Sweet flag is a fragrant wetland plant with a history of botanical confusion

Gardening · Published 3 October 2026 Swipe more like this →

In short: Known as sweet flag, Acorus calamus is a wetland perennial that has been used for centuries in perfumes and traditional medicine. Despite its long history, the plant is shrouded in mystery, from its disputed identity in ancient texts to modern concerns regarding the toxicity of its volatile oils.

Acorus calamus is a tall, sword-leaved wetland plant that thrives in marshes and along riverbanks. While it resembles the iris, it is easily distinguished by the fragrant, spicy odor released when its leaves are crushed and the presence of a solid, cylindrical spadix. The plant propagates primarily through its spreading rhizomes, which are knobby, finger-thick, and white on the interior. While it has been used for centuries in various cultures—from ancient Egyptian perfumes to traditional Chinese and Indian herbalism—its classification remains a subject of intense academic debate.

The plant's taxonomic status is complicated by its three distinct cytotypic forms: diploid, triploid, and tetraploid. The triploid form, which is the most common, is infertile and cannot produce fruit, relying entirely on asexual reproduction. Researchers often disagree on whether these forms represent a single species or distinct varieties. For instance, the Flora of North America treats the diploid form as a separate species, while the Flora of China argues that morphological overlap makes such distinctions impossible. This confusion is not new; historical European texts from the 15th and 16th centuries frequently misidentified native iris species as Acorus calamus, a mistake that persisted in botanical literature for generations.

Modern interest in the plant is tempered by significant safety concerns. The rhizomes contain beta-asarone, a compound that has demonstrated insecticidal and anti-fungal properties in laboratory settings. However, this same compound is linked to toxicity in mammals, leading the United States Food and Drug Administration to ban the plant from food use in 1968. While some traditional practices, such as the Ayurvedic process of boiling the rhizome in milk, aim to reduce beta-asarone levels, clinical evidence for the plant's safety or efficacy remains absent. Furthermore, despite persistent rumors of its psychoactive properties, experiments—including high-dosage trials—have failed to confirm any hallucinogenic effects.

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