[4] The generic name Lythrum is derived from the Greek 'lythron', meaning blood, in reference to the flower colour in some species.

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.

[2][8][9] 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 golden loosestrife beetle Galerucella pusilla is nearly identical to G. calmariensis, but usually lacks the black thoracic line.

The larvae emerge from their eggs and immediately burrow into the root of the plant, which they feed on continuously for over a year.

It has been used as an astringent medicinal herb to treat diarrhea and dysentery; it is considered safe to use for all ages, including babies.

[20] It is also cultivated as an ornamental plant in gardens, and is particularly associated with damp, poorly drained locations such as marshes, bogs and watersides.

[24] It has also been introduced in many areas of North America by bee keepers,[citation needed] due to its abundance of flowers which provide an important source of nectar.

[25] Purple loosestrife has been introduced into temperate New Zealand and North America where it is now widely naturalised (spreading on its own) and officially listed in some controlling agents.

Infestations may rarely result in dramatic disruption in water flow in rivers and canals, and the life cycles of organisms from waterfowl to amphibians to algae being affected.

In North America, purple loosestrife may be distinguished from similar native plants (e.g., fireweed Chamerion angustifolium, blue vervain Verbena hastata, Liatris Liatris spp., and spiraea (Spiraea douglasii) by its angular stalks which are square in outline, as well as by its leaves, which are in pairs that alternate at right angle and are not serrated.

Infestations of either of the Galerucella species is extremely effective in wiping out a stand of purple loosestrife, defoliating up to 100% of the plants in an area.