Reproducing only by seed.
Stems tall, erect, 60-120cm 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 long, finely hairy; upper leaves and those in the inflorescence usually alternate (1 per node) and smaller than the lower ones.
Flowers 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, 7-10mm 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.
Purple loosestrife on roadside growing in Central Ontario, August.
Dense terminal spikes
Flowers of purple loosetrife
Opposite leaf arrangement.
Leaf of purple loosestrife
Back of leaf.