Swamp Loosestrife is uncommonly found in marshes, swamps, and sphagnum bogs. It has large whorls of leaves in groups of three or four on the stem. Flowers form in clumps in leaf axils. They associate with Asclepias incarnata, Eupatorium perfoliatum, and Sagittaria latifolia. The stems can become woody, but the species will die back to the ground each year.