- Name also: Seashore Aster
- Latin synonym: Aster tripolium, Tripolium vulgare
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Biennial or short-lived perennial herb.
- Height: 10–60 cm (4–25 in.). Stem juicy, glabrous–virtually glabrous, usually reddish.
- Flower: Single flower-like approx. 2–3 cm (0.8–1.2 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets pink or blue (occasionally white), tongue-like; disc florets yellow, tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucral bracts overlapping in 2–3 rows, round-tipped. Capitula borne in a corymbose cluster.
- Leaves: Alternate, lowest stalked, upper stalkless, amplexicaul. Blade linear–narrowly lanceolate, glabrous–short-haired, with entire or toothed margins, fleshy.
- Fruit: Flat, hairy, yellowish brown achene, tip with light gray unbranched hairs.
- Habitat: Rocky seashores, crevices, clayey or silty sea-shore meadows.
- Flowering time: June–September.
Sea aster can hold its head up high in the company of American aster (Aster) and China aster (Callistephus) from Asia, which are both commonly grown as garden ornamentals. This is only more impressive when one considers that the plant grows among rocky outcrops at the water line, patches of salty land or cracks in coastal rocks almost entirely without companion plants. Typical accompanying plants such as sea arrow-grass, saltmarsh rush and sea plantain are unable to compete with regards to their flowers. Sea plantain’s assets in a merciless environment are its strong vertical rootstock, which it tightly embedded in the ground, and its narrow, thickly fleshy leaves that are immune to any potential harm from the salt and the waves. The plant defiantly resists the relentless battering of the waves and the sea’s attempts to bury it beneath the debris that it brings ashore, although stones that are tossed around in the water can damage the leaves and sometimes even break the stem.
Sea aster is absent from the outermost islands of the archipelago, which have rocky shorelines that have a gentle incline and would thus leave the plants too exposed to the elements. Sea aster is a halophyte, meaning that it grows in places that are affected by salinity, and indeed it is generally found in very salty areas – the Gulf of Finland and the Bay of Bothnia, however, are not very salty and the plant is thus rare. Its occurrence in the inner archipelago is limited by the lack of salt in inland waters and also the lack of suitable coastline in inhabited areas. On the largest islands and mainland coasts sea aster seeks out the most open and protected areas. The sea keeps its habitats wild long into the summer. The species can start to flower already around Midsummer along with most sea-shore flowers, but it often stays in bloom until well into the autumn. It is dangerous to bloom late: if autumn comes early the sea levels will rise and its pollen will be ruined. If there is a long Indian summer, however, it is able to produce an abundance of seed. Sea aster is biennial, although it is sometimes able to flower for three summers. The number of flowering plants is strongly affected by the autumn weather conditions two years previous.