Smooth Aster has flower heads intermediate in size between the large New England Asters and the many species of small asters. The heads have yellow centers, which turn reddish with age, and lavender blue rays. The stems and leaves are smooth, and the stem may have a purplish tint.
Asters are composite flowers, meaning that the flower heads are actually clusters of tiny flowers rather than a single flower. The center is made up of tiny yellow flowers called disk florets. Arranged around the center are ray florets, which have several petals (in the case of asters, three) fused into a single long ray.