Burls are “wart-like” knobby growths on the trunk or limbs of a tree. Their timber is different to that of the trunk. It is characterised by swirling interlocking grain and multiple gum veins. There are several theories about the cause of burl growth. Some are thought to be genetic while others may grow in response to injury or insect attack, or as a result of viral infection. Removal of burls from a growing tree has no adverse effect on it. To the contrary, large or multiple burls can kill or weaken a tree by restricting sap flow to the rest of its trunk and limbs.