A burl 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. Insect infestation and certain types of molds are the most common causes of this condition, although other stresses can also promote burl formation, such as physical injury, or viral or fungal infection.
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. In some tree species, burls can grow to great size. The largest, at 26 feet, occur in redwoods and can encircle the entire trunk. Burls yield a very peculiar and highly figured wood, prized for its beauty, strength, and rarity. It is sought after by furniture makers, artists, and wood sculptors.
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The Daily Omnivore 