The blue flag is also known as the wild iris, poison-flag, water flag, fleur-de-lis, flower-de-luce.
The blue flag is an erect, perennial herb from one to two feet high, springing from a thick, fleshy, horizontal rootstock. The flowers are from three to four inches across, violet blue variegated with yellow, green and white towards the centre, with purple markings. The three outer parts of the perianth are larger than the three inner, upright, narrow ones. The three stamens are concealed under the style branches. The leaves are bluish-green, sword-shaped, overlapping. The seed-capsule is upright, oblong, with a stout beak splitting into three pockets containing many reddish-brown seeds somewhat three-cornered and flattened. The flowers are in bloom in May and June, sometimes even in July.
The blue flag is native to Canada, and is found from Newfoundland to Manitoba in wet places, along the borders of streams and shallow waters.
The rootstock is poisonous. It contains the acrid, resinous substance irisin or iridin. When eaten, it produces nausea, vomiting, purging, and pain. It is often mistaken for the sweet flag (Acorus Calamus L.) which is not poisonous, and is masticated by some people as a cure for indigestion. When in flower, the two plants are so dissimilar that they could never be taken for one another, but in the autumn when the roots are gathered, nothing remains of the upper portion of the plants. Even then, however, they may be distinguished by their odour, the sweet flag being pleasant aromatic, while the blue flag is unpleasant and nauseous.