Plants perennial, matted. Tap-roots filiform; rhizomes weak, slender, 0.5–15+ cm. Stems 5–40+, flowering stems erect to ascending, green, 2–8(–10) cm; internodes terete to angular, 1/2–4 times as long as leaves, shiny, glabrous or occasionally sparsely stipitate-glandular at distalmost internodes. Leaves connate basally, with scarious sheath 0.2–0.3 mm, sessile; blade weakly to prominently 1-veined, narrowly oblong or elliptic to ovate-lanceolate, 2–12 × 1–3 mm, subsucculent, margins thickened, herbaceous, shiny, smooth, apex blunt to acute, prominently pustulate or not, glabrous; axillary leaf clusters absent. Inflorescences terminal, open, leafy, 1–3-flowered cymes or often flowers solitary. Pedicels erect in fruit, 1–10 mm, minutely retrorsely pubescent to stipitate-glandular. Flowers: sepals green, 3-veined, 2 lateral veins 1/2–3/4 times as long as midvein, more prominent in fruit, not keeled, ovate (herbaceous portion ovate), 2.5–4 mm, not enlarging in fruit, apex obtuse or somewhat acute, not pustulate, glabrous; petals oblanceolate, 3–4 mm, ± 11/2 times as long as sepals, apex acute. Capsules tightly enclosed by calyx, broadly ellipsoid, 4–4.5 mm, ca. 11/4 times as long as sepals. Seeds 12–18, brown, reniform, compressed, 0.5–0.7 mm, shiny, faintly rugulose. 2n = 40, 44 (both Europe).
Phenology: Flowering spring–summer.
Habitat: Moist, calcareous gravels and rock crevices
Elevation: 0-100 m
Greenland, St. Pierre and Miquelon, Man., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Europe.
Arenaria humifusa is similar to A. longipedunculata, but is smaller in some dimensions than that species.