These European natives are generally low, spreading plants that are useful as ground covers or in rock gardens. Best in lean, very well-drained soil.
S. x lempergii. GIANT- FLOWERED SOAPWORT. Handsome blue-green leaves to 12 in. long form a mat 8 in. tall, 112 ft. wide. Large clusters of bright pink, inch-wide flowers virtually smother the foliage for weeks in midsummer. 'Max Frei' has soft pink blooms.
S. ocymoides. ROCK SOAPWORT. Zones US, MS, LS; USDA 6-8. Trailing habit to 1 ft. high and 3 ft. across. Dark green, oval, 12-in.-long leaves. In spring, plants are covered with 14-in. pink flowers in loose bunches shaped much like those of phlox. Looks especially nice cascading over a wall. Selections include white 'Alba' and deep pink 'Rubra Compacta'.
S. officinalis. SOAPWORT, BOUNCING BET. Zones US, MS, LS, CS; USDA 6-9. Grows to 2 ft. tall, spreading wider by underground runners. Can be invasive in rich, moist soil. Dark green, oval, pointed leaves to 4 in. long; loose clusters of inch-wide red, pink, or white flowers in midsummer. When crushed in water, roots produce a sudsy, detergentlike lather. This is one tough plant; before the days of herbicides, it could be seen growing in the cinders along railroad rights-of-way. The selection 'Rosea Plena', with double light pink flowers, is the common garden form. 'Rubra Plena' has crimson blooms that turn paler as they age.
S. x olivana. CUSHION SOAPWORT. Zones US, MS, LS; USDA 6-8. Tiny, dark green leaves form a compact cushion spreading to 24 in. tall, 8 in. wide. In early summer, the foliage mound is covered with short-stemmed pink blooms to 34 in. across.