Soapwort (Saponaria officinalis) is an old cottage-garden perennial that reaches 50 to 90 cm and flowers pale pink from July to September. The name comes from the saponins in the root and leaves, which make the plant foam in water. It is one of the easiest perennials to grow, but it has one habit that decides where you put it: it spreads by underground rhizomes.
Two species are sold as soapwort
| Soapwort | Rock soapwort | |
|---|---|---|
| Species | Saponaria officinalis | Saponaria ocymoides |
| Height | 50 to 90 cm | 10 to 20 cm |
| Habit | Upright, spreads by rhizomes | Mat-forming, stays put |
| Flowering | July to September | May to July |
| Suits | Border, dry bank, cottage garden | Rock garden and wall tops |
The tall species is the scented one and the one that was used for washing. The low one is a rock garden plant with no spreading problem.
Sowing soapwort
Sow indoors in March or April and cover the seed with 0.5 cm of soil.
Hold 18 to 20 degrees. Germination takes 10 to 21 days.
Plant out in May with 40 cm between plants.
The first flowers come in year two. In year one the plant builds roots and a leaf rosette.
Sowing straight into the ground in May works too. The seed germinates at 15 degrees in the soil.
Position and soil
Sun or partial shade and free-draining soil that may well be poor and limy. Soapwort grows wild on railway embankments and gravelly slopes, and it takes drought once it has settled in.
Do not feed it. In rich soil the stems grow long and lie flat after rain. If it is already standing in fertile ground, cut it back to 20 cm in June so the plant stands firmer and flowers slightly later.
Keeping it in place
This is the most important section of the article. Soapwort spreads by rhizomes just under the surface and can take over several square metres in three to four years. It is not aggressive in the way ground elder is, but it does not stay where you put it.
Root barrier: sink an edging 25 to 30 cm deep around the spot.
Pot in the ground: plant it in a large pot with no base and bury the pot.
Dry, poor position: it spreads more slowly in gravel than in humus-rich soil.
Dig around the plant every spring and remove the runners with a spade.
If you want a plant with the same old-fashioned feel but without the spreading, baby's breath is an alternative, and it wants the same limy, dry soil.
The scent comes in the evening
The scent is faint in the middle of the day and only arrives towards dusk. That is no accident: the flower is built for moths, with a long tube and the nectar right at the bottom where only a long proboscis reaches.
So put it near a seating area or a window that stands open in the evening. More plants that draw insects are in the article on how to create a butterfly garden.
It lathers in water
Crush a handful of root or leaves, put them in lukewarm water and stir. After a few minutes you get a mild foam. That is the saponins, and it is the reason the plant was grown at monasteries and farms long before soap could be bought.
Textile conservators still use the method to wash old fabrics, because the solution is gentler on the fibres than modern detergent.
Two things to keep in mind. Saponins are toxic to fish and aquatic animals, so never pour the soap solution into a ditch or a watercourse. And do not eat the plant: the same compounds cause nausea.
Care through the year
June: cut back to 20 cm if the plant looks floppy.
July to September: remove faded flowers for a longer season and less self-seeding.
October: cut the whole plant back to 10 cm.
Every third year: lift, divide and replant a piece of the clump.
More perennials that cope with dry, sunny positions are in the article on perennials, and more species to sow yourself are among our flower seeds.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-08-31