UPDATED: An earler version of this post included a picture that was probably not Aster puniceus. Identifying asters is a fool’s game. Torrey & Gray’s Flora of North America lists 131 species of Aster, and the chances are good that the particular aster you’ve found by the roadside won’t exactly fit the description of any one of them. But this picture shows a plant that actually looks pretty much like what Aster puniceus is supposed to look like.
Now Symphyotrichum puniceum, but the generic name Symphyotrichum is still so little known that most people looking for this plant will still know it as Aster. These common blue asters like slopes above streams and squishy wet ground. They are quite variable: Britton & Brown say that “races differ in pubescence, leaf-form, and leaf-serration,” meaning that anything you say about the shape of the leaves or how rough or hairy they are has to be followed by the words “or not.” The leaves of these plants were rough and sandpapery, and the stem quite hairy. These grew on the bank of a brook near Wexford.
From Gray’s Manual of Botany: A puniceus, L. Stem tall and stout 3-7° high, rough-hairy all over or in lines, usually purple below, panicled above; leaves oblong-lanceolate, not narrowed or but slightly so to the auricled base, rough above, nearly smooth beneath, pointed; heads 4-6″ high, subsessile; scales narrowly linear, acute, loose, equal, in about 2 rows; rays long and showy (lilac-blue, paler in shade). —Low thickets and swamps, very common.