Lythrum salicaria or purple-loosestrife] is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the genus Lysimachia in the family Primulaceae. This herbaceous perennial plant is native to temperate regions of Europe, Asia, northern Africa, and eastern Australia.
Lythrum salicaria can grow 1–2 m (3 ft 3 in – 6 ft 7 in) tall, forming extensive clonal colonies, with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are lanceolate, 3–10 centimetres (1–4 in) long and 5–15 millimetres (3⁄16–9⁄16 in) broad, downy and sessile, and arranged opposite or in whorls of three.
The flowers are reddish purple, 10–20 millimetres (1⁄2–3⁄4 in) in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly in verticillasters in the axils of bracts or leaves. There are three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants. For instance, if the pistil is medium length, then the stamens will be long and short, but not medium.The flowers are visited by many types of insects, and can be characterised by a generalised pollination syndrome.
The fruit is a small 3–4 millimetres (1⁄8–5⁄32 in) capsule containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red autumn colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.
L. salicaria is very variable in leaf shape and degree of hairiness, and a number of subspecies and varieties have been described, but it is now generally regarded as monotypic with none of these variants being considered of botanical significance. The species Lythrum intermedium Ledeb. ex Colla has often been considered synonymous in the past, but is now treated as a separate species, from Central Asia, by the Plants of the World Online database.
