- Name also: Giant Chickweed
- Latin synonym: Stellaria aquatica
- Family: Pink Family – Caryophyllaceae
- Growing form: Annual–perennial herb.
- Height: 15–40 cm (6–15 in.). Stem limp (sometimes ascending), often many-branched, bristly, fragile, glabrous base, upper part straight and glandular-haired.
- Flower: Corolla regular (actinomorphic), white, approx. 1–1.5 cm (0.4–0.6 in.) wide; petals 5, deeply 2-lobed (appearance of 10 petals), 5–7 mm (0.2–0.3 in.) long, same length as sepals or slightly longer. Sepals 5, blunt, glandular-haired. Stamens 10. Gynoecium syncarpous, usually with five styles. Inflorescence lax; subtending bracts green.
- Leaves: Opposite, lower part stalked, upper part stalkless. Blade ovate–elliptic, cordate–blunt base, with entire and wavy margin, glabrous–glandular-haired, dark green.
- Fruit: Capsule splitting into 5 lobes, lobes shallowly pinnatifid.
- Habitat: Shores, shoreside hedgerows, alder swamps, broad-leaved shoreside forests, ditch banks, cultivated land, harbours, wasteland, yards.
- Flowering time: July–August.
Water chickweed has a genus all to itself. There have been attempts to put it in with genera Mouse-ear Chickweed (Cerastium) and Chickweed (Stellaria), but due to its many differentiating properties it was finally decided that it was better off in its own genus.
Water chickweed is highly reminiscent of wood stitchwort (Stellaria nemorum), which it is often confused with. There is a clear difference in their appearance however: water chickweed is always limp-stemmed, and its leaves are denser than wood stitchwort’s. Water chickweed’s stem has a number of stalkless leaf-pairs, while wood stitchwort’s leaves are stalked. The upper part of water chickweed’s stem, the flower-stalks and calyx are glandular-haired, and the calyx is matt. Wood stitchwort on the other hand is clearly more glabrous and its calyx is noticeably shinier. The matter can finally be decided by checking the gynoecium: water chickweed has five stamens and wood stitchwort has three.
Water chickweed can be found in many places, mainly in the south of Finland. It is even quite common in places such as south Häme and south-east Finland, while there are more northerly stands in Kainuu and northern Ostrobothnia. Water chickweed is, as its name suggests, fond of damp habitats, but they are quite broad-ranging. It can be found on the beaches, even rocky places that are pounded by waves, though it is most common among shoreside hedgerows and broad-leaved forests. It might appear close to harbours and other places where people are active, in ditch banks, walls, yards and gardens. Human activity has a positive effect on the appearance of water chickweed in the Finnish wild to the extent that it is suspected that it only appeared in the wild in Finland at the same time as people. It is however apparently at least partly native to southern Finland.