Lythrum salicaria aka Purple Loosestrife

Lythrum salicaria, or purple loosestrife, belongs to the loosestrife family, Lythraceae. This herbaceous perennial grows in temperate regions. Its native range spans temperate Eurasia, northwest Africa, Ethiopia and Australia. It grows beside lakes and slow rivers, in marshes and fens. It has spread beyond its native range into the United States. There, it invades wetlands and crowds out native plants.

The plant forms upright clumps and grows fast. RHS gives a garden height of 3.3–4.9 feet [1–1.5 metres]. Its listed spread reaches 1.6 feet [0.5 metres]. NC State gives larger dimensions, reaching 6 feet [1.8 metres] tall. Its spread can reach 4 feet [1.2 metres]. Thick roots produce runners that build a wide underground network. New shoots grow from this network. Straight green stems have four to six sides. The narrow, lance-shaped leaves lack stalks and feel soft. They grow in opposite pairs or whorls. The plant loses its leaves each year.

Small pink-purple flowers crowd the tops of stems in spikes. Each flower has five or six petals and a yellow centre. The spikes can reach 1 foot [0.3 metres] long. Missouri Botanical Garden lists flowering from May to September. NC State gives a shorter season, from July to September. Tan seed capsules can appear as early as July. They hold many tiny seeds, about the size of sand. Wind, water, animals and people spread these seeds.

Full sun and soil that stays moist suit this plant. It tolerates wet ground and poorly drained soil. RHS lists clay and loam, with acid, neutral or alkaline pH. It rates the plant H7 for cold tolerance. Missouri Botanical Garden lists USDA zones 4–9; NC State includes zone 3. Sources describe use in bog gardens, pond margins and flower beds. Yet its spread makes planting a serious concern. North Carolina and many other states ban its sale. Missouri Botanical Garden advises against planting the species.

Slugs, snails and Japanese beetles can damage the plant. RHS reports few disease problems. Removing spent flowers helps limit seed production. PFAF lists cooked leaves and roots as edible. The flowers yield a dye for food. It reports no known hazards, rather than proven safety. PFAF also records medicinal use for diarrhoea and dysentery. Other recorded uses include skin preparations and treatments to slow rot in wood and rope.