Purple Loosestrife

Lythrum salicaria

purple loosestrife flowers on several plants
 Image credit: Sigrid Resh

Characteristics

Purple loosestrife, a herbaceous perennial that is in the Lythraceae family, is a wetland invasive that prefers full sun and wet soil but is able to grow in shaded, drier conditions as well.

  • typically grows 6-9ft tall with a square shaped stem
  • produces multiple spike-shaped stalks with small purple flowers (5-7 petals each flower) that bloom from early July to September
  • generates about 2 million wind dispersed seeds per plant
  • dense, woody rootstock that are difficult to remove in wet soils, when broken off stem and root fragments are able to re-establish roots and create new plants

Note: Can also be identified by its whorled, slender, lance-shaped leaf that is about 4 inches long.

For more information visit MISIN.


KISMA Management Practices

As purple loosestrife forms thick monocultures, the plant is difficult to manage and remove. 

While small stands of purple loosestrife can be manually removed quite easily, big stands are much more difficult to remove. KISMA uses hand pulling and a form of biocontrol to manage this invasive. A leaf eating beetle (Galerucella pusilla) is sometimes used as  biocontrol agent on larger populations. These beetles will feed on the underside of the purple loosestrife leaf and can greatly decrease the population of the invasive. Manual management can be done following these steps: 

  1. loosen soil with shovel 
  2. pulling by hand or with the aid of tools such as a shovel is the most effective strategy, as long as care is taken to remove as much of the root mass as possible
  3. dispose of the plant in bags so as not to leave any stem fragments, seeds, or roots behind

Note: While removing purple loosestrife, it is extremely important to remove all root and stem fragments as they can reestablish themselves. It is also important to remember to wash your clothes and equipment after removal or treatment as to not track purple loosestrife seeds to other areas.


Native Alternatives

Species such as swamp milkweed, joe pye weed, blue vervain, and marsh blazing star are all tall herbaceous perennial flowers that are extremely similar to the looks of purple loosestrife. It is important to replace purple loosestrife with native alternatives to keep the purple loosestrife from growing back after removal and to cover up exposed topsoil.

 

purple loosestrife flowers

(image credit: Sigrid Resh) spike-shaped flowers of purple loosestrife

purple loosestrife overtaking a boardwalk
(image credit: Sigrid Resh) purple loosestrife invading a wetland boardwalk
purple loosestrife eaten by beetle
(image credit: Sigrid Resh) leaves being eaten by beetle (Galerucella pusilla)
purple loosestrife flowers

(image credit: Sigrid Resh) flowers of purple loosestrife

Sources

“Purple Loosestrife (Lythrum salicaria).” MISIN, 2020. 

Minnesota Department of Natural Resources, 2021.