Acorus americanus (Rafinesque) Rafinesque; A. angustatus Rafinesque; A. angustifolius Schott; A. asiaticus Nakai; A. calamus var. americanus Rafinesque; A. calamus var. angustus Besser; A. calamus var. angustifolius (Schott) Engler; A. calamus var. spurius (Schott) Engler; A. calamus var. verus Linnaeus; A. calamus var. vulgaris Linnaeus; A. cochinchinensis (Loureiro) Schott; A. griffithii Schott; A. spurius Schott; A. triqueter Turczaninow; Orontium cochinchinense Loureiro.

Rhizome stout, 4-10(-20) × (0.8-)1-1.5(-3) cm, aromatic; roots at lower side of rhizome. Leaves several, mid-green, often reddish at base, ensiform, (60-)70-100(-150) × (0.7-)1-2(-2.5) cm (mostly 1-1.5 cm wide), apex acuminate; midrib conspicuous on both sides. Peduncle compressed triangular, (15-)40-50 cm. Spathe mid-green, leaflike, 30-50 cm, acute. Spadix straight or slightly curved, erect, oblique, narrowly conic to subcylindric (tapering toward apex), 4.5-6.5(-8) × 0.6-1.2(-1.5) cm, densely flowered. Flowers yellowish green, 1.8-2 mm in diam. seen from above; tepals oblong, 2.5-3 × 1-1.2(-1.4) mm, keeled, membranous, apex triangular hooded; filaments oblong, flat, 2-2.5 × 0.3-0.5 mm, anthers cream-colored, 0.4-0.5 mm in diam.; pollen grains ca. 20 µm, exine shallowly and remotely foveolate; gynoecium obconic-cylindric, 2.5-3.5(-4) × (0.8-)1-2.3 mm, with conic, spongy apex and stigma very small. Infructescence 1.5-2 cm in diam., straw-brown at maturity, berries densely arranged. Berry oblong-obovoid, 1- to few seeded, (3.5-)4-4.5 × 2-3(-3.5) mm. Seed oblong-ellipsoid to ovoid, 2.5-3(-4) × 1-1.2(-1.8) mm, without bristles; testa light brown, subsmooth and slightly foveolate. Fl. (Feb-)Apr-Sep. 2n = 24, 36, 48.

For a full synonymy, see Govaerts et al. (World Checkl. & Bibliogr. Araceae, 545-553. 2002).

Acorus calamus is diploid, triploid, and tetraploid. Diploids are known to grow naturally in E Asia (Mongolia and C Siberia, at least) and North America; tetraploids are known only from Asia (India, E Siberia, and Japan); and triploids are typical for the plants in Europe, SW Asia, India (Himalayan region), and E North America. The triploid cytotype probably originated in the Himalayan region, as a hybrid between the diploid and tetraploid cytotypes. It then probably dispersed naturally or with humans to Sakhalin and with humans to Turkey, then to Europe, and finally to E North America as a medicinal plant (Evstatieva et al., Fitologiya 48: 19-22. 1996; Löve & Löve, Proc. Genet. Soc. Canada 2: 14-17. 1957).

The different cytotypes show a great morphological variability and also a large variation in the chemical composition of the essential oils from the rhizome and leaves. As a result, they have been considered as representing species or varieties: Acorus calamus var. calamus (or A. calamus var. vulgaris) for the triploids, A. calamus var. americanus for the diploids, and A. calamus var. angustus for the tetraploids. Acorus calamus is considered in this treatment as a variable species, and infraspecific taxa are not recognized because there is an overlap in the width of the leaves, from 0.8 cm to 1.5 or 2 cm, also from Asiatic collections from different geographical regions, and furthermore the length of the spadices is variable. It can be observed that several collections from Asia have somewhat narrower leaves, ca. 1 cm wide; but there are also broader ones, and there is a continuous series.

Swamps, pond sides, standing water, also cultivated; below 2800 m. Throughout China [Afghanistan, Bangladesh, Bhutan, India, Indonesia, Japan, Korea, Malaysia (Sarawak), Mongolia, Nepal, Pakistan, Russia (Far East, Siberia), Sri Lanka, Thailand, Vietnam; SW Asia, Europe (except S), North America].