Source: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller
BACKGROUND
Purple loosestrife (Lythrum salicaria) is a semi-aquatic plant native to Europe, probably introduced as an ornamental. Valuable riparian habitat is degraded, and food species are rapidly displaced by this weed, which has no wildlife value. Purple Loosestrife spreads both by seed and spreading rhizomesA creeping, underground stem that form dense, woody mats.
DESCRIPTION
Purple loosestrife is a semi-aquatic, hardy perennialPlant that lives for more than 2 growing seasons that can grow over 8 feet tall. Stems are usually 4 sided and much branched, bearing oppositeLeaves situated directly across the stem from each other or whorled3 or more similar structures arranged as spokes on a wheel, 4-inch long, lance-shaped leaves. Crowded flower spikesA narrow, nonspreading inflorescence develop at the stem tips in midsummer. Flowers are 1 inch in diameter with 5 to 7 reddish-purple, wrinkled petals that appear as if they have been crushed. A single plant may produce over 2.5 million tiny seeds per year.
DISTRIBUTION
Purple loosestrife is found throughout the northeastern U.S. and in some western states and in Idaho.
CONTROL
Biological control agents are available and include 2 leaf beetles that have worked well in Idaho. Herbicides are available, but effectiveness is inconsistent and application is difficult. .