Sweet Flag is a botanical puzzle. Most people in the UK will know it as a garden centre plant commmonly described as the Japanese Sweet Flag. But it has long been known for its medicinal value, and has been cultivated in Asia for this reason.

The species Acorus calamus is native to the southeastern United States, growing in wet areas in marshes and ditches. It looks rather grass-like, until closer inspection reveals the arum-like cluster of flowers (the spadix). Until recently, it was just another arum among others, in one of the larger and more complicated monocot families. Upon investigation of its morphology and its DNA sequences, it now appears that Acorus may represent an early stage in the evolution of the monocots.


The genus includes as many as six species:
  • Acorus americanus, formerly known as A. calamus var. americanus. This is the American Sweet Flag; occurring in Alaska, Canada and northern USA. Diploid plants in Siberia and temperate Asia may also belong here, but have not been fully investigated. Recently recognised as a distinct species by the Flora of North America.
  • Acorus calamus. - Common Sweet Flag is probably of cultivated origin. It is native to Europe, temperate India and the Himalayas and southern Asia, widely cultivated and naturalised elsewhere.
  • Acorus gramineus - Japanese Sweet Flag or Grassy-leaved Sweet Flag; occurring in the Himalayas to Japan, Myanmar, Thailand, the Philippines. This species is becoming popular as a tough groundcover garden plant.
  • Acorus triqueter -; occurring in eastern Asia, Japan and Taiwan.
  • Acorus latifolius: native to China
  • Acorus xiangyeus: native to China



Because this is such a small group (only one genus), it is not surprising that the fossil record is poorly known. Some identified some fossil Eocene spadices from Tennessee as belonging to this group. The preservation is good enough to reveal details of tje anatomy, both of which are consistent with modern Acorus. Despite its sparse fossil record, the group has great importance for paleontologists, in that it gives us one picture of what early monocots may have looked like, and where they may have lived.

In the garden

Gerald Klingaman, wrote the following article in 'Ornamentals' about the Japanese Sweet Flag (Acorus gramineus) in
Extension News - April 21, 2006

"The more I know about a plant, the more I appreciate it. The Japanese sweet flag (Acorus gramineus) is becoming more common in our gardens, but its’ small stature doesn’t necessarily indicate the importance it may have played in the evolutionary history of the world.

Japanese sweet flag is a small grass-like herbaceous perennial that spreads by means of a shallow, much-branched rhizome. From these rhizomes are produced a number of narrow, glossy, foot-long leaves arrayed at the ends of the rhizomes in a fan-like fashion. The leaves resemble those of some irises. It’s native in a wide swath from India to the Philippines, but most of the forms in cultivation are selections made by Japanese gardeners.

A number of cultivars are available, each with distinctive growth forms. ‘Ogon’ is a golden-leafed form with leaves about a foot long and is currently most common in nurseries. ‘Variegatus’ has a creamy yellow stripe down the length of the blade. ‘Pusillus’ is a dwarf, 3-inch tall clumping form; ‘Minimus Aureus’ is a golden foliaged dwarf. These last two are too small to use in the average garden because they would be overrun by more vigorous plants.

Acorus flowers are a curiosity in themselves. In early spring, a white, finger-like flower is pushed out of the side of a normal looking leaf about half way along its length. The floral spike (spadix) is about 2-inches long and resembles that of the philodendron family, where Acorus has been traditionally placed.

But Acorus always seemed to be an odd fit in the philodendron tribe, so in 1987 it was finally moved from that family to one of its own, the Acoraceae. Plant reclassification has been rampant in recent years, partly because of increased international collaboration between botanists and partly because DNA analysis has become a powerful new tool to aid in the understanding of botanical relationships.

Using DNA typing, scientists looked at a number of marker genes in Acorus and tried to match it in a DNA database. The more matches, the more closely the plants are related. Acorus DNA didn’t match well with the philodendron family nor any of the other monocots, but it did match somewhat with members of the pepper family, a dicot.

Some scientists now think that Acorus may be the first monocot - plants that produce only one seed leaf at germination and include grasses, palms, orchids, philodendrons, bromeliads and the like. Evolutionarily speaking, monocots are more modern than dicots, plants producing two seed leaves at germination and the largest overall group of flowering plants. So, this lowly little plant may have had earth-changing biological significance, being an ancient ancestor to the most important of our world food crops.
Japanese sweet flag has long been grown as a houseplant but, since water gardening returned to fashion, it has been touted as a aquatic plant for use in shallow pools. We now realize it is hardy outside throughout Arkansas and as far north as zone 5 in just average garden soils. It grows in full sun or moderate shade.

Because of its aquatic heritage, it will grow in heavy clay soils, but does best in fertile, reasonably moist locations. It’s evergreen throughout the state, but leaf tips may burn during periods of extreme cold or if the plants get bone dry in the summer. Propagation is easy by division at any time of the year. I have seen no indication of pests or diseases."

In other words there is much scope for collection for experimentation with biodiversity within this species and within the genus Acorus as a whole.