Invasive Species: Pesky Plants by Chris Kjolhede

Lose Purple Loosestrife!

Purple Loosestrife (Lythrum salicaria) is an invasive planet species native to Eurasia. It was introduced to North America in the early 1800s and now has become widespread. This invasive species was first reported in New York in 1843. It outcompetes native plants and thus reduces biodiversity. Purple loosestrife has been used as a medicinal for diarrhea, menstrual problems and bacterial infections, but scientific evidence of its value is lacking. The states of Minnesota, Wisconsin and Illinois prohibit the sale of purple loosestrife.    

Purple loosestrife is beautiful, especially when it blooms en masse, in a single-species stand around a pond or in a wetland area. The flower stalks are pink more than purple and very pretty. The trouble is that it is an invasive species that none of our local birds, mammals or fish need to survive. And, this competitive invasive species takes the place of many plants, including cattails, important to wetland habitats. Loosestrife can decrease food sources, nesting sites and shelter for many animals.  This pretty flower is more than just a nuisance; it can be a threat to local and migrating water birds, such as ducks, geese, and shorebirds. And in dense stands, Purple loosestrife can even disrupt water flow.      

To control purple loosestrife, you can locate it easily while it is flowering in late June, in July or early August. The large number of seeds and the dense plants make it hard to completely free a site that has been overwhelmed by purple loosestrife. Look for many stalks of many small florets of bright pink on upright plants about three feet tall. Purple loosestrife in small patches can be pulled out, but this should be done at the onset of the bloom to avoid spreading the seeds. A mature plant may produce more than one million seeds in one growing season; some seeds survive at least three years. Pulled plants should be dried and burned well away from water. Efforts to eradicate purple loosestrife may take several years of rigorous, mid-summer pulling out of mature plants.  

There is hope for controlling purple loosestrife. An introduced leaf-eating beetle, of the genus Galerucella, feeds only on purple loosestrife. This beetle’s native habitat is the same range in Europe and Asia as the purple loosestrife’s native habitat; neither the plant nor the insect is invasive in those settings. There have been some success with this method of control in North America. This beetle may provide an avenue for eradication going forward and early studies predict a 90 percent reduction in the loosestrife population. 

Purple loosestrife has been sold in nurseries as a home garden plant.  There’s a problem with having it in your garden. It’s about those millions of seeds. If you live near a wetland or pond, you may jeopardize the native ecosystem, the native plants amphibians, fish and birds.

Chris Kjolhede, MD, MPH, is an emeritus pediatrician at Bassett Healthcare Network.