Not exactly a rock-garden-sized Aster (yes, I still call them Aster) is Aster pilosus (Symphyotrichum pilosum), native to eastern North American and disjunctly in British Columbia. Typically growing to 3' or more, the species is highly variable and I have plants that reach 6' tall, and as well, have observed plants growing only 18" tall and could be admissible to a larger rock garden.
It is known by several common names; Frost Aster, White Heath Aster, Hairy Aster, but I like to call it Deven's Aster ;D based on a remarkably variable population found growing at Devens, Massachusetts, the destination of the NARGS Eastern Study Weekend 2010. This area is a partially decommissioned military base formerly known as Fort Devens in Ayer, Massachusetts, but became its own town, Devens, Massachusetts, a growing business community.
This Aster species has many exceptional attributes including: extended season of bloom (Sept - Dec), graceful yet showy blooms, ease of cultivation, deliciously scented blooms (rare for Aster) like vanilla butter cookies, the floral display immune to adverse weather such as rain, snow, or freezing... the flowers just keep on going. One of the very last plants in bloom in the late autumn garden.
This photo essay explores the great variability of Aster pilosus found growing at Devens; the range of plant form, height, flower size, floriferousness, and even flower color (some light pink forms here), is remarkable. The first two photos show a lower growing type with high flower count that I hope to test under garden conditions; cuttings have been rooted. All photos taken on November 03, 2010, very late indeed, considering some are just starting to open flowers!