Purple loosestrife, a beautiful garden plant with an aggressive nature, was introduced into North America in the early 1800's. The plant was sold in North Dakota by its genus name, Lythrum, for at least 50 years. The garden varieties were thought to be sterile, not producing seeds, but now have been shown to cross-pollinate with the wild Lythrum type and sometimes with other Lythrum cultivars.
The most destructive impact of purple loosestrife invasions is on the ecology of aquatic sites. Purple loosestrife forms dense monotypic stands as it displaces native wetland plants. When purple loosestrife replaces native vegetation, it also can displace wildlife. Waterfowl production decreases as suitable nesting habitat is eliminated. The plant's growth is generally too compact to offer cover. Purple loosestrife is not a food source of waterfowl, songbirds and other wetland organisms. Therefore, the area can become an environmental desert with no wildlife when completely taken over by purple loosestrife.
Purple loosestrife is a rhizomatous perennial forb. Wild infestations are associated with moist or marshy sites. The stems are erect (1.5 to 8 + feet tall) and four to six angled, and can be smooth or pubescent with few branches. Leaves are simple (.75 to 4 inches long, .2 to .5 inches wide), entire, and can be opposite or whorled.
The most identifiable characteristic of purple loosestrife is the striking rose to purple flowers. The flowers are arranged on a spike, which can be a few inches to 3 feet long. Each flower has five to seven petals. The plant usually flowers from early July to mid-September.
Control in non-crop lands:
Cultural: Plants in a flower bed or small infestations can be controlled by digging up the root mass. Be sure that you dig up the entire root ball which may be quite large if the plant is many years old. New plants may sprout up from the root mass the following year so monitoring the site will be needed.
Chemical: Several herbicides can be used to control purple loosestrife infestations. Glyphosate (Roundup) can be applied to targeted plants within a flower bed. Garlon is a selective broadleaf herbicide that will not kill cattail or other desirable monocot species in wetlands.
Biological: Three biocontrol insects have been released with two leaf -feeding beetles (Galerucella spp.) being the most successful. These insects overwinter as adults and lay eggs in early June. The adults and larva feed on the leaves and flowers. Several summers of heavy feeding greatly reduces infestations. However, most large infestations are in urban areas where mosquito control programs have kept these insects from becoming well established.