Its four-angled stem is glabrous to pubescent. Margins are entire. Leaf surfaces are pubescent. Calyx lobes number six. They differ from purple loosestrife by having solitary or paired flowers in the leaf axils, rather than many flowers in terminal spikes, and six to eight stamens rather than twelve.
Purple loosestrife is a highly aggressive, perennial wetland plant native to Eurasia.
It was introduced to the northeast coast of North America in the early 1800's as a contaminant of European ship ballast and as a medicinal herb. It has subsequently spread to most of the temperate parts of the United States and Canada and now occurs in dense stands throughout the northeastern United States, southeastern Canada, the Midwest, and in scattered locations in the western United States and southwestern Canada. Purple loosestrife is found in wetlands such as cattail marshes, sedge meadows, and bogs. It occurs along stream and river banks and lake shores. Purple loosestrife is also found in ditches and other disturbed wet soil areas.