Burdock is any of a group of biennial thistles in the genus Arctium, 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 18" (45 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 October. The prickly heads of these plant… (More on Arctium)