Standing tall along streams, roadsides and marshes, purple loosestrife (Lythrum salicaria) is easily spotted in the summer months with its erect, purple spikes. The square stem, like those of all plants in the mint family, is sturdy, growing 3-10 feet in height. The leaves are narrow, 1-4 inches long, and attached directly to the stems.
The mass of purple blossoms may be beautiful to behold, but this invasive plant displaces native plants, eliminating nesting sites, shelter and food for native wildlife. Dense stands can have negative impacts on local fish populations and waterfowl habitat. Loosestrife also impacts recreational uses, creating problems for boaters, fishermen and hunters. These effects have the potential to harm local economies. Finally, purple loosestrife affects agriculture by blocking drainage and irrigation ditches, thus decreasing crop yield and quality.
How did this plant arrive in North America? Native to Europe and Asia, it was introduced in the mid-1800s. It could have been brought accidentally, in ship’s ballast or wool; or deliberately, as an ornamental plant.
The plant prefers moist, organic soils, but can survive many conditions associated with disturbed sites. It can tolerate a wide range of temperatures, sunlight and soil acidity. With no natural predators, purple loosestrife can outcompete native vegetation.
One mature plant can produce more than 2 million seeds in one season. Roughly the size of ground pepper grains, these seeds are viable in the soil for years, waiting for the right conditions to germinate.
Purple loosestrife can resprout from broken stems, plant fragments or underground roots. Mowing the plant leaves many fragments that may take root. The sale and propagation of purple loosestrife was banned in Maine in 2018.
Controlling this invasive plant is difficult. Mechanical methods are often combined with both chemical and biological controls. Dig or pull the entire plant in early spring, but beware — root fragments will resprout.
When the plant flowers, cut and bag the flower spikes, or cut the entire plant at the start of the flowering season, before seeds are set. Dispose of the bagged materials in the trash, not in compost. Both digging and cutting methods should be repeated for three or more years, and the sites should be monitored for regrowth.
A 2% solution of glyphosate may be applied if the plants are more than 25 feet from water and use of chemicals is permitted by local ordinances. Apply the glyphosate when the plant is in flower, but before seeds are formed. Monitor and repeat for at least three years.
Efforts to introduce natural insect predators from loosestrife’s native homes have had mixed results. Species of insects have been tested in North America: two types of beetles and two weevils. Cornell University has done extensive testing and determined that the introduction of the insects is safe.
New Hampshire’s Department of Agriculture has introduced beetles in areas infested with purple loosestrife. In Wisconsin, the Department of Natural Resources has reported significant control of purple loosestrife when the beetles are introduced. While the plants are not eradicated, their impact on the environment is reduced.
What native plants would provide the same colors or spikes seen in loosestrife? Try planting liatris, known as blazing star or gayfeather. The plant’s flowering spikes grow up to 5 feet tall, depending on the species. While most varieties are purple, some are white. The flowers are great for arrangements and, once established, the plants are drought-tolerant.
Other options include blue vervain, which grows in moist soils, reaching 2-6 feet in height. The blue-spiked flowers are attractive to pollinators, especially bees. Blue vervain also is used as a medicinal herb. Joe-pye weed is another native plant recommended as a replacement for purple loosestrife. Tall joe-pye weed is an impressive plant, growing up to 8 feet high and topped with large clusters of bright pink flowers. Baby joe-pye weed is shorter. All varieties attract numerous butterflies and pollinators.
So don’t envy those spikes of purple loosestrife. Plant a native and enjoy the results.
