Growing form
Biennial herb.
Height
5–20 cm. (2–8 in.). With many stems. Stem with max. upper part lightly branched, glabrous.
Flower
Corolla wheel-shaped, light red, approx. 10 mm (0.4 in.) wide, fused, 5-lobed, long and constricted tube; throat lacking fringe. Calyx almost with separate leaves, 5-lobed, same length as calyx-tube. Stamens 5, anthers protruding from calyx-tube. Gynoecium composed of 2 fused carpels. Inflorescence a corymb, flowers often in small groups.
Leaves
In basal rosette and opposite on stem, stalkless. Rosette leaf blades obovate, with entire margin, parallel-veined, stem leaf blades linearly lanceolate, 1-veined.
Fruit
2-parted capsule.
Habitat
Gravelly and stony sea-shores.
Flowering time
July–August.

Seaside centaury grows on stony and shingly sea-shores, where the environment creates many challenges. During long and deep periods of low pressure the sea water can rise so high that the plants are submerged and flower unsuccessfully. If the conditions on the shore are ideal, the shore’s perennial salt-loving plants form such dense stands that biennial seaside centaury is unable to compete successfully on its own. The species have an underground seed bank, however, which awaits more favourable conditions. Overcrowding can be relieved by a harsh winter in which the sea ice erodes the plants’ stands. An exceptionally dry summer also thins the ranks of perennial competitors, and during the autumn rains sea century starts to grow and form its leaf rosette.

Sea centaury is one of the smallest sea-side plants. Although centauries typically grow in low-growing open places in waterside meadows, they are not so easy to notice when they are not flowering. After they have opened their flowers are a startling neon red, although this pales slightly as the inflorescence ages. Finland’s most northern sea centaury flowers decorate the shoreline on islands near Vaasa. Their northern limit is defined mainly by the length of the growing season: coastal centauries only start to flower at the end of the summer and are unable to ripen their seeds in the north before winter arrives. Although sea centaury is undoubtedly a shore plant in Finland, it is not dependent on salt.

Sea centaury is different from its close relative lesser centaury (C. pulchellum) with regards to the stem and also the size of the flowers. Lesser centaury’s calyx is also shorter than the calyx-tube, while on sea centaury it is at least the same size. Annual lesser centaury doesn’t have a leaf rosette, while biennial sea centaury does, and its remains can still be seen while it is flowering.

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