common soapwort

The plant possesses leafy, unbranched stems (often tinged with red). It grows in patches, attaining a height of 70 cm (28 in). The broad, lanceolate, sessile leaves are opposite and between 4 and 12 cm long. Its sweetly scented flowers are radially symmetrical and pink, or sometimes white. Each of the five flat petals have two small scales in the throat of the corolla. They are about 2.5 cm (1 in) wide. They are arranged in dense, terminal clusters on the main stem and its branches. The long tubular calyx has five pointed red teeth.

The individual flowers open in the evening, and stay open for about three days. They produce a stronger scent at night and supplement nectar production during the night. The flowers are protandrous: on the second night of blooming, the pollen is released, and the stigma develops to its final position by the third night. Much of the seed production comes from self-pollination. The flowers are visited by various insects including Noctuidae, Sphingidae, bumblebees, and hoverflies. In the Northern Hemisphere Saponaria officinalis blooms from May to September, and in the Southern Hemisphere October to March.