Purple loosestrife (Lythrum salicaria) is usually found in wetlands, marshes, river and stream sides, roadside ditches, culverts, and lake and pond shores. However, loosestrife can also grow in drier soils such as those of meadows, pastureland, and home gardens. In Vermont, purple loosestrife has the highest densities in Essex county but can also be found in Chittenden, Addison, Washington, and Windsor counties.
Purple loosestrife can grow quite large, up to 4.5 ft. tall with mature plants having many stems from a single rootstock. Stems are woody, square, and ridged with five or six sides.
Leaves are downy, narrow, and smooth-edged. They grow oppositely arranged in pairs that alternate down the stem at 90° angles.
Purple loosestrife can also be identified by its flower spikes made up of many bright purple or magenta colored individual flowers. These flowers have five to seven petals that bloom midsummer.
Purple loosestrife can quickly overwhelm and displace native plants. This can be especially damaging in wetlands whose native grasses and sedges provide important habitat, nesting opportunities and food for hundreds of species.
The dense roots and stems also trap sediments and can clog waterways. Decaying loosestrife leaves also create a highly acidic environment that has been shown to increase the mortality rate of American toad tadpoles.
Control and Prevention
Purple loosestrife invasions can be managed by a few methods. Small infestations can be hand pulled; the best time to do this is when the plants are in flower between June and August but before they go to seed.
Hand pulling and cutting at appropriate times can also help manage large infestations by controlling their spread, especially near drainage ditches or streams where seeds can easily spread and establish colonies in new areas. It is important to clean equipment used in removal and to check sites periodically for new growth.
Chemical control can also be effective. Make sure to use an herbicide permitted for wetland use or contact a professional for chemical treatment. For large infestations, the USDA has approved a few beetle species for biological control.
Early detection is the best method to prevent the spread of purple loosestrife.
Loosestrife spreads rapidly; a single plant can produce more than one million seeds with most able to successfully grow into mature plants. Catching and treating invasions early is critical. When pulling plants, remove the entire root system to prevent further spread.
You should also avoid planting loosestrife on your property. Some wildflower seed mixes may contain purple loosestrife seeds and even sterile varieties of loosestrife that are sold in nurseries can cross pollinate with wild purple loosestrife to produce viable seeds.
Purple loosestrife is on the Vermont Noxious Weed Quarantine list, and it is illegal to purchase, plant, or transport it in the state.
Native Plant Sources
Check out Native Plant Sources for places to purchase plants native to Vermont.
For more on purple loosestrife management options
Vermont Department of Agriculture
Vermont Department of Environmental Conservation
Visit the Vermont Department of Environmental Conservation for rules regarding management in and around wetlands and contact (802) 490-6177 for more information