A spreading herbaceous species which forms clumps of up to 1 metre in height and 1.5 metres in diameter. Spreads by underground rhizomes and often forms dense swathes. Leaves are tough or leathery and can reach 85 centimetres in length, though 30-50 cm is more usual. Flowers are carried in loose clusters on tall flower spikes which exceed the foliage in height. Each flower is blue with yellow filaments and brown or black anthers. Flowers are followed by green fruits which turn a vivid blue at maturity. Each contains several dark, glossy seeds.