I never realized there was more than one species of blue cohosh but it seems there are two. The maroon flowers were an indication of Northern Blue Cohosh (Caulophyllum giganteum) whereas the species I was more familiar with, Caulophyllum thalictroides, has more numerous, light green flowers.
Both plants overlay much of the same native range though C. thalictroides ranges a little farther south and west than its sister species. (See http://plants.usda.gov for range details.) As for how to tell which species it is other than by its flower? Well, C. giganteum blooms 10-15 days earlier and has larger and less pinnately divided leaves than C. thalictroides. Exact differences according to one source were:
The two species differ somewhat vegetatively. The ultimate leaf segments of C. giganteum are 5–10 cm long and first leaf is 2- or, more commonly, 3-times pinnately divided. The ultimate leaf segments of C. thalictroides are 3–8 cm long and the first leaf is 3- or, more commonly, 4-times pinnately divided.It was a good day for something new indeed!
Northern Blue Cohosh (Caulophyllum giganteum) in flower:
Blue Cohosh (Caulophyllum thalictroides) in flower:
Blue Cohosh foliage:
Blue Cohosh fruits (green) early/midsummer:
Blue Cohosh fruits (ripe) late summer: