An upright, unbranched, perennial herb that can form small colonies by sending out long, slender runners (stolons). Flowers arise singly from upper leaf axils with 5 yellow, pointed lobes; showy but nodding. Blooms May–August. Leaves opposite, to 5 inches long, pale green on the undersurface, closely spaced; the lower ovate, becoming lanceolate higher up on the stem, much narrowed toward the base; upper leaves sessile or with very short leaf stems.
Similar species: Nine species in this genus are recorded for Missouri. Fringed loosestrife (L. ciliata) occurs nearly statewide, in moist or wet areas in open woods or open valleys. It reaches 3 feet tall and has petal lobes that are minutely fringed, hairy stems, and spreading flower clusters. The leaves are broadly ovate with 6-inch, long-hairy leaf stems.