A lump on a tree trunk is not one diagnosis. A wood knot is associated with a branch base incorporated into the trunk, while a burl is an irregular swelling with unusual growth. Woundwood, galls and other changes can also create raised areas. Identify the feature before deciding whether the tree needs attention.
What a wood knot represents
In cut wood, a knot records where a branch passed through the trunk. The branch and surrounding trunk grew together over time; knots are not limited to branches that were already dead. The relationship between the branch and later growth affects how the knot appears in a board.
On a standing tree, a branch attachment or an old branch wound may be visible, but a large exterior swelling should not automatically be described as the same feature as a knot in lumber. A photograph of bark alone may not establish what is happening underneath.
How a burl differs
Burls are rounded or irregular swellings that can have striking grain patterns. The University of Minnesota’s tree-condition guide notes that their exact cause is uncertain. Do not present every burl as proof of a particular infection, injury or a tree’s last attempt to survive.
Some trees have particular regeneration adaptations. The UC Botanical Garden’s account of fire and plants discusses sprouting from redwood burls after serious damage. That example should not be generalized to every swelling on every tree species.
Look at the tree around the swelling
Record whether the feature is old and stable or changing, and whether there is a former branch at its center. Look for associated cracks, cavities, loose bark, dieback or other visible changes without cutting into the trunk.
New decline or a swelling near an important branch attachment deserves closer assessment. Conversely, an old rounded feature on an otherwise sound-looking tree does not by itself establish an emergency. Tree condition and risk depend on more than the presence of a bump.
Do not remove a burl to improve the tree
Cutting off a swelling can create a major wound. Do not drill, carve or apply chemicals merely to find out whether a lump is hollow. Keep mowers and other equipment away from the bark, and use appropriate pruning methods for branches that actually need management.
If a tree is near a house, path or other occupied area and has concerning structural symptoms, arrange an assessment by a qualified arborist. Keep away from hanging limbs or an obviously unstable tree while that assessment is arranged.
Knots are not termite evidence
A knot or burl alone does not show that termites are present, and it does not call for termite treatment. Holes, damaged wood and insect signs require their own identification. Do not infer an infestation from a natural growth feature or treat the tree and surrounding garden without evidence.
What knots mean for wood use
Knots and irregular grain can influence how a piece of wood behaves, while also creating decorative patterns. The effect depends on size, position and the intended use. Attractive knotty wood is not automatically suitable for a structural application; use appropriately graded material for that purpose.
For a living tree, the practical question is its health and structural condition, not the potential value of a burl. Leave the feature intact and seek an informed assessment when accompanying symptoms suggest a problem.
