The line between our two species is not clear. Intermediate plants [called V. ×illinoensis Gleason by many authors] are relatively frequent (known from Allegan, Ingham, Kalamazoo, Kent, Lenawee, Livingston, Monroe, and Washtenaw Cos.) These mostly have either the small, few-flowered heads of V. gigantea with the leaf pubescence of V. missurica, or the larger heads of V. missurica but the very slightly pubescent or glabrate leaves of V. gigantea. Reports of V. baldwinii from Michigan are based on specimens of V. missurica.
The Prairie Ironweed, Vernonia fasciculata Michx., has been collected at a wetland mitigation site in Wayne Co., in 2004, but was likely planted there. It differs from our two native species in having the underside of the leaf essentially glabrous and punctate with small, shiny glands.
1. Flowers (and achenes) fewer than 30 per head; involucres ca. 5–7 mm broad; leaves nearly glabrous beneath, with minute, sparse, straight (often slenderly conical) hairs less than 0.3 mm long (sometimes slightly longer, denser, or crinkly hairs on midrib).
1. Flowers (and achenes) 33–42 per head; involucres ca. (6–) 6.5–9 (–10) mm broad; leaves ± pubescent beneath, especially on main veins, with crinkly hairs, some at least 0.3 mm long.
All species found in Vernonia
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. August 18, 2017. http://michiganflora.net/genus.aspx?id=Vernonia.