Species of Euthamia seem far from distinct, and more work is needed despite the small size of the genus.
1. Principal cauline leaves with 1 strong longitudinal vein (i.e., the midrib) and sometimes with a pair of weak veins, the widest blades less than 3 (–3.5) mm broad; leaves and upper part of stem glabrous; uppermost leaves and leafy bracts glistening as if varnished and with conspicuous, dense, dark glandular dots.
1. Principal cauline leaves with 3 strong longitudinal veins, the widest blades (3–) 3.5–8 (–13) mm broad and often with an additional pair of weaker veins; leaves and upper part of stem usually short-hairy to scabrous-hispidulous; uppermost leaves and leafy bracts dull in aspect, the glandular dots often obscure.
All species found in Euthamia
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 14, 2017. http://www.michiganflora.net/genus.aspx?id=Euthamia.