- Name also: Three-nerved Sandwort, Apetalous Sandwort (USA)
- Family: Pink Family – Caryophyllaceae
- Growing form: Annual, sometimes biennial, occasionally short-lived perennial herb.
- Height: 10–20 cm (4–8 in.). Stem limp–ascending, abundantly branched, finely haired.
- Flower: Corolla regular (actinomorphic), white, approx. 0.5 cm (0.2 in.) broad; petals usually. 5, entire, approx. 2 mm (0.08 in.) long, shorter than sepals. Sepals 5, tapered, clearly (1–)3-veined, broadly membranous margins. Stamens 10. Gynoecium syncarpous, with 3(–5) styles. Flowers 1–20.
- Leaves: Opposite, lower stalked, upper stalkless. Blade ovate–elliptic, taper-tipped, with entire margins, with hairy underside and margins, clearly 3-veined.
- Fruit: Spherical, yellowish, 6-valved, 3–4 mm (0.12–0.16 in.) long capsule. Seeds black, glossy, with small, white, oily elaiosome.
- Habitat: Stony ridges in rich forests, precipices, rocks in lush woods, on top of rocks, stream banks, clearfell sites, roadsides in forests, casually on lawns.
- Flowering time: (May–)June–July.
Three-veined sandwort is highly reminiscent of genus Arenaria plants, although the former has larger leaves (three-veined Sandwort’s leaves are at least 1 cm long, Arenaria leaves are 6 mm long at the most). It can be most easily confused with thyme-leaved sandwort (A. serpyllifolia) – Arctic sandwort (A. norvegica) and fringed sandwort (A. pseudofrigida) are rare, protected Lapland plants. Plants in both genera have entire, white petals, and the gynoecium has three bodies and a 6-valved capsule. Three-veined sandwort has a small, white elaiosome on its seeds which thyme-leaved sandwort lacks. This appendage contains oil which attracts ants, and as they eat it they drag the seed along with them and give it a ride to a new place to grow. Sometimes the seeds end up in the ant’s nest, and the plant might grow beside the nest.
Three-veined sandwort likes rich, damp forests and semi-shade, and it can be found growing alone or in small patches on moss-covered rocky ledges, on boulders, crags and beside streams – places that are too small for tree roots, and where the leaf canopy is therefore open. The upper parts of sea-shores where the waves have deposited e.g. algae offer the plant an open and nutritious habitat – it is however no salt-lover. Sometimes it grows on open, calciferous rocks. It is quite common in the south of Finland, but gets quickly rarer in the middle of the country, and its northernmost limit is around Oulu. Three-veined sandwort has benefitted from logging sites, the construction of forest roads and in general all human activity that fractures the forest. It is not really a plant that favours human activity, unlike chickweed, which it is often confused with.
Three-veined sandwort and common chickweed (Stellaria media) initially look a lot like each other, but on closer inspection a whole host of differences become apparent. Chickweed’s stem is only hairy on one side, while three-veined sandwort is short-haired all over; chickweed’s leaves are stalked and the leaf blade is feather-veined, three-veined sandwort’s stalks are almost non-existent and the leaf blade is clearly three-veined. More differences can be found in the small, white flowers: chickweed’s sepals are blunt and the petals are so deeply lobed that its five petals look like ten; three-veined sandwort’s sepals are sharp and its petals are unlobed.