Non-Native Invasive Species

Purple loosestrife

Lythum salicaria

Purple loosestrife

Purple loosestrife
Photo: Ohio State Weed Lab Archive, Ohio State University, Bugwood.org

The horticultural industry used to advertise purple loosestrife as the perfect perennial - easy to grow, very hardy, no major pests or diseases, and gorgeous. But perfect it's not. Instead, a purple pestilence is what it is - a beautiful purple plague of wetlands.

The species, a native of Europe and Asia, was first discovered in North America in 1814. By the 1830s it was so common most people thought it was a native plant. Now, thanks to canals and railroads, irrigation projects and the interstate system, and the hungry eyes of unwitting gardeners, the plant is spread across much of the northern part of the country. It has taken over wetlands, filled roadside ditches, colonized pond edges, even invaded tidal flats - just about any place that's moist and disturbed. And we can't make it go away.

Purple loosestrife

Purple loosestrife
Photo: Ohio State Weed Lab Archive, Ohio State University, Bugwood.org

Why would we want to make it go away? When in bloom - as it is from July through August - stands of loosestrife are stunningly beautiful, and bees and some butterflies love the plants. (It is said to make good honey.) But loosestrife is a botanical bully, with a "me or the highway" kind of attitude (if plants could be said to have attitudes). It sets so much viable seed, reproduces readily from root fragments, and generally grows so aggressively in such thick stands that purple loosestrife soon replaces native species, and the organisms that depend on them. Vervains, golden alexanders, joe pye weeds, coneflowers, asters and others can't compete with purple loosestrife.

Many governmental agencies and hundreds of other organizations sponsor loosestrife control programs, including research into biological controls. Beginning in the 1990s, several organisms which prey on purple loosestrife were released throughout the northeast, Midwest, and Canada. (Scientists reassure us they'd been carefully tested to make sure the organisms posed no threat to desirable species.) To date, two species of leaf-eating beetles seem to have settled in nicely, and seem to be eating heartily. Researchers hope they will spread throughout the range of the plant, someday.

Purple loosestrife

Purple loosestrife
Photo: Randy Westbrooks, U.S. Geological Survey, Bugwood.org

Until they do, landowners can and should remove loosestrife but hand pulling or by herbiciding. (Most wildlands managers use Rodeo™, a formulation of Roundup™ made for use in wetlands.) Where bare earth is left, plant a substitute species, and monitor for loosestrife seedlings. It will take years for all the seeds left in the soil to germinate.