Leaves of erigeron subtrinervis
grow at the base and quite densely along the stem, in an alternate arrangement, but the former tend to be withered at flowering. Leaves have fine hairs along the margins and on both surfaces, in contrast to the essentially hairless leaves of the similar erigeron speciosus
. Another distinguishing factor is that leaves of erigeron subtrinervis have three veins (nerves); the central one is prominent, the two at either side rather less so.
Flower heads have between 100 and 150 ray florets, colored pale blue, pale purple or lavender. They form branched, corymbiform arrays; flowers lower down the main stem have longer stalks, so all attain a similar height. A plant may have a single flower, or as many as 21.