Perennial herb 0.5 - 1.5 m tall Stem: stout, erect, usually with some hairs, especially near the branching inflorescence. Leaves: opposite or in whorls of three, stalkless, non-toothed, more than 5 cm long, somewhat lance-shaped, widest below the middle, gradually tapering to a pointed tip, and sometimes with a clasping base. Inflorescence: a terminal, 10 - 40 cm long, elongate "spike" or wand-like cluster with numerous, crowded flowers in the axils of two, pointed, hairy bracts. There are usually at least two, and normally more, flowers per bract axil. Flowers: bright purple-red, stalkless, radially symmetric, with sepals and petals fused together most of the sepal length into a slender, eight- to twelve-nerved, hairy sepal-petal tube (hypanthium). Sepals: five to seven, but fused for most of their length with each other and the petals into a long, slender tube (hypanthium). The separate sepal lobes are short-triangular, and they alternate with much longer (around two times longer), very narrow appendages. Petals: five to seven, but fused for less than half their length, then separating into ascending or slightly spreading, 0.7 - 1.2 cm long, somewhat oblong or elliptic lobes. Stamens: usually twelve (always more than ten ), in two distinct whorls or series, with each whorl a different length from the other (short or tall) and relative to the style length (short, medium, or tall). Pistil: with a single, superior, two-chambered ovary; a single, slender style; and a rounded stigma. The style of each flower can be one of three possible lengths (short, medium, or tall) in relation to the two different lengths of the whorls of stamens. This condition (tristyly) produces three different floral morphs. Fruit: a many-seeded, two-chambered capsule enclosed by the sepal-petal tube (hypanthium), and breaking apart through the side walls of the chambers, not directly into the chambers (septicidal).
Similar species: Lythrum salicaria is very similar to another Eurasian species, L. virgatum, which is often cultivated (and escapes), but that species is not hairy, where as L. salicaria is normally hairy, especially in the flowering "spikes". The other species in our area, the native L. alatum, is usually hairless, and less than 0.8 m tall with a four-angled stem, alternate leaves less than 5 cm long, and always less than ten stamens per flower.
Flowering: July to October
Habitat and ecology: Introduced from Eurasia, found in wet places of all kinds, and increasingly abundant since it is an invasive plant.
Occurence in the Chicago region: non-native
Notes: Listed as one of the top 100 of the "World's Worst" invaders (see link below), this plant is native to Eurasia and has escaped cultivation to become an aggressive weed in our natural wetlands and even roadside ditches. Large patches of this species are seen throughout the Chicago Region. Though seemingly "pretty" due to the bright purple-red flowers, this plant spreads quickly and chokes out high-quality native wetland plant species, which consequently makes wetlands less useful for wildlife. These plants persist year after year as well as produce copious amounts of seed. Due to the threat this plant poses to our nation's wetlands, the USDA has spearheaded efforts to combat this species by releasing natural biological enemies, beetles from Eurasia, which feed on purple loosestrife.
Stout, erect perennial herb 5-15 dm, glabrous or more often pubescent or, especially upwards, even subtomentose; lvs opposite or ternate, sessile, lanceolate to nearly linear, 3-10 cm, the larger ones cordulate at base; spikes 1-4 dm, interspersed with foliaceous, lanceolate to ovate bracts; fls trimorphic as to the style and stamens; appendages of the hypanthium linear, fully twice as long as the sep; pet red-purple, 7-12 mm; stamens mostly 12, alternately longer and shorter; disk wanting; 2n=30, 50, 60. Native of Eurasia; in wet places from Nf. and Que. to N.D., s. to Va., Mo., and Kans., and occasionally w. to the Pacific. July-Sept.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.