- Name also: German Knotweed, German Knotgrass, German Moss
- Family: Pink Family – Caryophyllaceae
- Growing form: Annual or biennial herb. Sometimes tufted.
- Height: 5–25 cm (2–10 in.). Stem limp–quite erect, branched, rigid, sometimes lower part reddish.
- Flower: Corolla lacking. Calyx regular (actinomorphic), green, approx. 3–4 mm (0.12–0.16 in.) broad, fused, 5-lobed, lobes tapered, narrowly membranous margins. Stamens 2–5. Pistil of 2 fused carpels, styles 2. Flowers in dense terminal clusters, sometimes also axillary. Subtending bracts usually extending above inflorescence.
- Leaves: Opposite, stalkless, united in pairs, often slightly curled. Blade linear–needle-like, with entire margins, basal part hairy, bright green.
- Fruit: 2–4 mm (0.08–0.16 in.) long achene attached to calyx. Calyx lobes ascending oblique–curved inwards.
- Habitat: Rocks, Meadows, sandy areas, arable land, wasteland, streets, roadsides, banks.
- Flowering time: June–September.
Annual knawel’s flowers are so small and modest that many people can miss them completely. They lack a colourful corolla and do not secrete nectar so insects are not interested. Through self-fertilisation, however, it is able to produce achenes, seeds and fruits that don’t open. The receptacle hardens around the achene into a stiff, hard surface, which has given the plant its scientific name: skleros, ‘hard’, anthos, ‘flower’. Despite its species name annuus, meaning annual, it can overwinter as a young plant beneath the snow.
In Finland annual knawel is follows people, and it is so old and established in Finland that it is hard to believe that it is an alien. In densely populated areas it often replaces natural lichen. If it gets trampled too much it disappears from the busiest places.
Annual knawel mutates a lot and there are many stands with hereditary differences. Ssp. polycarpos grows compactly and has short internodes and leaves on the stem. It’s quite broad, short calyx lobes are turned inwards and are closed. It is more clearly a field and rock plant than ssp. annuus, which grows in Finland like a weed and is straggly branched, long-leaved and jointed. Its flowers are more open and also axillary, unlike ssp. polycarpos. Its close relative perennial knawel (S. perennis) can be distinguished by its broadly white-margined calyx lobes; annual knawel’s membranous margin is narrow in all the subspecies. Perennial knawel is mainly greyish green, while annual knawel is mostly bright green. The species rarely grow together as they prefer different habitats. Perennial knawel thrives beside other plants in sandy areas and on meadows, but it doesn’t do well in the culturally influenced places that annual knawel likes.