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CommentsAcross its range, Tall Ironweed is a rather variable species. Different populations of plants can vary in the number of disk florets per flowerhead, the width of the leaves, and the hairiness of the stems and leaf undersides. Tall Ironweed also hybridizes with other Vernonia spp. (Ironweeds), especially Vernonia missurica (Missouri Ironweed). The latter hybrid is referred to as Vernonia × illinoensis (Illinois Ironweed). Illinois Ironweed typically has flowerheads with 30-35 disk florets and is more hairy than Tall Ironweed. Missouri Ironweed has flowerheads with 35-50 disk florets and it is more hairy than Tall Ironweed; both stems and leaf undersides of Missouri Ironweed are often white-woolly from the abundance of these hairs. Another species, Vernonia fasciculata (Smooth Ironweed), has about the same number of disk florets in its flowerheads as Tall Ironweed (less than 30), but it is usually smaller in size and its stems are hairless. Unlike Tall Ironweed, Smooth Ironweed often has dark dots on the underside of its hairless leaves. A scientific synonym of Tall Ironweed is Vernonia altissima.