A burl is an irregular, rounded outgrowth that appears on the exterior of a tree, often resembling a large, woody tumor. This growth characteristic is not specific to one species but can occur in many types of trees, including maple, walnut, oak, and redwood. This abnormal formation creates a unique wood grain pattern, making it highly valued by woodworkers for decorative veneers and furniture. The science behind this formation is a defensive biological reaction involving cellular and hormonal changes within the tree’s structure.
The Anatomy of a Burl Growth
A burl is fundamentally a mass of disorganized tissue resulting from localized, multiplied cell division. Instead of aligning in the uniform, vertical pattern that forms straight-grained wood, the cells in a burl proliferate chaotically. This disorganized growth is a type of callus tissue, which is the tree’s protective response to damage. Burls can form anywhere on a tree, including the trunk, branches, or even underground attached to the root system, where they are sometimes called lignotubers.
The unique internal structure of a burl is defined by the proliferation of dormant, undeveloped adventitious buds. These buds form outside of the normal leaf axil or shoot tip and often remain suppressed for years. As the burl expands, these latent buds become engulfed, compressed, and folded into the growing mass of wood without fully sprouting. The resulting wood features highly contorted xylem and phloem, creating a dense, hard, and swirling pattern distinct from the surrounding wood.
Biological Causes and Triggers
The initiation of a burl is caused by various stressors that trigger an uncontrolled growth response in the tree’s cambium layer. This meristematic layer, responsible for all secondary growth, is sensitive to disruption, and sustained irritation can lead to the formation of abnormal tissue. The tree’s reaction is a localized effort to seal off the damaged area, but the process results in a persistent, excessive proliferation of cells.
One of the most well-documented causes involves bacterial infection, particularly the soil bacterium Agrobacterium tumefaciens, which causes crown gall. This bacterium infects the tree through a wound, such as a crack from frost or an insect bore hole. Once inside, the bacterium transfers a small segment of its own DNA, called T-DNA, from a tumor-inducing plasmid into the tree’s cells.
The foreign T-DNA contains oncogenes that alter the synthesis and sensitivity of the tree’s growth hormones, specifically auxins and cytokinins. This genetic manipulation causes a localized hormonal imbalance, compelling the cambium cells to divide rapidly and uncontrollably, essentially creating a plant tumor. This mechanism explains why burls continue to enlarge over the life of the tree, even after the initial stressor is gone.
Other triggers include injury from fire, pruning, or mechanical damage, as well as infestation by certain fungi or insects that feed on the cambium. In some species, such as the coastal redwood, a predisposition for burl formation is also linked to genetic factors. Regardless of the specific trigger, the result is the disruption of normal cell regulation in the cambium, leading to the erratic and dense growth of the wood mass.
Why Burl Wood is Unique
The chaotic cellular division and the compression of undeveloped buds translate directly into the material properties of burl wood. Unlike the uniform, straight grain of a normal trunk, burl wood features a visually striking, swirling, and interlocked grain pattern. This irregular pattern often contains small circular marks called “eyes,” which result from the trapped adventitious buds.
Because the wood fibers do not run in a single, parallel direction, burl wood is dense and resistant to splitting. This lack of traditional grain structure makes the material more dimensionally stable, though it can be challenging for woodworkers to carve and shape. The coloration is often richer and more varied than surrounding wood, sometimes incorporating mineral deposits or fungal spalting. Burls are primarily harvested for decorative purposes, such as veneers, tabletops, and small specialty items, where their unique appearance is the main appeal.