Acorus calamus (also called sweet flag or calamus, among many common names) is a tall perennial wetland monocot of the Acoraceae family, in the genus Acorus.

Sweet Sedge is a perennial herb, 30 to 100 cm tall. In habit it resembling the Iris. It consists of tufts of basal leaves that rise from a spreading rhizome. The leaves are erect yellowish-green, radical, with pink sheathing at their bases, sword-shaped, flat and narrow, tapering into a long, acute point, and have parallel veins. The leaves have smooth edges, which can be wavy or crimped. The sweet sedge can easily be distinguished of those of the Iris, and other similar plants, by the unusual crimped edges of the leaves, the fragant odour the emits when crushed, and the unusual flower spadix.

The solid, triangular flower-stems rise from the axils of the outer leaves. A semi-erect spadix emerges from one side of the flower stem. The spadix is solid, cylindrical, that tapers at each end, and is 5 to 10 cms. in length. A covering spathe, as is usual with Acoraceae, is absent. The spadix is densely crowded with tiny greenish-yellow flowers arranged in diamond-shaped pattern. Each flower contains six petals and stamens enclosed in a perianth with six divisions and surrounding a threecelled, oblong ovary with a sessile stigma. The flowers are sweetly fragrant. In Europe it flowers for about a month in late spring or early summer, but usually does not bear fruit there. Only plants that grow in water bear flowers. The fruit is a berry filled with mucus, which when ripe falls into the water and thus disperses. Even in Asia it fruits sparingly and propagates itself mainly by spreading its rhizome, forming colonies.

The branched, cylindrical, knobby rhizome is about a finger thick and has numerous coarse fibrous roots below it. The exterior is brown and the interior white.

Sweet Flag is native to India, central Asia, southern Russia and Siberia, and perhaps Eastern Europe. It also grows in China and Japan. It was introduced into Western Europe and North America for medicinal purposes. Habitats include edges of small lakes, ponds and rivers, marshes, swamps, and wetlands.

In addition to “sweet flag” and “calamus” other common names include beewortbitter pepper rootcalamus rootflag rootgladdonmyrtle flagmyrtle grassmyrtle rootmyrtle sedgepine root, rat root, sea sedgesweet cane, sweet cinnamonsweet grasssweet myrtlesweet rootsweet rush, and sweet sedge. Common names in Asia include: “Changpu 菖蒲” (Mandarin Chinese); “shoubu 菖蒲” (Japanese); “vacha”; “changpo 창포” (Korean); “bacch” (Unani); “bajai”, “gora-bach”, “vasa bach” (Hindi); “vekhand” (Marathi); “vasambu”/வசம்பு (Tamil); “vadaja”, “vasa” (Telugu); “baje” (Kannada); “vayambu” (Malayalam); Haimavati, “bhutanashini”, “jatila” (Sanskrit), “kâmpean” កំពាន (Khmer) and “bojho बोझो” (Nepali).