Purple loosestrife, Lythrum salicaria L., [LYTSA, salicaire, Spiked loosestrife, salicaire commune, lythrum salicaire] Perennial, reproducing only by seed. Stems tall, erect, 60 -120 cm (24 - 48 in.) high, somewhat branched, usually finely hairy, more or less square in cross-section, especially where the leaves are opposite; leaves opposite (2 per node) or sometimes whorled (3 or more per node), stalkless, broad near the base and tapering towards the tip. 3-10cm (1 ¼ - 4 in.) long, finely hairy; upper leaves and those in the inflorescence usually alternate (1 per node) and smaller than the lower ones; lowers in dense terminal spikes; sepals united into a column with 8 to 10 or 12 prominent green veins and ending in several, long, thin, pointed lobes; petals 5 to 7, red-purple, 7 - 10 mm (¼ - 2/5 in.) long, very showy; stamens several and 1 pistil; seedpod small, containing many tiny seeds. Flowers from June to autumn.

Purple loosestrife was introduced from Europe but is now widely naturalized in wet meadows, river flood-plains, and damp roadsides throughout most of Ontario.

Its opposite leaves and square stems resemble plants of the Mint Family but it is distinguished by having separate petals, a seedpod with many fine seeds, and it lacks the minty odour.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)