- Name also: Yellow Flag, Yellow Flag Iris, Pale-yellow Iris, Water Flag
- Family: Iris Family – Iridaceae
- Growing form: Perennial herb. Rootstock thick, creeping.
- Height: 60–120 cm (25–50 in.). Stem sparsely branched, round, full.
- Flower: Perianth regular (actinomorphic), bright yellow, up to 10 cm (4 in.) broad; tepals 6 in two separate whorls. Outer tepals 3, downward-curved; inner tepals 3, smaller than outer tepals, narrow, erect. Stamens 3. Styles 3, stigmatic lobes big, like tepals, inner perianth longer than leaves. Inflorescence 4–12-flowered, cyme-like.
- Leaves: On base, stalkless, 50–90 cm (20–35 in.) long, smaller than stem-leaves. Blade sword-shaped, with entire margins, parallel-veined, greyish green.
- Fruit: Ovoid, 3-lobed, 4–5 cm (1.6–2 in.) long capsule.
- Habitat: Clay, sludge and mud shores, alder thickets, swamps, ditches, shallow water. Also an ornamental.
- Flowering time: June–July.
- Endangerment: Protected in the provinces of Oulu and Lapland.
Handsome-flowered yellow iris’s more characteristic habitats are rich sludge and mud shores from the water line to the wet surfaces of alder groves. It is most typically found in gently-flowing river banks and sheltered bays – in open parts of inland waters it is already clearly rarer and the outermost stands cling on to rocky outcrops in the archipelago. Yellow iris grows commonly as far as southern parts of northern Häme and northern Savo, but it grows as far north as northern Ostrobothnia. Yellow iris’s own ways of spreading would hardly have been efficient enough to give it such a large habitat if humans had not been involved: it is one of Finland’s most impressive wild flowers and is often transplanted from natural wetlands to summer cottages and lakeside villas, and even into flower beds. The best places for it in the garden are the edges of water features and garden ponds, especially if the soil is rich.
Yellow iris is much loved in Finland, as can be seen in the common names it goes by. Its flower was probably used as a model for the lily in coats-of-arms, and it featured in the crests of e.g. the French royal family and the Crusaders. Nowadays the best known version is probably as the lily that represents the World Organization of the Scout Movement. The flower, which can be up to over 10 cm across, would start to be a waste of resources, so it works as three separate units. The pollinating bumblebee has to visit all three parts of the flower and on the other hand three insects can visit the flower at the same time. It is worthwhile keeping an eye on yellow iris after it has flowered as the leaves start to make strange-looking oval, elongated capsule fruits. The flat seeds float well, and the plant also spreads through pieces of its rhizome that have broken off. Yellow iris is the provincial flower of Kymenlaakso.