A burl (British bur or burr) is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from dormant buds.
A burl results from a tree undergoing some form of stress. It may be environmental or introduced by humans. Most burls grow beneath the ground, attached to the roots as a type of malignancy that is generally not discovered until the tree dies or falls over. Such burls sometimes appear as groups of bulbous protrusions connected by a system of rope-like roots. Almost all burl wood is covered by bark, even if it is underground. Insect infestation and certain types of mold infestation are the most common causes of this condition.
Burls yield a very peculiar and highly figured wood, one prized for its beauty by many; its rarity also adds to its expense. It is sought after by people such as furniture makers, artists, and wood sculptors.