What is Purple Loosestrife?

Purple Loosestrife

Purple loosestrife (Lythrum salicaria) is not native to North America, but was introduced from Europe.  It was likely introduced in the 1800s unintentionally with shipments of livestock, and intentionally for its medicinal value and use in gardens. It began to be noted as problem in the 1930s in the United States as it spread as a result of habitat disturbances due to the creation of canals, highways and drainage ditches.  The lack of natural predators, and beginning in the mid-20th century, its increased use in the gardening industry, may have further contributed to the invasion of purple loosestrife into our natural landscapes. 

The ornamental varieties of purple loosestrife produced for the horticultural industry in the mid-1900s were incorrectly thought to be sterile.  In the 1990s, it was discovered that these horticultural varieties were capable of crossing with wild purple loosestrife plants as well as a loosestrife native to parts of North America, winged loosestrife (L. alatum).

Common garden varieties of purple loosestrife that were available in Manitoba that people still may be familiar with are ‘Dropmore Purple’, ‘Morden Pink’, ‘Morden Gleam’ and ‘Morden Rose’.  Research also showed that these varieties could produce viable seeds and were not sterile as thought.  The sale of all lythrum species has been prohibited in Manitoba since 1996; it is still seen in gardens in the province.

Purple loosestrife spreads into natural areas and competes for resources with native vegetation.  Native vegetation provides food, shelter and habitat for wildlife whereas an introduced species, like purple loosestrife, usually has limited value to waterfowl, insects and other animals in Manitoba.

Purple loosestrife produces a lot of seeds (hundreds of thousands to millions per plant) that spread into rivers, streams, canals and drainage ditches and are transported by water to areas beyond the initial infestation. The Assiniboine River in Winnipeg demonstrates this process.  From an area of initial infestation just west of the city limits, purple loosestrife has spread east along the Assiniboine to where it flows into the Red River. In the city, purple loosestrife is found along the Red River north (downstream) of where the Assiniboine connects.  

Purple loosestrife can also be spread by its plant parts; pieces of stem and roots are able to form a new plant.  This makes controlling the plant particularly challenging.

How to Identify Purple Loosestrife

Purple loosestrife plants are generally one to two metres (3 to 6 feet) tall and made up of several stalks. The most striking feature which makes the plant easy to identify are the flower stalks covered with pinkish-purple flowers, each with five or six petals.

Before the plant is in flower the plant can be identified by its square stem and the lance-shaped leaves that are arranged in opposite pairs along the plant.

Purple loosestrife can be easily confused with other plants. These include the native plants: fireweed (Epilobium angustifolium), blue vervain (Verbena hastata), dotted blazing star (Liatris punctata) and a plant native to eastern North America, winged loosestrife (Lythrum alatum).  It can also resemble another invasive, dame’s rocket (Hesperis matronalis).  On closer examination, the similarities between these plants and purple loosestrife are superficial.  

  Square Stem                                 Lance-Shaped Leaves

              

Photo Credits: Close-up of flowers: Manitoba Purple Loosestrife Project; Stem: H. Catton, ISCM; Close-up of leaf: G.H.S./USFW

What can YOU do about it?....

Manitoba Conservation Ducks Unlimited City of Winnipeg Invasive Species Council of Manitoba