Purple loosestrife (Lythrum salicaria) is an herbaceous perennial wetland plant.
Purple loosestrife is native to Eurasia. It was intentionally introduced in the U.S. because of its lovely purple flowers and perceived beauty.
Loosestrife is an invader of wetlands, drainage canals, and roadside ditches.It can adjust to a wide range of growing conditions such as, moist, sandy and clay soils, in full sun or partial shade, and it can survive flooding up to 18 inches in depth.
Purple loosestrife forms homogeneous stands that out compete and replace native wetland plants that are necessary to support wildlife. Furthermore, it is prolific seed producer, an individual plant may produce up to one million seeds in a single season.
Height: 3′ – 7’ or more.
Leaves: simple, usually opposite on stems but sometimes alternate or bunched in whorls. Lanced shaped, without petioles. Edges are smooth, sometimes downy.
Flowers: showy, individual flowers have 5 or 6 pink petals that are 0.5″ – 0.75” across. Petals surround small yellow centers and are closely attached to the stem. Flowers bloom from the bottom of the flower spike to the top from early July to September.
Seeds: born in capsules that burst at maturity in late July or August. Single stems can produce 100,000 to 300,000 seeds annually. The seeds can live 20 months submerged underwater. Water, animals, boats and humans assist in transporting the seeds.
Stems: 1 – 50 per plant, upright, stiff, usually four sided, green to purple, often branching, making the plant bushy in appearance.
Root system: consists of a large woody tap root with fibrous rhizomes. Rhizomes spread rapidly and aid in plant reproduction.
Biological control: considered the best option for large-scale infestation. Four insects have been approved by the USDA as biological control agents for purple loosestrife: a root-mining weevil (Hylobius transversovittatus), two leaf-feeding beetles (Galerucella calmariensis and Galerucella pusilla), and a leaf-eating weevil (Nanophyes marmoratus). We recommend Galerucella beetles, which feed almost exclusively on purple loosestrife.
Pulling/Cutting: infestations less than 100 plants can be controlled by pulling or cutting just before the plants begin flowering to avoid spreading seed. When pulling, all root fragments should be removed and plants should be properly disposed of.
Mowing: not recommended because it will spread plant segments and seeds.
Cut-stem treatment: apply a solution of 30% glyphosate to the raw area of freshly cut stems, after the flowering portion has been removed.
Foliar treatment: spot treatment with glyphosate is effective on older plants. It is most effective when applied as plants are preparing for dormancy, but mid-summer and late-season treatments may be needed to reduce seed production.
For more information
Title photo: Eric Sweeney, http://www.aquaticweedremovers.com/purple-loosestrife-treatment/.
Purple loosestrife leaves: http://minnesotaseasons.com/Plants/purple_loosestrife.html
Purple loosestrife blooms: Lesley J. Mehrhoff, University of CT, http://www.nyis.info/index.php?action=invasive_detail&id=64
Purple loosestrife stem: Rob Routledge, Sault College, Bugwood.org.
Purple loosestrife roots: http://www.pbase.com/crocodile/image/31905356