It has short, branching, stout, woody rhizomes. They can be up to 1.5 cm in diameter. Underneath, they have fleshy roots. Over a long period, they can form large clumps of plants. It has basal leaves, that are erect, between deep green and dark green, stiff and (lanceolate) sword-like. They can grow up to 60–90 cm (24–35 in) long and 1–2 cm wide. The leaves are not usually as long as the flowering stems. The flower stalk begins to grows in April and then reaches maturity in May. The solid, slightly flattened stems, about 1-cm wide, can generally grow up to between 40–90 cm (16–35 in) long. Although, larger forms are known to be much bigger, growing up to between 90–170 cm (35–67 in) long. It has normally 1 or more short branches. The stems have 2–3 spathes (leaves of the flower bud), which are papery (in form) and 9–12 cm (4–5 in) long. The stems (and branches) hold 2–5 terminal (top of stem) flowers, which open in succession, blooming in late spring, or summer, between May and July. The large flowers are 8–10 cm (3–4 in) in diameter, and are white or near-white. It has 2 pairs of petals, 3 large sepals (outer petals), known as the 'falls' and 3 inner, smaller petals (or tepals, known as the 'standards'. The falls are spreading, arching downwards, with a large egg-yolk yellow central area. They are 8–10 cm (3–4 in) long and 3–6 cm wide. The standards are erect, spatulate (spoon-like), with a yellow centre section surrounded in white. They are 4–8 cm (2–3 in) long and 1-1.5 cm wide. The flowers are pollinated by insects. It has 1–2.5 cm long, white, funnel-form or cup shaped perianth tube, 4–6 cm (2–2 in) long, white style branches, and 2 lobed stigmas. After the iris has flowered, it produces an ovoid to oblong-elliptic, triangular in cross section, seed capsule, 4–6 cm (2–2 in) long and 2-2.5 cm wide, with a beaked top. The brown seed capsule has 2 ribs. Inside the seed capsule, are 2 rows of papery, wrinkled, white, flattened or wedged-shaped seeds, that are 4-5mm across.