Purple loosestrife (Lythrum salicaria) is a semiaquatic perennial plant that is native to Europe and can be purchased (illegally) as an ornamental. This plant outcompetes native habitat along waterways and has no food value for wildlife. A single plant can produce approximately 250,000 seeds per year and can destroy river and ditch banks by impeding water flow. This noxious weed is so invasive that it can choke out cattails. It can grow close to 8 feet tall, and for the most part has to have its roots in moist-to-wet soil. It produces a beautiful purple flower that grows up the top foot of the woody, four-sided (square) stem. It blooms from mid-July to mid-August. Purple loosestrife can be found across Idaho along the banks of rivers, ditches, ponds and other watered areas.
Control and management options
Controlling purple loosestrife is a challenge because this plant grows in and near water. Digging it up can be done with a lot of effort, but remember to try to get most of the root, as it will grow back if some roots are left behind. There is a limited number of herbicides that can be legally used over water to help control this noxious weed. These include specialized herbicides such as Habitat, Rennovate, Aquamaster or Weedar 64. One of the best tools for managing populations of this noxious weed is the use of a defoliating beetle called galerucella. This insect only attacks the purple loosestrife plant and has done a great job of removing large populations of purple loosestrife around the state. Galerucella can be purchased from the Nez Perce Bio-Control Center in Lapham as well as from other USDA/APHIS centers that carry the insect.
For more information on purple loosestrife and Idaho’s other listed noxious weeds, go to the Idaho Weed Awareness Campaign website.