Purple loosestrife

Conservation Status

IUCN Red List

not listed

NatureServe

NNA - Not applicable
SNA - Not applicable

Minnesota

not listed

Weed Status

Prohibited Control Noxious Weed

Wetland Indicator Status

Great Plains

OBL - Obligate wetland

Midwest

OBL - Obligate wetland

Northcentral & Northeast

OBL - Obligate wetland

Description

Purple loosestrife is an erect, stout, perennial forb that rises on 30 to 50 stems from a taproot and shallow, fibrous roots that develop rhizomes. It can be 12″ to 72″ tall, though in Minnesota it is usually less than 48″ in height.

The stems are erect, square, hairy within the inflorescence, variably hairy above the middle, hairless below. Mature plant stems may be 5- or 6-sided and woody near the base in late summer.

The leaves are opposite below the inflorescence, sometimes alternate within the inflorescence. Sometimes they appear in whorls of 3. They are lance-shaped to nearly linear, 1″ to 4″ long, and 3 ⁄16″ to ⅝″ wide, becoming gradually smaller as they ascend the stem. They taper to a point at the tip and are rounded at the base. The larger leaves are somewhat heart-shaped at the base and slightly clasp the stem. The upper and lower surfaces are more or less softly hairy, especially those near the top of the stem. The margins are untoothed.

The inflorescence is a dense, 6″ to 14″ long, spike-like cluster of numerous flowers at the end of the stem. The flowers are in whorled clusters rising from a pair of hairy, leaf-like bracts. The clusters have 1 or more stalkless flowers and there are 3 to 9 flowers per whorl.

Individual flowers are ½″ to 1″ wide. There are 5 or 6, sometimes 7, pink to purple petals and the same number of green to purple sepals. The petals are about twice as long as the sepals. The petals are fused at their base with the sepals for most of the sepal length into a hairy, 12-nerved, 3 ⁄16″ to 5 ⁄16″ long, 1 ⁄16″ wide tube (hypanthium). There are always at least 10, usually 12, stamens, in 2 whorls of 5 or 6 each. There are three types of flowers, distinguished as those with short, medium, or long styles. Flowers with short styles have a whorl of long stamens and a whorl of medium length stamens. Flowers with medium-length styles have a whorl of long stamens and a whorl of short stamens. Flowers with long styles have medium to short stamens. The longer stamens protrude well beyond the tube.

The fruit is a two-chambered capsule about ⅛″ long and 1 ⁄16″ in diameter containing many dust-like seeds.

Height

12″ to 72″

Flower Color

Pink to purple

Similar Species

Fireweed (Chamerion angustifolium) stem is round. The leaves are alternate. The flower spike is 4″ to 5″ wide at the base.

Swamp loosestrife (Decodon verticillatus) stems are usually arched and rooting at the tip, not erect. The leaves are usually in whorls of 3 or 4 and are on short leaf stalks. The flowers are in dense clusters in the upper leaf axils.

Winged loosestrife (Lythrum alatum var. alatum), a native plant, is usually hairless. The middle and upper leaves are alternate. The flowers are smaller, ½″ across or less, and appear singly in the upper leaf axils. There are always fewer than 10 stamens per flower.

Habitat

Wet. Fens, marshes, meadows, shores, shallows. Full to partial sun.

Ecology

Flowering

July to September

Pests and Diseases

Biological Control

Several insects that are native to the historic range of purple loosestrife have been intentionally introduced into North America to control the spread of this invasive plant species.

black-margined loosestrife beetle (Neogalerucella calmariensis)

flower bud weevil (Nanophyes marmoratus)

Goeze root-boring weevil (Hylobius transversovittatus)

golden loosestrife beetle (Neogalerucella pusilla)

Use

Distribution

Sources

2, 3, 5, 7, 24, 28, 30, 83.

Biodiversity occurrence data published by: Minnesota Biodiversity Atlas (accessed through the Minnesota Biodiversity Atlas Portal, bellatlas.umn.edu. Accessed 3/10/2026).

Midwest Herbaria Portal. 2026. https://midwestherbaria.org/portal/index.php. Accessed 3/10/2026.

Lythrum salicaria L. in GBIF Secretariat (2023). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org. Accessed 3/10/2026.

EDDMapS. 2013. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at www.eddmaps.org/. Accessed 3/10/2026.