5. Lythrum hyssopifolia Linnaeus, Sp. Pl. 1: 447. 1753.
Lythrum adsurgens Greene
Herbs annual or short-lived perennial, slender, 1.5–6 dm, gray-green glaucous, glabrous. Stems often from creeping rhizome, erect or weakly erect, sparsely branched distally. Leaves mostly alternate, sometimes opposite proximally, overlapping and scarcely smaller distally, equal to or longer than internodes; sessile; blade oblong to linear, 5–30 × 1–10 mm, base rounded. Inflorescences racemes. Flowers alternate, subsessile, monostylous; floral tube without red spots, obconic, becoming cylindrical, 4–6 × 0.5–1 mm; epicalyx segments 2 times length of sepals; petals pink or rose, oblong to obovate, 1.5–3(–5) × 0.7–1.3 mm, 1/2 floral tube length; nectary absent; stamens (2–)4–6(–12). Capsules septicidal or septifragal. Seeds ca. 20, obovoid, 0.8 × 0.5 mm. 2n = 20.
Flowering summer–fall. Disturbed moist or seasonally flooded ground, drying pond margins, vernal pools, marshes; 50–1600 m; introduced; B.C., Ont.; Calif., Maine, Mass., Mich., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Wash.; Europe; Asia; introduced also in South America (Argentina, Chile), Africa, Pacific Islands (New Zealand), Australia.
Lythrum hyssopifolia has been present in the eastern United States since at least the early 1800s and now has a scattered and disjunct distribution in the eastern and western states and Ontario. Successful establishment is attributed in part to its self-compatible, monostylous breeding system. In Australia, L. hyssopifolia has been responsible for the poisoning death of young sheep that grazed on canola stubble contaminated with it (B. Crawford 2002).