Lythrum salicaria is a perennial herb, 0.5-2.7 m tall, densely colonial with as many as 30-50 stems arising from a mature woody root crown. No mycorrhizae have been found associated with the roots (Mal et al. 1992); stems erect, branches opposite, each pair at right angle to next up stem, candelabra-like, stems 4-sided, usually somewhat hairy.
Leaves opposite or in threes, stalkless, blade 3-10 cm long, to 2 cm wide, narrowly lance-shaped (Yatskievych 2006), tip pointed, base often slightly lobed, surface rough, margin entire.
Flowers rose-purple, 1-1.5 cm long, petals 5-7, 0.5-1.2 cm long, 0.3 cm wide; floral tube to 0.7 cm long, 0.3 cm wide, 12 ribbed, narrowly cylindrical, densely hairy, calyx lobes 6, pink, 0.1 cm long, alternating with greenish appendages 0.2-0.3 cm long from rim of floral tube; stamens 10-12, one set of 5-6 shorter than the other set, longer stamens bright pink with purple anthers, shortest ones greenish, ovary superior; stamens and styles vary in length (Mal et al. 1992); flowers 3-9 per node, interspersed with small, leaf-like bracts on an inflorescence of dense spikes 10-40 cm long (Yatskievych 2006); blooms July-Sept.
Fruit dry, a 2-parted capsule 0.3-0.4 cm long, 0.2 cm wide, enclosed in persistent, floral tube, eventually splitting open to release up to 130 tiny, orange seeds; stems persistent through winter.Seeds germinate in late spring and plants can flower in 8-10 weeks and set fruit the same autumn (Mal et al. 1992).
Wetland status: FACW+.
Frequency in NYC: Frequent.
Habitat: Open wetlands, pond edges, tolerant of up to 50% shade. Nutrient deficiencies, particularly lack of nitrogen decreased flowering (Mal et al. 1992).
Notes: Formerly used as medicinal plant, and is attractive to honey bees. It has been widely used as an ornamental and was noted as growing wild as early as 1814. Likely original introduction in sandy ship ballast (Mal et al. 1992). Colonization apparently limited in acid soil (Young 1996) but Mal et al. (1992) report that invasion of L. salicaria is more likely in infertile wetlands. Listed by New York State DEC Aquatic Invasive Species Eradication Grant Program as an invasive alien plant (Sanford 2007). Listed as a “noxious weed” in 34 other states, not including New Jersey (USDA, NRCS 2006; Douce et al. 2007; Wooten et al. 1996). Purple loosestrife displaces native species, decreases plant diversity, and degrades habitat for water fowl, black terns and marsh wrens (Mal and Lovett-Doust 1997; Mal et al. 1992). Since about 1992, several insect predators, have been introduced on an experimental basis to control L. salicaria. These are a root-mining weevil, Hylobius transversovittatus (Malecki and Blossey 2007), a flower-feeding weevil, Nanophyes marmoratus, two leaf-eating beetles, Galerucella pusilla and G. calmariensis and the gall gnat Bayeria salicaria. These show promise as controls on the spread of this exotic plant (Cornell U. 1997; Mal et al. 1992; NYSDEC. 2007; Sanford 2007).