Soapwort is a common plant that occurs in most counties of Illinois. It was originally introduced into the United States from Europe for horticultural purposes. Habitats include sloping banks of streams, gravel bars and sand bars along streams, areas along roadsides and railroads, prairie remnants along railroads, weedy meadows, and waste areas. Soapwort is still cultivated in flowerbeds and herbal gardens. While this plant is typically found in disturbed areas, it also occurs in some natural areas, where it can become a pest. This introduced perennial plant is about 1–2½' tall and more or less erect. From the axils of the upper leaves, some short side stems are produced, otherwise it is little branched. The stems are round and hairless. The opposite leaves are up to 4" long and 1¾" across. They are ovate or broadly lanceolate, smooth along the margins, and hairless. There are 3 conspicuous veins on the upper surface of the larger leaves. The leaves are sessile against the stem, otherwise they have short broad petioles. The central stem and some of the uppermost side stems terminate in small clusters of flowers. Each flower is about ¾–1" across, consisting of a long cylindrical calyx that is green and a corolla with 5 spreading petals that are white or light pink. The calyx is hairless and about 1" long. At the base of each petal, there are 2 claws that are small and slender. The 10 stamens are strongly exerted from the throat of the corolla; they are white with pale yellow anthers. As the flowers age, the petals begin to curve backward. The blooming period occurs during the summer and lasts about 2 months. There is a pleasant floral scent. A sack-like capsule of seeds lies within the calyx of each flower. These seeds are somewhat flat and nearly round along the perimeter, with an outer surface that is granular or pebble-like. The root system produces abundant orange rhizomes. This plant spreads by reseeding itself vegetatively through the rhizomes. It often forms colonies.
Sources: Soapwort (Saponaria officinalis), http://www.illinoiswildflowers.info/weeds/plants/soapwort.htm (accessed January 23, 2017). Saponaria officinalis | Encyclopedia of Puget Sound, https://www.eopugetsound.org/species/saponaria-officinalis (accessed January 23, 2017).