Other Common Names: Wild Iris, Poison Flag, Water Flag, Fleur-de-lis.

The Blue Flag may cause trouble if its rootstock is eaten, as sometimes happens, in mistake for that of the Sweet Flag, Acorus calamus L. Violent vomiting, purging and burning, and congestion of the intestinal tract follow its ingestion.

The plant is erect, with thick, sword-shaped, bluish-green leaves and large flowers whose perianth of three large outer and three smaller, inner divisions is blue, changing to yellow towards the centre, and veined with purple. The pistil has three thin, petal-like stigmas, which arch closely over the stamens.