AIS July 2014 : Purple Loosestrife

Best Detected when blooming in late July and early August. Plants have angular stems, usually opposite pairs of leaves, and multiple magenta flower spikes. Best controlled by biological control, Galerucella weevils, which are propagated and then released in July.


  • Hardy perennial found in moist soil: wetlands, stream/river banks, lake shores, and disturbed
  • Established plants tolerate dry conditions:roadsides, ditches, and fields
  • 3-9 foot tall with angular stalks
  • Lance shaped, 1-4” leaves usually on opposite sides, with each pair at 90° angle
  • Magenta flowers with 5-6 petals and a yellow center on numerous long spikes
  • Seeds smaller than pin head; easily transported
  • Dense root mats several feet across
  • Introduced by European settlers in the 1800’s for garden use

Life History

  • Each flowering plant releases 2.7 million seeds annually that remain viable in the soil for years!
  • Can establish from bits of root or broken stem fragments
  • Water drawdowns accelerate spread by providing open substrate and sun for seed germination

Why Is It A Problem?

  • Quickly spreads, outcompeting and shading out native plants
  • Wildlife and fish decline significantly through reduction of habitat and food sources: Baltimore butterflies, marsh wrens, black terns, pied-billed grebes, and least bitterns disappear completely
  • Alters decomposition rates and nutrient cycling
  • Wetlands store and filter less water
  • Thick growth impedes boat travel
  • Encroaches on agricultural fields

What Can Be Done?

First Steps!

  1. Identify locations while blooming in late July and report
  2. Cut/ bag flower heads before they put out seed to prevent spread (dispose of properly)

Biological Control Methods

  • Best method & long-term
  • Galerucella weevils feed exclusively on loosestrife
  • Eat leaves & shoots, weaken plants and reduce flowering
  • Propagated and released through a citizen action program in WI
  • Beetles reduce loosestrife by 80-90%

Chemical and Mechanical Methods

  • Require follow-up and impractical/ expensive on large sites. Burn plants or place in landfill but do NOT compost. Do not disturb seed bank.
  • Gently pull or dig young, unestablished plants: roots must come out intact
  • Apply approved herbicide in dry, upland areas on private property on individual plants by selective hand spraying. DNR permit required near or in the water. Some chemicals are nonselective and effect native plants.

Note: DNR permits are required for chemical treatments, mechanical treatments, some manual treatments, biological control, bottom screening, and buoy/barrier placement.