Acorus is a small genus of monocot flowering plants comprising the entirety of the family Acoraceae and the order Acorales. Phylogenetic studies place it as the sister group to all other monocots — the oldest surviving lineage within the monocot clade — making it of considerable interest in understanding early flowering plant evolution. Although historically classified within Araceae (aroids) under systems such as the Cronquist system, molecular systematics have demonstrated that Acorus is not closely related to Araceae or any other monocot family, a finding now formalised by the APG III and APG IV systems.
The genus contains a small number of species (the Kew Checklist accepts four) characterised by sword-like, parallel-veined leaves and a distinctive spadix inflorescence. Plants grow in wetland habitats — particularly marshes, shorelines, and floodplains where water levels fluctuate seasonally — and spread through thick creeping rhizomes. Like many marsh plants, Acorus relies on aerenchyma tissue to transport oxygen to the rooting zone. The dried leaves of some species yield a sweet fragrance from ethereal oils, a quality that made strewing the cut leaves across floors a common practice in medieval Europe.
The best-known member is Acorus calamus (common sweet flag), a sterile triploid of likely cultivated hybrid origin that has been widely naturalised across Europe, South Asia, and eastern North America. Acorus gramineus (Japanese sweet flag) is a fertile diploid native to East and Southeast Asia and widely grown as an ornamental. Products derived from A. calamus were banned as food additives in the United States in 1968 due to the carcinogenic compound β-asarone; notably, diploid strains from North America and parts of Asia do not produce this compound.
Etymology
The genus name Acorus derives from the Greek acoron, a term used by the ancient physician Dioscorides, which itself came from coreon meaning "pupil." The name reflects the plant's historical use in herbal medicine as a treatment for inflammation of the eye.
Distribution
Acorus is native to North America and northern and eastern Asia, and has been naturalised in southern Asia and Europe through centuries of cultivation. Plants grow in wetlands — particularly marshes, shorelines, and floodplains where water levels fluctuate seasonally. Wild populations include diploids in North America and eastern Asia, and tetraploids in eastern Asia; widely cultivated plants are sterile triploids of probable hybrid origin.
Ecology
Acorus species occupy wetland and marsh habitats, spreading via thick rhizomes and relying on aerenchyma tissue to deliver oxygen to submerged root zones. They frequently colonise gaps in marsh vegetation — a growth strategy described in ecological literature as "tussock interstitial," analogous to Iris versicolor. Seed banks may not accumulate in some wetlands due to low seed production; seeds appear adapted to germinate in open clearings following cold stratification, requiring a period of fluctuating temperature and light.
Cultural Uses
The aromatic, parallel-veined leaves of Acorus species contain ethereal oils that produce a sweet scent when dried. Cut leaves were commonly strewn across floors in medieval Europe for fragrance and as a presumed pest deterrent. Acorus calamus has a long history of use in traditional herbal medicine across Asia and Europe; however, products derived from it were banned as food additives in the United States in 1968 because of the carcinogenic compound β-asarone found in triploid and tetraploid strains.
Taxonomy Notes
Acorus was traditionally included in Araceae under most classification systems, including the Cronquist system, following the original description of Acoraceae in 1820. Molecular systematic studies demonstrated that it is not closely related to Araceae or any other monocot family. The APG III system (unchanged in APG IV) formally recognises order Acorales as distinct from Alismatales and as the sister group to all other monocots. Some taxonomists still place it as a subfamily of Araceae within Alismatales, though modern phylogenetic evidence does not support this. Naming confusion persists in older literature where Acorus calamus was incorrectly applied to the North American Acorus americanus.
