Purple loosestrife (Lythrum salicaria) is an herbaceous perennial wetland plant native to Eurasia. Because of its lovely purple flowers and perceived beauty, this plant continues to be sold in the nursery trade. A prolific seed producer, an individual mature plant may produce up to one million seeds in a single season.
Loosestrife is a prolific invader of wetlands, drainage canals, and roadside ditches. It forms homogeneous stands that outcompete and replace native wetland plants that are necessary to support wildlife.
Habitat: moist soil to shallow water. Sand to clay soils. Sun to partial shade. Can adjust to a wide range of growing conditions including flooding up to 18”.
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 and surround small yellow centers. Closely attached to the stem. 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. Somewhat woody and die back each year.
Root system: consists of a large woody tap root with fibrous rhizomes. Rhizomes spread rapidly and aid in plant reproduction.
Biological, manual and chemical controls are effective.
Biological control: considered the best option for large-scale control. 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.
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