Purple Loosestrife

(Lythrum salicaria)

Progressive Control Pest

Purple Loosestrife is an erect perennial herb, with a woody four-sided stem, with leaves in whorls, growing up to 3 metres high. It reproduces prolifically by both seed dispersal and vegetative propagation. Mature plants can have up to 50 stems emerging from a single rootstock and produce as many as 3 million seeds annually. The magenta-coloured flowers, carried on a long spike, with five to seven petals, are produced throughout most of the summer. The woody stems remain standing during the winter, when the plant is readily identified by its brownish colour and spiral-shaped capsule clusters.

Reasons for the Strategy

Purple loosestrife plant featuring tall magenta flowers.

Purple Loosestrife is capable of invading a variety of wetland habitats, including marshes, river and stream banks, pond edges, lakes, roadside ditches, and reservoirs. Disturbed areas are more prone to invasion because exposed soil is ideal for germination. It can out-compete and replace native grasses, sedges, and other flowering plants that provide a higher quality food source and habitat for wildlife. Its dense stands can deleteriously impact wildlife habitat used by birds. If left untreated, it can almost entirely eliminate open water habitat. The recreational and aesthetic values of wetlands and waterways diminish as they become choked by dense stands of Purple Loosestrife and biodiversity decreases. The seeds are distributed by wind, water, wildlife and by construction vehicles and can remain viable in the soil for many years.

Purple Loosestrife is assessed at “1” on the infestation curve. The low incidence of Purple Loosestrife in the Tasman- Nelson region, extensive areas of suitable habitat, high natural values of wetland habitats, and the potential for it to cause significant adverse effects, mean the benefits of progressive control far outweigh the costs.


To reduce the distribution and density of Purple Loosestrife where it has become naturalised in wetland habitat in the Tasman-Nelson region during the term of the Strategy.

Alternative Measures

The alternative option of “do nothing” or relying on voluntary control will not achieve the objective of reducing the distribution and density of Purple Loosestrife, and will result in significant additional costs to the community through lost production, and the increased cost of control in the future. Requiring total control is not practical given the uncertainty about its distribution.

Strategy Rule for Purple Loosestrife

The occupier shall destroy all adult and juvenile forms of Purple Loosestrife that threatens wetlands. A breach of Strategy Rule 5.9.5 is an offence under Section 154(r) of the Biosecurity Act 1993.

Biosecurity Act Requirement

No person shall knowingly sell, propagate, breed, release, or commercially display Purple Loosestrife, under Sections 52 and 53 of the Act.