Lobelia siphilitica is a perennial herb by basal offshoots, 0.5-1.5 m. tall (usually shorter), stout, erect, mostly unbranched, sap milky (Radford et al. 1968).
Leaves alternate, stalkless, blade 5-12 cm long, 1-6 cm wide, elliptic, pointed at both ends, hairy above, margin irregularly toothed, upper leaves merging into floral bracts.
Flowers blue, to 3 cm long, bilaterally symmetrical, tubular, corolla two lipped, lower lip 3-lobed, upper lip 2-lobed; inflorescence a raceme in upper axils; blooms and fruits Aug.-Sept.
Fruit a dry capsule opening near top.
Wetland status: FACW+.
Frequency in NYC: Very infrequent.
Habitat: Open marshes or swamp forests.