Roundup and similar glyphosate formulations can be used to remove purple loosestrife from large plantings or infestations away from water. For example, the Rodeo and Glypro formulations of glyphosate can be used in water. With the Rodeo or Glypro formulations, a nonionic surfactant approved for aquatic sites at 0.25% vol/vol must be added to the spray solution. Many formulations of glyphosate are sold but only those labeled for aquatic use can be applied in or near water. Glyphosate will provide good control of purple loosestrife when applied from July to early September. Although the root crown expands and produces more shoots each year, the maximum growth of the root crown diameter is limited to about 20 inches. The aerial shoots die in the fall and new shoots arise the following spring from buds at the top of the root crown. The roots become thick and woody in mature plants. The seed capsule is two celled and contains many very small seeds (1 mm long or less). The plant usually flowers from early July to mid-September in North Dakota.
Each flower has five to seven petals arising from a cylindrical green tube.
The flowers are arranged on a spike, which can be a few inches to 3 feet long. The most identifiable characteristic of purple loosestrife is the striking rose to purple colored flowers (Figure 4). Leaves are simple (0.75 to 4 inches long, 0.2 to 0.5 inches wide), entire, and can be opposite or whorled. The stems are erect (1.5 to 8 or more feet tall), four to six angled, and can be smooth or pubescent with few branches. Wild infestations are associated with moist or marshy sites. Purple loosestrife is a rhizomatous perennial forb introduced to North America from Eurasia and Africa. The plant’s growth is generally too compact to offer cover, and cover may be as crucial to wildlife as food. In addition, overall waterfowl production decreases as suitable nesting habitat is eliminated. Waterfowl, especially ducks, avoid wetlands that have become dominated with purple loosestrife. Muskrats use cattails to build their homes, and they show a preference for cattail over purple loosestrife for food.
For example, songbirds do not consume the small hard seed. When purple loosestrife replaces native vegetation it also can displace wildlife. Under optimum conditions, a small isolated group of purple loosestrife plants can spread to cover aquatic sites in just one growing season (Figure 3). Purple loosestrife forms dense monotypic stands as it displaces native wetland plants (Figure 2). The most destructive impact of purple loosestrife invasions is on the ecology of aquatic sites. State law requires all plants to be removed to prevent this plant from becoming a major weed problem in the wetlands of the state. The weed has slowly spread over time and currently infests approximately 1200 acres in 20 counties. These infestations can be traced to escapes from public or private horticultural plantings, often from seed that finds its way to streams and rivers through storm drains.Purple loosestrife was added to the North Dakota Noxious Weed list in 1996 after it was found on 37 acres in 11 counties. Known purple loosestrife infestations in North Dakota are small and generally found in or downstream of urban areas. Many landscapes and gardens in North Dakota use Lythrum as a highlight of the planting.
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