Yellow Flag is a tall handsome perennial. The flowers are a strong yellow colour. The leaves are stiff and sword-like. It grows by freshwater, river and lakesides, marshes and ditches; sometimes it takes over large parts of wet fields. This can be a wonderful sight in June. In the old days leaves of this plant were sometimes used for thatching and bedding. Country people traditionally placed flowers of Yellow Flag outside their houses as a decoration for the Corpus Christi festival. Caleb Threlkeld called the plant Yellow Water Flower de Luce. There are cultivated varieties of this Iris, 'Bastardii', with pale yellow flowers, a deeper yellow 'Golden Queen' and 'Variegata' with yellow stripedfoliage.
The flowers are in bloom from June to August. Yellow Flag grows in the same conditions in Britain and in Europe, North Africa and western Asia.