- Name also: Slender Bog Arrow-grass, Marsh Arrowgrass
- Family: Arrowgrass Family – Juncaginaceae
- Growing form: Perennial herb. Rootstock short. With runners, overwintering bud appears on runners in autumn.
- Height: 5–50 cm (2–20 in.). Stem unbranched, approx. 1 mm (0.04 in.) thick to base.
- Flower: Regular (actinomorphic), 2–3 mm (0.08–0.12 in.) wide. Tepals 6, like sepals, green. Stamens 6. Pistils 6. Inflorescence long, a lax raceme, flower-stalks almost erect, parallel to flower stem.
- Leaves: With basal rosette, stalkless, erect. Blade linear, semi-cylindrical, 0.5–2 mm (0.02–0.08 in.) wide.
- Fruit: Narrowly club-like, 6–9 mm (0.25–0.35 in.) long, schizocarp splitting into 3 from base.
- Habitat: Shores, springs, swamps, damp meadows, ditches.
- Flowering time: June–July.
Marsh arrow-grass is in every way more delicate and modest than its relative sea arrow-grass (T. maritima) – and it is also less demanding in relation to its habitat, being happy not just on seashores but also rich lake shores, springs, damp meadows and rich swamps. It also grows in man-made habitats like shoreside meadows, damp fields, beside paths and at the bottom of ditches. Marsh arrow-grass is quite common – indeed it can be quite abundant – across Finland, but it can be easily missed by those whore are not keeping their eyes peeled. It is however easy to recognize: an erect stem, which is terminated in a modest flower, rises out of a leaf rosette formed by narrow, ascending oblique leaves.
Like the other arrow-grasses, marsh arrow-grass is wind-pollinated and flowers in three phases: first the ripe pistils, then the outer stamens open and, as these fall away, finally the inner stamens open. Marsh arrow-grass is very distinctive, and even when it is ripening its seeds it has a long raceme full of erect, narrow, 3-lobed schizocarps. The structure of marsh arrow-grass’s schizocarps is the basis of the species’ scientific name: Triglochin means three tips. The sharp spines on the underside of the carpels easily find their way onto feathers and fur, spreading the species to new habitats.