Lythrum alatum is a perennial herb 40-80 cm tall, erect, branches wand-like, stem 4-angled.
Leaves mostly alternate, opposite on lower stem, below branches, stalkless (sessile), blade to 4 cm long ca 1 cm wide, narrowly lance-shaped to elliptic, tip pointed, base rounded to slightly lobed, thick, surface dark green, crowded along branches, reduced to bracts in inflorescence.
Flowers pink-purple, radially symmetrical, 6-parted, petals to 0.5 cm long, 0.2 cm wide, midrib darker purple, arising from inside rim of calyx tube (hypanthium); calyx narrowly tubular, 0.4-0.7 cm long, tube 12-ribbed, lobes to 0.5 cm long, tips pointed, calyx lobes alternating with short, linear appendages 0.1 cm long (Yatskievych 2006); stamens 6, unequal, shorter than petals; ovary superior; flowers solitary or paired in axils of leafy bracts; blooms June-Aug. (Hough 1983).
Fruit dry, a 2-parted capsule enclosed in persistent calyx tube, splitting along suture to release numerous seeds; fruits Sept.-Oct (Hough 1983; Levine 1995).
Wetland status: FACW+.
Frequency in NYC: Very infrequent.
Habitat: Moist to wet soil of marshes, ditches.
Notes: There is concern that the insects released as biological control agent for L. salicaria, may possibly decrease populations of native Lythraceae species.