- Name also: Calamus
- Family: Sweet-Flag Family – Acoraceae
(formerly Arum Family – Araceae)
- Growing form: Perennial herb. Rootstock almost horizontal, thick. Forming dense stands.
- Height: 60–150 cm (24–60 in.). Stem leafy, flatly 3-edged.
- Flower: Perianth regular (actinomorphic), green, very small; tepals 6, like sepals. Stamens 6. Gynoecium fused. Inflorescence a dense, abundantly flowered, slightly arched, green–brownish, 6–10 cm (2.4–4 in.) long spadix, which looks halfway up stem. Inflorescence’s subtending, leaf-like, sword-shaped spathe looks like a continuation of stem. Rarely flowers in Finland.
- Leaves: On base alternate, stalkless, 80–150 cm (32–60 in.) long. Base sheath-like. Blade sword-shaped, linear, often slightly arched, with tapered tip, rigid, with entire margins, wavy on one edge, glabrous, shiny.
- Fruit: Dry berry. Seeds are not produced in Finland.
- Habitat: Banks of shallow, clay-bottomed lakes, rivers and ponds.
- Flowering time: July–August.
Sweet flag is native to China and the Himalayas. It was brought to Europe by Tatars in the middle of the 16th century as they used it to clean their drinking water, and later it spread mainly as a medicinal herb, for which it has one of the best reputations all over the world. The herb’s rhizome, which has a fragrance of lemon or mandarin, is distilled into an aromatic oil which was valued for its ability to heal stomach troubles. Chewing a piece of the root was believed to ward off infectious disease, even plague, and it was also believed that cravings for nicotine could be alleviated in the same way. In many countries it has been used to flavour beer and wine, and it has also been made into jam. Sweet flag made itself at home in Finland during the Age of Utility in the 18th century, and it still grows as a legacy of its ancient cultivation as a herb on clay shores in shallow water in the south-west of the country as far north as the River Kokemäenjoki and to the west of Helsinki.
Sweet flag used to be classed in the same family as bog arum, and their relationship can be seen especially in their inflorescence: their immeasurably small individual flowers form a thick-columned spiked inflorescence, i.e. a spadix. Bog arum’s impressive inflorescence is enhanced by the pure whiteness of the inside of the spathe – sweet flag’s green spathe looks a lot like a stem and the spadix seems to be halfway up. Plants in Finland do not produce any seed, so the species spreads vegetatively, and bits of the root that are broken off by the ice can travel larger distances. Sweet flag is not able to spread to new water systems in this way, however, and with regards to suitable habitats it demands nutritious places.
When it is not flowering, sweet flag looks a lot like yellow iris (Iris pseudacorus). Sweet flag seldom flowers and is often easiest to identify by the pleasant lemony smell of its aromatic oil.