Moderately tall plant, 2 to 5 ft. Leaves opposite, toothed. Lower ones unite across the stem so that stem appears to pierce through a single long leaf ("perfoliate leaves"). Flowerheads with tiny white flowers, forming small clusters united into large, flattish clusters. Flower buds pure white. Blooms in autumn.
A common native of wet to damp places. In Wildwood occasional around the entrance wetland.
Members of the genus Eupatorium and Ageratina with white flowers are called bonesets, snakeroots or thoroughworts. A number of them occur in the Park, but no other has the distinctive pierced leaves of this species, and all prefer more shady places.