A burl (American English) or bur or burr (British English) 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. Insect infestation and certain types of mold infestation are the most common causes of this condition.