Blue flag  (Iris versicolor)

The rhizome, or underground stem, of the blue flag (indicating its showy blue flowers) is used medicinally. Blue flag and closely related species (particularly Iris missouriensis, western blue flag) grow across North America.

Uses
It is used as a nonspecific immune enhancer, as a laxative, and to detoxify the intestinal tract. Topical application of fresh, sliced rhizomes to the sores of impetigo (a common bacterial skin infection in children) has been recommended by herbalists. Traditional herbalists have used blue flag to treat poor digestion characterized by fat malabsorption.

Side effects:

Blue flag can cause nausea, vomiting, and loose stools if too much is taken. People should not exceed the recommended amount given above. Fresh rhizome should only be applied topically and never taken internally, since it can irritate the mouth6 and is much more likely to cause nausea and diarrhea. Blue flag should only be taken on the advice of a physician or herbalist trained in its use. Blue flag is unsafe for use during pregnancy or breast-feeding. People should not give blue flag to children