Flower:

Single flowers on short stalks blooming from each leaf axil, typically 2 to 5 blooming at a time in a cluster slowly ascending the branch as newer buds mature. Individual flowers are ¼ to ½ inch across with 6 petals, pale lavender, purple or rose-pink with a darker mid-vein, rounded at the tip and fused at the base into a short tube, forming an off-white throat. Petals are textured like wrinkled tissue paper. The calyx holding the flower forms a narrow tube and has strong parallel veination and 6 sharp tips curved outward. 6 purplish brown stamens extend out of the throat with the single greenish style hidden inside the tube. A plant has several racemes on erect branching stems in the upper part of the plant.

Leaves and stems:

Leaves are elliptical to lance-shaped, rounded at the base and tapered to a point at the tip, toothless and hairless with no leaf stalk. Lower leaves are up to 2 inches long and ½ inch wide, more lance-like and opposite, becoming smaller, more oval and alternate in upper portions of the plant. Stems are 4-sided, have slightly raised ridges or wings that run parallel the length of the stems, and are hairless.

Notes:

Winged Loosestrife is the native next of kin to the widely invasive and destructive Purple Loosestrife, Lythrum salicaria that was introduced by gardeners via the global nursery industry and is now ranked among the most highly problematic invasive species in North America. While perhaps L. alatum was once more widely encountered than it is now, much of its habitat is highly vulnerable to invasion by its cousin as well as problem plants like Reed Canary Grass, which easily displace it and everything else. Where Winged Loosestrife does persist it is unlikely to be confused with Purple Loosestrife, as it behaves itself and does not have the dense flower spikes of its cousin, and for those reasons it is not always easy to pick out of the landscape.