Flower:

Short-stalked flowers, usually single or sometimes paired, in the leaf axils on branching stems, 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 rounded at the tip and fused at the base into a short tube. Petals are pale lavender, purple or rose-pink with a darker mid-vein and textured like wrinkled tissue paper. The calyx holding the flower forms a narrow tube slightly longer than the petals, has strong parallel venation and 6 short, sharply pointed lobes that curve outward. Inside the tube are 6 purplish stamens and a single greenish style. A plant has several racemes on erect branching stems in the upper part of the plant.

Leaves and stems:

Leaves are narrowly egg-shaped to oblong-elliptic, mostly rounded at the base and tapered to a point at the tip, toothless, hairless and stalkless. Lower leaves are up to 2 inches long and ½ inch wide, mostly opposite, becoming smaller, more oval-elliptic and mostly 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.

Fruit:

Fruit is a capsule that develops in the persistent calyx, containing numerous seeds.

Seeds are less than 1 mm long, narrowly lance-elliptic, pale straw-colored to golden brown.

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.