Idaho OnePlan recommends the website of the Idaho Weed Awareness Campaign as the best resource for up-to-date information about Idaho's noxious weeds, and their control.
Source for this page: 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.

(Thumbnail images are linked to a larger photo)

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. .