- Name also: Sea Milkweed, Black Saltwort
- Latin synonym: Lysimachia maritima
- Family: Primrose Family – Primulaceae
- Growing form: Perennial herb. Rootstock creeping, branching.
- Height: 3–25 cm (1.2–10 in.). Stem ascending–erect, unbranched–branched at base, glabrous.
- Flower: Corolla lacking. Corolla-like calyx regular (actinomorphic)–campanulate, light red and dark-spotted, 3–6 mm (0.12–0.24 in.) wide, fused, 5-lobed till halfway, lobe margins white, membranous. Stamens 5. Pistil a fused carpel. Flowers solitary in axils.
- Leaves: At most opposite, upper part alternate, stalkless, slightly amplexicaul. Lowest leaves scaly, brown. Upper leaves with blade ovately lanceolate–elliptic, fleshy, glabrous, bluish green, faintly dark-spotted.
- Fruit: Spherical, 3 mm (0.12 in.) long capsule.
- Habitat: Beaches, waterside meadows, rocky outcrops and strips of grassy land in cracks in the rocks.
- Flowering time: June–July.
Sea milkwort thrives, as its name suggests, on shores, the rocky lower parts of seaside meadows that get flooded by sea water, cracks in stones, gravels and among mixed grasses. The species can stand cold seawater, a salty growing medium and even occasional floods. Its thick, juicy leaves and stems cope very well with dampness and the wind is unable to damage the low, creeping stem. Further up the shore the influence of the sea diminishes, the soil changes and more plants grow, but plants like milkwort do not thrive there amongst the increased competition. In Finland milkwort is common all along the coast, while elsewhere it can be found also inland, around salty springs, salt flats and even lake shores.
Milkwort increases efficiently vegetatively and uses its runners to form wide, pure stands that can cover several square metres. The part that is above the ground dies in the autumn, but in the spring the buds on the tips begin to grow again. This usually repeats at least a couple of times in the plant’s life before it flowers for the first time. The structure of the flower is clearly different from other primrose plants: its petals have withered to nothing, but its campanulate calyx is colourful like a corolla. The flowers’ pollinators appear to be ants. The seeds lose their viability quite soon if they are floating in salt water, but the species also propagates itself through pieces of floating rootstock.