Flower:

Branching cluster of 20 to 300+, stalked, daisy-type flowers at the top of the plant and arising from upper leaf axils. Flowers are about ½ inch across with 7 to 15 petals (ray flowers) surrounding a creamy yellow center disk that turns reddish with age. Ray color ranges from nearly white to pale blue to lavender.

Surrounding the base of the flower are 4 to 6 layers of narrow, sharply pointed bracts (phyllaries), the outermost typically green, the inner greenish white at the base with long, diamond shaped green tips that are often red at the apex. Phyllaries are usually hairless, sometimes with a few minute hairs. Flower stalks are up to ¾ inch long with a few, small, leaf-like bracts and may have sparse hairs around the edge.

Leaves and stems:

Leaves are thin, coarsely toothed around the edges, and mostly strongly heart-shaped. Both surfaces are hairless to variously hairy, sometimes just hairy on the veins. Basal and lower stem leaves are up to 6 inches long and 3 inches wide, typically wither away by flowering time, and have long stalks with narrow wings. Farther up the stem, stalks become shorter and may be somewhat clasping, wings become slightly broader, and the blades less strongly heart-shaped to rounded at the base

Uppermost leaves and into the flower clusters are reduced to stalkless, lance-elliptic bracts that are toothless or nearly so. Stems are single or a few from the base, mostly erect to ascending but may flop over, usually hairless except for short hairs in the flower cluster, and becoming somewhat woody at the base with age.

Fruit:

Flowers become seed heads with tufts of light brown to pinkish hair to carry them off in the wind.

Seed is light brown to purplish, up to 2.5 mm long, angled with 4 or 5 veins.

Notes:

Asters can be difficult to ID. Blue Wood Aster is distinguished by a combination of traits: stem usually hairless below the flower cluster; thin, mostly strongly heart-shaped leaves with coarsely toothed edges and narrowly winged leaf stalks, upper leaves more rounded at the base and lance-elliptic with broader wings, the uppermost leaves into the flower clusters stalkless; ½-inch pale blue to lavender (or whitish) flowers with 7 to 15 rays, and a panicle with 20 to 300+ flowers.

Of the other blue/violet asters with heart-shaped leaves, Drummond's Aster (Symphyotrichum drummondii) is consistently more densely hairy, Lindley's Aster (S. ciliolatum) leaves are less strongly heart-shaped with broader wings on the stalks,  Short's Aster (S. shortii) has nearly toothless leaves that are proportionately narrower, Big-leaf Aster (Eurybia macrophylla) lower leaves are persistent through flowering, proportionately broader with clasping stalks. Arrowleaf Aster (S. urophyllum) has heart-shaped leaves with narrow wings on the lower stem, but has smaller white flowers arranged in tighter clusters.