What is Loosestrife?
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 all mint family plants, is sturdy, growing 3’ to 10’ in height. The leaves are narrow, 1” to 4” 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 the native wildlife. Dense stands can have negative impacts local fish populations and waterfowl habitat. Loosestrife also impacts recreational uses in the affected area, creating problems for boaters, fishermen and hunters. These, in turn, have potential to harm local economies. Finally, the purple loosestrife affects agriculture by blocking drainage and irrigation ditches, thus decreasing crop yield and quality.
 Loosestrife spreading in field |  Loosestrife has a distinctive square stem |
How did this plant arrive in North America? Native to Europe and Asia, it was originally introduced in the mid-1800s, either accidentally in ship ballasts or from seeds tangled in imported wool and sheep, or deliberately brought 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 of disease or insects, purple loosestrife can out-compete native vegetations. One mature plant can produce over 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 re-sprout from broken stems, plant fragments or underground roots. Mowing the plant down means many fragments are on the ground to potentially take root. Maine banned the sale and propagation of purple loosestrife in 2018.
Mitigating Loosestrife Infestation
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 re-sprout. 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 the compost. Both digging and cutting methods should be repeated for three or more years; monitor the sites 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 home 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 with these insects (beetles Galerucella calmariensis and Galerucella pusilla; weevils Nanophyes marmoratus and Hylobius transversovittatus) 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 reports significant control of the purple loosestrife when the beetles are introduced. While the plants are not completely eradicated, their impacts on the environment are reduced.
What to Plant Instead of Loosestrife
What native plants would provide the same colors or spikes seen in the loosestrife? Try planting liatris (blazing star or gayfeather). The plant’s flowering spikes are up to 5’ tall, depending on the species selected. While most varieties are in the purple hues, some white varieties are available. The flowers are great in arrangements and once established, the plants are drought tolerant.
Other options include Blue Vervain (Verbena hastata), which grows in moist soils, reaching 2’ to 6’ in height. The blue-spiked flowers are attractive to pollinators, especially bees. Blue vervain is also used as a medicinal herb. Joe Pye Weed is another native plant recommended as a replacement for the invasive purple loosestrife. Tall Joe Pye Weed (Eupatorium fistulosum) is an impressive plant growing up to 8 feet high and topped with large clusters of bright pink flowers. Shorter varieties of Baby Joe Pye weed are available, all attracting numerous butterflies and pollinators to the garden.
So don’t envy those spikes of purple loosestrife. Plant a native and enjoy the results.
