Agalinis tenuifolia is an annual herb, 20-60 cm tall; a root hemiparasite; stems thin, smooth, much branched.
Leaves opposite (alternate near branch tips), linear, 0.1-0.6 cm wide.
Flowers purple with dark markings, yellow throat stripes, slightly bilaterally symmetrical, tubular, 1-1.5 cm, tube longer than petal lobes, upper lip curved forward over stamens, much longer than lower lobes, smooth inside, calyx tube 0.2-0.4 cm, lobes about 0.1-0.2 cm, flower stalks 1-2 cm long, very fine; blooming and fruiting Aug.-Oct.
Fruit dry, hollow, a round capsule 0.3-0.5 cm, splitting open to release numerous tiny seeds.
Wetland status: FAC.
Frequency in NYC: Infrequent.
Habitat: Moist to wet open meadows, edges.