The idea that a tree burl is a form of plant malignancy, a kind of “tree cancer,” is a common misconception. These abnormal, rounded growths appear on the smooth surfaces of trunks and branches, suggesting an internal biological failure. A burl is essentially a localized mass of disorganized woody tissue that a tree produces in response to a stimulus, creating a distinct bulge covered in bark.
What Burls Are and How They Form
A burl is a dense, knobby outgrowth representing a surge of highly localized, chaotic cell division within the tree. These formations result from the abnormal proliferation of the vascular cambium, the layer responsible for producing new wood and bark. Normally, the cambium creates organized tissue with straight grain; in a burl, it produces a mass of contorted, interlocked fibers. This growth is a type of hyperplasia, an overgrowth of cells, often incorporating numerous dormant buds. If cut open, this chaotic internal structure is visible as a unique, swirling wood grain prized by woodworkers. Burls commonly appear on the main trunk, near the base, or at branch junctions.
Triggers for Burl Growth
Burls are initiated by an external stressor that disrupts the tree’s hormone balance. Physical injury is a common trigger, such as a deep pruning cut, equipment damage, or abrasion from falling branches. The tree attempts to seal off the wound by producing a dense boundary of callus tissue, which can develop into a burl. Infections from various pathogens also prompt this defensive growth response. Fungi, bacteria, or viruses enter through minor wounds and stimulate the cambium to divide uncontrollably to isolate the affected area. Insect infestations, particularly those that bore into the wood, can also trigger burl formation as the tree walls off the invading larvae.
Why Burls Are Not Malignant Cancer
A burl is not considered malignant cancer due to fundamental differences in plant and animal cell biology. Animal cancer is defined by metastasis, where genetically mutated cells spread through the bloodstream to colonize distant organs. Plant cells, however, are encased in rigid cellulose cell walls, which prevents them from migrating throughout the organism. Burls are localized growths that remain confined to the area of origin, even when caused by an infectious agent. While they are a form of tumor, they lack the invasive, systemic nature of animal malignancy. Most burls are triggered by an external environmental factor or pathogen, not an internal, unchecked genetic failure, which is the hallmark of human cancer.
Tree Health and the Survival of a Burl
For most trees, a burl is a benign feature and a testament to the plant’s defense system. The tree uses the dense, disorganized wood of the burl to effectively compartmentalize the original injury or infection. These growths can remain on a tree for decades without affecting its overall health or longevity. While generally harmless, a very large burl on a branch can weaken the structural integrity of that limb, making it susceptible to breakage in high winds. Removing a burl from a trunk is ill-advised, as it creates a massive, open wound that the tree cannot easily seal, inviting decay and pathogens into the core wood.