads that reduce size), especially when forces are applied perpendicular to the grain. The extent of this weakening depends on factors such as the knot’s size, type, location within the wood, and how many knots are present.
Despite their impact on lumber strength, knots have minimal effect on the health of a living tree. They are a natural part of tree growth and an adaptation to its environment. A tree might form a burl, a knot-like growth, as a response to stress or injury, as a protective measure. Knots can be denser and harder than the surrounding wood, making them challenging to cut, but their presence in a living tree is a common occurrence.
Knots in Lumber and Their Value
When a tree is milled into lumber, knots appear as distinct circular or oval features, often darker than clear wood. These visible characteristics play a significant role in how lumber is graded. Wood intended for structural applications, where strength is a primary concern, is graded with stricter limits on knot size and placement. Large or numerous knots can lower the grade and commercial value of lumber used for load-bearing purposes.
Conversely, for aesthetic applications like furniture or paneling, knots can be highly desirable. People appreciate the unique character and rustic charm knots add to wood products. Knotty pine paneling, for example, is sought after for its distinctive appearance. The presence of knots can also make some wood more affordable for projects where structural integrity is not the primary concern and visual appeal is embraced.