Leaves usually 60–120 cm long, 2–3 cm wide. Inflorescence with scape up to 150 cm long, 1- or 2-branched (sometimes simple), with 2–4 condensed 2–6-flowered clusters. Primary bracts 10–40(–60) cm long. Flowers predominantly magenta to purple. Perianth tube 1.5–2 cm long; sepals 6–10 cm long, 3.5–5 cm wide, spreading to slightly recurved, with distinctive yellow 'signal' of 3 or 5 papillose lines extending just beyond the stigma lobes; petals smaller and relatively narrower than sepals, 5.5–7 cm long, c. 3 cm wide, ascending to spreading, glabrous. Capsule 6-angled in section. Seeds brown. Flowers Oct.–Nov.
North-central to south-eastern U.S.A., and southern Canada; widely cultivated. Currently known from the Broken Creek at Nathalia, north-central Vic. where growing in water to c. 1 m deep. Reportedly established from discarded cultivated plants sometime before 1990.
The 'Louisiana hybrid' group of Iris is derived from five closely related aquatic or semi-aquatic species, I. brevicaulis Raf., I. fulva Ker Gawl., I. giganticaerulea Small, I. hexagona Walt. and I. nelsonii Randolph. Some hybridisation occurs in nature, but extensive breeding in horticulture has produced a wide array of attractive cultivars. The Nathalia plants approach I. x vinicolor, a hybrid between I. giganticaerulea and I. fulva.