Sp. Pl. 1: 446. 1753.
Herbs perennial, or subshrubs, robust, 5–15(–30) dm, green, gray-puberulent, sometimes glabrate. Stems erect, much-branched distally. Leaves opposite proximally, alternate distally, sometimes whorled, gradually smaller and overlapping in inflorescences; sessile; blade lanceolate, 2–10(–14) × 0.5–2 mm, base cordate or rounded. Inflorescences spikelike with flowers mostly in densely clustered, whorled cymes. Flowers whorled, sessile or subsessile, tristylous; floral tube cylindrical, 4–6 × 2 mm; epicalyx segments equal to or to 2 times length of sepals; petals rose purple, oblong to obovate, 6–14 × 2.5–4 mm; nectary encircling base of ovary; stamens 12, of 2 lengths. Capsules septicidal or septifragal. Seeds ca. 90, obovoid. 2n = 30 (Asia), 60.
Phenology: Flowering summer–fall.
Habitat: Wet areas, marshes, ditches, swamps, shores.
Elevation: 0–2200 m.
Introduced; St. Pierre and Miquelon, Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Sask., Ala., Alaska, Ark., Calif., Colo., Conn., Del., D.C., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., Eurasia, introduced also in temperate zones nearly worldwide.
Lythrum salicaria is one of the ten most frequently listed noxious weeds in North America. It forms extensive, showy monocultures that, although beautiful to the eye, crowd out native vegetation in wetlands, severely degrade wildlife habitats, and eliminate wildlife food sources. It was first reported in North America in 1814, possibly arriving in ship ballast, and was well-established along the New England coast by the 1830s (R. L. Stuckey 1980; D. Q. Thompson et al. 1987). It slowly spread throughout the northern United States and Canada, reaching the Pacific Northwest by 1940. In recent years, the spread has been exponential, aided by its wide tolerance of soil and climate, expansion of the highway system, increased availability of disturbed and degraded habitats, and horticultural escapes. Reproductive success is due to immense seed production, estimated at over 2.5 million seeds per year for a single mature plant. Seeds have been shown to be 80% viable after two years and to form extensive seed banks. The woody roots are difficult to eliminate and easily resprout if not removed completely. Biological control using host-specific beetles introduced from Europe (Galerucella and Hylobius species that attack leaves and roots, respectively) is showing some success, but whether these insects will effectively control L. salicaria throughout North America and remain specific to it remain to be fully determined. At least 33 U.S. states and many Canadian provinces have declared purple loosestrife a noxious weed, prohibiting sale or distribution of the species and its cultivars. Putative fertile hybrids with the closely related, possibly conspecific, L. virgatum, also a European introduction, has caused L. virgatum also to be placed on the prohibited list in several states. Results of genetic comparisons among L. salicaria, L. virgatum, their cultivars, and native L. alatum using isozymes indicate that L. salicaria and L. virgatum cannot be distinguished, whereas L. alatum is distinct from the introduced species. The F1 generations of L. salicaria cultivars × native L. alatum have been found to be highly fertile (N. O. Anderson and P. D. Ascher 1993; M. S. Strefeler et al. 1996).