Purple Loosestrife (Lythrum salicaria) — identification, habitat & look-alikes

Purple loosestrife is a Eurasian wetland plant that has become one of North America's best-known invasive weeds. In summer its tall spikes of magenta flowers crowd ditches, marshes, and lakeshores, and a single plant can release up to 2.7 million tiny seeds a year. It is also called spiked loosestrife.

Appearance

Growth form: A perennial 3–6 ft (1–2 m) tall with many upright stems rising from a single woody root mass.
Leaves: Lance-shaped, stalkless, downy leaves with smooth edges, set in opposite pairs or whorls of three on a square stem. They often turn bright red in early fall.
Flowers: Reddish-purple flowers about ½–¾ in (10–20 mm) across, usually with six petals and 12 stamens, packed in long spikes. Each plant has one of three flower types with different style and stamen lengths, so pollen has to move between plant types.
Flowering time: Summer
Fruit: Small capsules full of tiny seeds; one plant can release up to 2.7 million seeds a year.

Look-alikes

Winged loosestrifeLythrum alatum

This native usually bears its flowers one at a time in the upper leaf axils, spaced out along the stem rather than packed into a dense spike. Its square stems can have narrow 'wings,' and its narrow leaves are smaller and not hairy.

Why they get mixed upSame genus, similar color, and both grow in wet places.

FireweedChamaenerion angustifolium

Fireweed flowers have four petals and a long stigma split into four, and its leaves alternate along a round stem. Purple loosestrife has a square stem with leaves in opposite pairs or threes, and it thrives in wetter ground than fireweed.

Why they get mixed upBoth have tall magenta flower spikes in summer.

Butterfly bushBuddleja davidii

Butterfly bush is an arching shrub up to 16 ft (5 m) with honey-scented flowers in cone-shaped clusters; purple loosestrife is a non-woody wetland plant with six-petaled flowers along square stems.

Why they get mixed upBoth have long purple flower spikes in summer.

Where it lives

Typical habitats: Marshes
Wet meadows
Ditches
Lake and river edges
Range: Native to Europe, temperate Asia, and North Africa. In the US it was established by the 1830s, arriving in ships' ballast and as a garden plant, and it has since spread through wetlands in many states.
Preferred conditions: Full sun and wet ground. In some places it crowds into sedge meadows and can slow the flow of water in canals and streams.

Growing it

Propagation: Spreads by seed and by forming clonal colonies from its rootstock.

Conservation

IUCN Red List: LCLeast ConcernLow risk of extinction
Status: Some states restrict it. For example, Washington lists it as a Class B , and California's noxious weed list covers the genus Lythrum. It is on 's list of 100 of the world's worst invasive alien species.
Threats: Dense stands can crowd out native wetland plants.

Culture and stories

Culture and stories: The English name 'loosestrife' was coined by the naturalist William Turner in his 1548 herbal; 'salicaria' refers to its willowlike leaves.
It was used in Europe as an astringent herbal remedy for diarrhea.