Weed of the Month
Purple Loosestrife
What is that beautiful purple plant we see at the edges of the Delaware River, creeks, and lakes? It’s probably purple loosestrife (Lythrum saliccania L.), an invasive plant native to Eurasia, Great Britain, Europe, Russia, Manchuria, China Southeast Asia and northern India. It was introduced into the United States in the 1800’s because of its beauty and its medicinal qualities. Unfortunately, it is very invasive, crowds out other plants, and replaces native plants that provide a higher quality of food for wildlife. At this time, purple loosestrife has invaded every state except Florida.
Description
Purple Loosestrife is a tall upright plant with a square woody stem that can grow from four to ten feet high. Its leaves are lance-shaped on the stem and heart-shaped or rounded at the base. Very showy magenta-colored flower spikes with five to seven petals bloom from summer to fall. Mature plants can have from 30 to 50 stems from a single root. It spreads by seeds that are carried away by insects that enjoy its delicious nectar. One plant may have as many as thirty flowering stems that can produce two to three million seeds per year. The seeds remain viable for at least two years! Loosestrife also spreads underground through roots that grow about one foot per year.
Control
The home gardener can hand-pull small plants. This is best done before the seeds appear. More mature plants can be treated with a glyphosate herbicide such as Rodeo for wetlands and Roundup for drier areas. These are most effective when applied late in the season. In addition, a mid-summer application will reduce the amount of seeds produced. Heavy infestations of the plant may require biological control. The U.S, Department of Agriculture has approved using weevils, and flower-feeding beetles are under investigation. These are best left to the professionals. The best control is not to plant purple loosestrife, even though they are available in nurseries and gardening centers. Even the ones labeled “sterile†can reproduce freely and combine with native Lythrum plants.
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