- Family: Pink Family – Caryophyllaceae
- Growing form: Perennial herb.
- Height: 5–20(–25) cm (2–10 in.). Stem limp–erect, sparsely branched, glabrous or with glandular hairs.
- Flower: Corolla regular (actinomorphic), white–reddish, 8–10 mm (0.32–0.4 in.) broad; petals 5, approx. 2–3 times as long as sepals, 3–6.5 mm long. Sepals 5, often reddish. Stamens usually 10 (sometimes 8 or 11). Gynoecium syncarpous, with 5 (occasionally 4) styles. Flowers solitary or in sparse groups. Buds spherical.
- Leaves: In basal rosette and on stem opposite, stalkless, united in pairs, becoming smaller towards top, bulbils in upper axils. Blade thread-like, with entire margins, glabrous, sometimes fleshy.
- Fruit: Egg-shaped, yellowish gray, 5 or occasionally 4-valved capsule.
- Habitat: Gravelly and sandy seashore meadows, rocks and lawns, sometimes lake shores, dry lime or serpentine rocks, wasteland.
- Flowering time: July–August.
Low-growing, prostrate and delicate knotted pearlwort cannot grow in an overcrowded environment, but it has found its place in cracks in rocks by the seaside and pockets of mineral soil which the ice and the breakers keep open. The species is predominantly an archipelago plant, and on the mainland it can be found mainly on exposed coastline.
Knotted pearlwort protects itself against the swell of the sea with narrow, needle-like leaves and a deep-reaching rootstock. Sometimes the ice or a storm can break the flower-stems, but a dense leaf rosette against the ground protects it from the forces of nature and produces new shoots to compensate. In bad weather knotted pearlwort closes its flowers, in which case it can be hard to find. When its white petals open to their full extent they look relatively large compared to the generally delicate structure of the plant and the bareness of the surrounding land. Insects do not visit much, although some bees and flies pop in, and self-pollination appears to be normal. The plant produces seeds but it also propagates itself vegetatively through axillary bulbils which develop into new plants after they break away from the parent plant. Sometimes they only become independent when the parent plant dies. The bulbils’ compact, bulb-like leaf bunches make the shoot look delightfully knotted, and have given the plant its name.
Knotted pearlwort is not just a seashore plant: there are a handful of separate stands inland in southern and central Finland along lake shores, and in Lapland beside lakes and streams. The species also grows on calciferous rock in Lohja and ultra-alkaline rocks in Kaavi, and industrial carbon waste sometimes offers the plant suitably hostile conditions. Many other Pink family members are known for their ability to adapt to grow on special soils. Knotted pearlwort is not a particular alkaline serpentine plant, but variations occur within the species: plants that grow in Lapland have less bulbils than those that grow by the sea in the south.