Flower:

Pinkish purple flowers whorled in a dense cluster around the stem at the leaf axils. Flowers have 4 to 7 crinkled petals (usually 5), each about ¼ inch long, elliptical but often fleeting and not always easy to distinguish. 8 to 10 white-tipped pink stamens arch out in a spidery array extending nearly double the length of the petals. Over-all flower structure is nearly 1 inch across.The calyx holding the flower is a thin walled, paper-like cup with gnarly appendages. Multiple flowering stems, rarely branched, shoot from the base of the plant.

Leaves and stem:

Leaves are opposite or more often in whorls of 3 or 4, elliptical to lance-like, tapered similarly at both ends, 2 to 8 inches long, ½ to 2 inches wide, toothless, finely hairy on the underside and smooth on the upper surface, the mid-vein reddish near base, on a short stalk. Stems are long and arching, up to nearly 10 feet in length but the overall plant height rarely exceeds 3 to 4 feet. Stems root at the tips as they re-meet the substrata, are sometimes slightly hairy, angled with 4 to 6 sides, often pinkish red on blooming canes and woody at the base. 

Fruit:

Fruit is a globular capsule up to 1/3 inch across, containing numerous seeds. Capsules persist through winter. Minnesota plants rarely form set seed but spread as clonal mats via re-rooting stems.

Notes:

Being at the northwest fringe of its natural range, Swamp Loosestrife is restricted to lakeshores in east central Minnesota. Because this region has seen such heavy development over the past decades it should come as no surprise that another native species has been put at risk. According to the DNR it has been listed as a Special Concern species since 1984.