Purple Loosestrife

Purple Loosestrife (Lythrum salicaria) is an invasive wetland plant with showy purple flowers arranged on flower spikes. Originally from Europe and Asia, the species was introduced to the United States’ Great Lakes through contaminated cargo ship ballast and the importation of seeds for decorative purposes. Due to its negative impacts on native plants and its ability to escape from cultivation, purple loosestrife is now illegal to sell in most states.

Environmental Impact

Purple loosestrife impacts:

  • Dense growth along shoreland areas makes it difficult to access open water.
  • Overtakes habitat and outcompetes native aquatic plants, potentially lowering diversity.
  • Provides unsuitable shelter, food, and nesting habitat for native animals.
  • Dense root systems change the hydrology of wetlands.

Mitigation Strategies

Mitigation Strategies:

The most effective way to control the population of purple loosestrife is by implementing biological controls in the form of leaf-eating beetles. These beetles were carefully selected by the United States government from native species in Europe, where purple loosestrife originates. Several years of quarantined testing was performed in the late 1980s before the beetles were released into the ecosystem. To accelerate the introduction of the beetles, the Minnesota Department of Natural Resources recruited many partner organizations to rear insects statewide. Minneapolis Park and Recreation Board (MPRB) staff were part of the initial biocontrol rearing and release efforts in Minnesota in the late 1990s.

MPRB’s current control of purple loosestrife mainly relies on the several types of biocontrol insects that are now present throughout Minnesota. Mowing, cutting, and pulling proved to be costly and largely ineffective control methods requiring continual maintenance, as did herbicide treatments. MPRB wetlands that were most impacted by loosestrife currently have vastly reduced populations compared to the time prior to the release of the biocontrol. Where the bio-control insects are established, purple loosestrife is now a long-term but low-level presence. The current state of purple loosestrife growth in Minnesota represents an incredible success in the development and use of biocontrol insects to control invasive species.

Resources

Contact

For more information, contact the Forestry Department at 612-313-7710 or forestry@minneapolisparks.org.

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