- Family: Pink Family – Caryophyllaceae
- Growing form: Perennial herb.
- Height: 5–15 cm (4–8 in.). Stem erect–ascending, often purple.
- Flower: Corolla lacking. Calyx regular (actinomorphic), green, 3.5–4.5 mm (0.14–0.18 in.) wide, fused, 5-lobed, lobes blunt, with broadly membranous margins. Stamens usually 10. Gynoecium syncarpous, with 2 styles. Dense flower clusters terminate stems. Subtending bracts not usually extending above inflorescence.
- Leaves: Opposite, stalkless, united in pairs, often curved in one direction. Blade needle-like, with entire margin and hairy base, greyish bluish green.
- Fruit: 3–4 mm (0.12–0.16 in.) long achene attached to calyx. Calyx lobes erect–curved inwards.
- Habitat: Roadsides, paths, fields, embankments, ballast soil heaps, sandy areas, rocky meadows.
- Flowering time: June–September.
- Endangerment: Endangered.
Perennial knawel grows in Finland at the northernmost reaches of its habitat: there is a small, fragmented stand in the south-westernmost corner of the country, on the Åland Islands. The plant has also settled in well in Hamina, an old garrison town, where it most likely arrived from Russia. Nowadays it grows on the mainland both in Hamina and nearby Vehkalahti, and as a casual species as far north as Oulu. Perennial knawel has always grown close to human activity, on ballast soil deposits, roadside embankments, playing fields and rocky outcrops, and beside streets and paths. It is easily choked out by larger competitors, but its stout, deep-reaching rootstock helps it thrive in dry, open terrain. In drier summers it can disappear from sight, but it reproduces itself from its seedbank in more favourable times. Overgrown grazing lands, asphalted roads and grassed-over recreational areas have made urban environments look tidier but have encroached upon perennial knawel’s habitats.
Usually in the world of plants, the modesty of flowers is a sign that they self-pollinate. Perennial knawel’s flowers don’t really look like anything special to the human eye, but the plant is in fact insect-pollinated: pollen is carried mainly by ants, which also transport the ripe seeds at the end of the summer.
Perennial knawel can be easily confused with its more common relative annual knawel (S. annuus). It is an annual herb which favours more disturbed soil than perennial knawel. Annual knawel is bright green while perennial knawel is greyish bluish green. Both knawels’ flowers are very modest, but perennial knawel’s calyx lobes are blunt with a wide, white membranous margin; annual knawel’s calyx lobes are tapered and have only narrow membranous margins.