Sweet Flag: The Plant That Conquered Half of Europe Without Ever Setting Seed

Sweet flag almost never flowers in Central Europe and sets no seed – yet over the past 450 years it has spread along ponds, pools and slow-moving waters across the continent. For centuries the plant stood in cottage gardens, in liqueur recipes and in perfumery. Today it is the sole entry on the list by which Swiss food law excludes source materials from flavouring production altogether. A portrait of a plant whose botany and whose regulation are equally unusual.

How an Asian marsh plant reached European waters

Sweet flag (Acorus calamus) is native to Asia. In Switzerland and the rest of Central Europe it counts as a neophyte – a plant that arrived in the region only after 1500. Info Flora, the national data and information centre on Swiss flora, accordingly lists it as a naturalised, originally non-native aquatic plant of the colline to montane zone.

Sources differ on how exactly it reached Europe. The name cited most often is the Flemish botanist Carolus Clusius, said to have cultivated a rhizome received from Constantinople near Vienna in 1574. Even older reference works phrase this cautiously – "reportedly", "is said to have" – and other accounts credit the Italian physician Pietro Andrea Mattioli and the 1560s. What is certain is only that sweet flag appeared in Central European gardens during the sixteenth century and spread rapidly along watercourses from there. As late as the eighteenth century, dried rhizome was still partly imported from Asia.

Why sweet flag is a botanical special case among flowering plants

Sweet flag was long placed among the arums (Araceae). Molecular work has corrected that picture: the genus Acorus now forms its own family (Acoraceae) in its own order (Acorales) – and stands as sister group to all other monocots. It is therefore the oldest surviving lineage of the monocotyledons, the group that also includes grasses, orchids and palms.

The plant reaches roughly 60 to 120 centimetres, occasionally more. The stem is three-sided, the leaves sword-shaped and arranged in two ranks. The spadix appears lateral because the bract continues the stem beyond it. Flowering time is June to July – an event rarely observed in Central Europe.

The reason is remarkable: the lineage established in Europe is triploid and therefore sterile. It sets no fruit and reproduces exclusively by vegetative growth of its creeping rhizome. What grows along European banks today traces back to the clonal spread of a few introduced rootstocks – a plant that has colonised half a continent without ever setting a seed.

How to tell sweet flag apart from yellow flag iris

Outside the flowering season, sweet flag is regularly confused with yellow flag iris (Iris pseudacorus), which occupies the same habitat and bears similarly shaped leaves.

Feature	Sweet flag	Yellow flag iris
Crushed leaf	intensely aromatic	no marked scent
Leaf margin	usually slightly wavy	smooth
Inflorescence	inconspicuous green-yellow spadix, apparently lateral	large yellow iris flower
Flowering in Central Europe	rare, sterile	regular, fruiting
Stem	three-sided	rounded to flattened

What the rhizome contains – and why the plant's origin changes everything

Depending on the source, the rhizome contains roughly 1.5 to 5 per cent essential oil. It holds monoterpenes such as camphor and alpha-pinene, sesquiterpenes such as acorone and shyobunone, and phenylpropanes – among them alpha- and beta-asarone. Added to these are a bitter principle described in older literature as "acorin", tannins, mucilage and starch.

What matters here is not the species but the lineage. The Committee on Herbal Medicinal Products (HMPC) of the European Medicines Agency compiled the figures in a public statement in 2005 – and the differences are stark.

Lineage	Ploidy	Main distribution	Beta-asarone in rhizome essential oil	Beta-asarone in dried rhizome
var. americanus	diploid	North America	not detected	not detected
var. calamus	triploid	Europe, western Asia	9–19 %	around 0.3 %
var. angustatus	tetraploid	South and East Asia	85–95 %	4.4–8.3 %

Orders of magnitude therefore separate the North American from the Asian lineage – in plants that are barely distinguishable morphologically. Recent taxonomic work consequently proposes treating the diploid and tetraploid lineages not as varieties of Acorus calamus but as species in their own right. For practical purposes little changes: without knowing the origin, the asarone content of a rootstock cannot be estimated.

How sweet flag was used in Europe over the centuries

Sweet flag was long an everyday plant in Europe, and its folk names reflect that. German records include Magenwurz, Ackerwurz and Gewürzkalmus, alongside the label "German ginger"; Swiss dialect forms such as Chalmis (St Gallen) and Kalmuswürzen (Bern) are documented. The Latin epithet calamus derives from the Greek kálamos, meaning reed or cane.

The peeled and dried rootstock ranked among the classic bitter drugs of European plant lore. Boiled with sugar it yielded a candied confection. The essential oil was used to flavour liqueurs and bitters and found its way into perfumery, where it still plays a role. In earlier centuries the cut leaves were strewn on floors indoors as a fragrant covering – a use that has essentially disappeared, and with it a good part of the plant's former range.

These uses are historically documented. They say nothing about whether any of the expectations attached to them would hold up under present-day scrutiny.

What Swiss food law states about sweet flag

The FDHA Ordinance on Flavourings and Food Ingredients with Flavouring Properties (Aromenverordnung, SR 817.022.41) addresses beta-asarone in three places:

• Annex 4, section 1: beta-asarone is among the substances that may not be added to foods as such.
• Annex 4, section 2: for naturally occurring beta-asarone in alcoholic beverages, a maximum level of 1.0 mg/kg applies.
• Annex 5, section 1: the tetraploid form of Acorus calamus may not be used to produce flavourings or food ingredients with flavouring properties. It is the sole entry on that list.

The background is the toxicological assessment of the asarones. The HMPC states that alpha- and beta-asarone are genotoxic and hepatocarcinogenic in animal models. Its recommendation is to reduce their concentration in herbal medicinal products to a minimum and to prefer diploid varieties; for exposure from herbal medicinal products the committee names a provisional reference figure of around 115 micrograms per day, pending a full benefit-risk assessment.

No HMPC monograph on Acorus calamus exists. There is therefore no harmonised European assessment of the plant as a traditional herbal medicinal product – unlike lemon balm, valerian or chamomile.

What to bear in mind with sweet flag

Origin cannot be determined. Standing at a waterside, there is no way to tell which lineage a stand belongs to. Anyone collecting rootstocks themselves does not know the asarone content of the material.

Risk of confusion. Yellow flag iris grows in the same locations and its underground parts are toxic.

Pregnancy, breastfeeding, children. The specialist literature consistently advises against using sweet flag preparations in these situations.

Essential oil. Calamus oil is a raw material of the fragrance and beverage industries, not a food. It has no place in self-administration.

In the garden. As a pond-margin plant, sweet flag is unproblematic and decorative. As a kitchen or tea herb, it is not.

The bitter-plant tradition sweet flag once belonged to

For centuries sweet flag belonged to the same group as a range of other European bitter plants – gentian, dandelion, yarrow, fennel, caraway. These plants appear in the same old herbals, and they have remained unproblematic under food law. The European bitter-plant tradition is carried today by them, no longer by sweet flag.

Conclusion: a plant whose history offers more than its present-day use

Sweet flag is botanically extraordinary – the oldest surviving lineage of the monocots, represented in Europe by a sterile clone that has spread for centuries by rhizome division alone. In cultural history it was one of the defining aromatic plants of the early modern period, from confectionery through liqueur production to perfumery.

As a plant for consumption, it is now largely a matter of history. Asarone content depends on a ploidy level that cannot be seen in the plant; the toxicological assessment is clear enough that Swiss law excludes the tetraploid form – as the only source material of any kind – entirely from flavouring production. No recognised monograph exists. Anyone interested in sweet flag will therefore find in it above all one thing: an exceptionally well-documented piece of European plant history.

This article provides general information about a plant and its cultural-historical and regulatory context. It is not a recommendation for use and does not replace medical or nutritional advice. Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle.

Sources

• FDHA Ordinance on Flavourings and Food Ingredients with Flavouring Properties (Aromenverordnung), SR 817.022.41, as at 1 July 2025 – fedlex.admin.ch (German)
• Committee on Herbal Medicinal Products (HMPC): Public Statement on the Use of Herbal Medicinal Products Containing Asarone. EMEA/HMPC/139215/2005, European Medicines Agency, 2005 – ema.europa.eu
• Info Flora: Acorus calamus L. – species profile of the national data and information centre on Swiss flora – infoflora.ch
• EPPO Global Database: Acorus calamus (ACSCA) – gd.eppo.int
• Soukup A., Seago J. L., Votrubová O.: Developmental Anatomy of the Root Cortex of the Basal Monocotyledon, Acorus calamus (Acorales, Acoraceae). Annals of Botany 96(3), 2005, 379–385. doi:10.1093/aob/mci190 – PMC4246772
• Flora of the Southeastern United States: Acorus calamus – taxonomic treatment following Sokoloff et al. (2024) – fsus.ncbg.unc.edu