Arctium is a genus of biennial plants commonly known as burdock, family Asteraceae. Native to the Old World, several species have been widely introduced worldwide. Plants of the genus Arctium have dark green leaves that can grow up to 28" (71 cm) long. They are generally large, coarse and ovate, with the lower ones being heart-shaped. They are woolly underneath. The leafstalks are generally hollow. Arctium species generally flower from July through to October. The prickly heads of these plants (burrs) are noted for easily catching on to fur and clothing (being the inspiration for Velcro), thus providing an excellent mechanism for seed dispersal. Burrs cause local irritation and can possibly cause intestinal hairballs in pets. However, most animals avoid ingesting these plants.