Purple Loosestrife (Lythrum salicaria)
Purple loosestrife (Lythrum salicaria), native to Eurasia, was accidentally introduced by way of ship ballast or infested sheep and raw wool. Purple loosestrife may also have been intentionally introduced by horticulturists for ornamental and medicinal uses. It is a wetland perennial herb that can grow up to nine feet tall. Each spring, some 30 to 50 stems arise from a perennial, woody rootstock. In flooded areas, purple loosestrife can form dense, fibrous rootmats. Stems have short, slender branches and evenly spaced nodes. Leaves are elongate, smooth-edged, rounded or heart-shaped at the base and attach close to the stems. Showy purple flowers grow on long spikes and occur in pairs or clusters. Purple loosestrife can easily be confused with other native wetland plants such as fireweed. Look for purple loosestrife’s squarish stems, opposite leaves, and flowers with five to seven narrow petals.
Purple loosestrife poses special challenges to management due to its ability to reproduce from plant fragments or by seed. A plant can produce 2.7 million seeds each year, each about the size of a grain of sand. It also grows in poor soil and is aided by nutrient runoff from surrounding farmlands.
The impact of purple loosestrife invasion is enormous. The weed displaces the resident plant community, reduces and degrades available wildlife habitat, decreases flow in irrigation canals and ditches, degrades hunting and fishing areas, and reduces the quality of wetland pastures.
Management Techniques No individual method will control purple loosestrife in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. Some methods found to be effective are listed below. Because of the aquatic nature of this plant, chemical treatment is often inadvisable or impossible in many situations. Purple loosestrife has proven to be an extremely difficult weed to control.
Biological controls. At least five different insects have been studied and used to control purple loosestrife including leaf, seed and root feeder beetles. The black-margined loosestrife beetle (Galerucella calmariensis) and the golden loosestrife beetle (Galerucella pusilla) appear to be the most effective. These insects have both been established in Oregon.
Hand pulling is most effective on young plants, one to two years old, and as soon as flowers appear (mid July to August). Roots extend about one foot into the soil. A hand shovel may be necessary to remove as much of the rootmass as possible with minimal soil disturbance. Remove all plant parts from the site and bag them to prevent sprouting. Hand pulling is not feasible on older plants.
Smothering. Covering dense populations of plants with black plastic can block sunlight and kill plants. This technique works best for pure stands of seedlings. Plants should be cut or mowed and then covered with black plastic for at least five consecutive months, beginning in early spring (April or May). Young plants will be killed, but older plants and seeds may still survive. Smothering is more effective on a small scale, and constant monitoring is necessary.
