Eutrochium

1. Florets mostly (8–) 9–20 (–22) per head; inflorescence flat-topped or of flat-topped subunits; stem distinctly spotted with red-purple or continuously red-purple.

E. maculatum

1. Florets 3–7 per head; inflorescence ± dome-shaped (convex); stem greenish (except at nodes) in common species (purplish in the other).

2. Corollas 5 mm long or less; middle (if not all) internodes ca. (7–) 10–20 mm or more thick, hollow over half their diameter, ± reddish throughout or spotted with elongate red dashes; leaves mostly 6 or more per whorl.

E. fistulosum

2. Corollas 5–6 (–6.7) mm long at maturity; middle internodes less than 5 (–8) mm thick, with solid pith (or a very slender hollow), greenish; leaves 3–5 per whorl.

E. purpureum

All species found in Eutrochium

Eutrochium fistulosumHOLLOW-STEMMED JOE-PYE-WEED 
Eutrochium maculatumJOE-PYE-WEED 
Eutrochium purpureumGREEN-STEMMED JOE-PYE-WEED 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 11, 2017. http://www.michiganflora.net/genus.aspx?id=Eutrochium.