This common perennial aster is noted for its purplish stem covered with bristly white hairs. This aster is variable though, and can have slightly pubescent or glabrous stem. New growth branches may be misleading since they are sometimes green. The leaves are rough and usually lightly toothed.  At their base they clasp the stem but do not surround it.  The leaves are up to ten inches long and lance-shaped.  They may vary somewhat from plant to plant.  A line of white hairs runs down the middle vein on the underside of the leaves. The stem may zigzag a bit and may be mistaken for the crooked-stemmed aster (S. prenanthoides).  It may also be confused wit the late purple aster but the latter has shorter, more rounded and consistently toothless leaves. 

The composite flowers of this species have numerous (30-60) light violet or violet ray florets (petals) and numerous creamy white or yellow disk florets.  The latter turn purple as they age.  The flower heads are 1 to 1 ½ inches in diameter. The blooming period of this species is from August to late October.  Because of the color of the flowers it is sometimes also mistaken for the New England aster (S. novae-angliae), but the latter usually has more ray florets and a deeper purple/blue coloration.  It also lacks the purple stem.

The purplestem aster tends to be a wetland plant, growing in swamps and wet thickets and along shorelines.  It prefers an area where it can get a fair amount of sunlight.  It is found in Canada and eastern United States except Florida, but is found mostly in the mountains in the South. It is found throughout Pennsylvania.  It is also called the red-stemmed aster, the swamp aster or the glossy-leaved aster.