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Plants of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States


Common Name
Sweet Flag (Calamus, Flag-root)


Scientific Name
Acorus calamus L. and
Acorus americanus Raf.


Plant Family
Sweet Flag (Acoraceae)

Garden Location
Historic - not extant


Prime Season
Late Spring - Early Summer flowering



Two species of Sweet Flag are found in North America. A. calamus is a European introduced and naturalized perennial emergent iris-like herb growing in moist environments with the leaves erect and up to 5 feet in height, shorter in environments with only moderate moisture. A. americanus is a native North American species with similar overall characteristics. The differences are given below.

The leaves are erect and flat, with a thick center, and sword-like, bright green, rising fan-like from a white base that can have pink to red colors. The lower section of the leaf (proximal part) is more swollen, while the upper part (distal part) flattens out. In calamus the midvein only is prominently raised and usually off-center. Leaves are 1 inch or less wide. The leaf is also slightly longer and slightly wider than americanus, which has an additional 1 to 5 major parallel veins. These may not be immediately evident on fresh leaves, but holding the blade to the light, or when dry, will show them. Margins of both species are smooth but sometimes wavy. Cut or bruised leaves are sweetly aromatic with a scent resembling tangerine.

The inflorescence occurs on certain outer leaves in the form of a spadix without a spathe. Unlike the other leaves, this leaf is triangular in cross-section at the approximate mid-point of the leaf, and at that point a solitary dense 2 to 4 inch long pointed spadix angles outward and the remainder of the leaf then partially encircles this spadix, then flattens and extends upward like the other leaves. This inflorescence structure was once considered to be a spathe but no other plants, other than the Acorus genus, in North America have such a structure where the inflorescence is borne midway on the leaf (which is termed a sympodial leaf). This difference has caused the Acorus genus to be moved out of the Araceae family into the newer Acoraceae family.

Flowers: The spadix is packed with tiny 6-parted yellowish-green flowers. These are arranged in a diamond pattern and each flower has a perianth of 6 divisions with 6 tepals (petal and sepal combined) enclosing a large green 3-celled ovary. Around the ovary are 6 stamens. The tepals can be a light brown in color, are very small with squarish tips; anthers of the stamens are yellow (see drawing below).
In North America A. calamus is sterile and A. americanus is fertile. A test is that the pollen grains of the latter will stain deeply in aniline blue whereas those of the former do not. In England A. calamus does fruit but does so sparingly (Ref.#7)

Seed: The fruit of the Acorus americanus species is a brown to reddish berry, full of mucus, with, usually, 6 embedded seeds.


Habitat: Sweet Flag grows from creeping rhizomes that allow the plant to spread vegetatively. Habitat is usually ponds, marshes and shallow areas with water less than 20 inches deep. The rhizomes branch frequently, are aromatic and have a long history of medicinal use (notes below). Sweet Flag can be grown in rich moist soil if frequent watering is provided but plants will be shorter. Propagation by rhizome involves cutting 2 to 4 inch sections in spring or fall and plant them 4 to 6 inches deep.

Names: The genus Acorus is taken from the Greek, akoron, the Greek name for a plant used by Dioscorides, and that name is said to be derived from Coreon, the pupil of the eye, and for treating diseases of that organ, the plant was used by the ancients (Ref. #7), As to the common name of Sweet Flag, the name is thought to come from the resemblance in form to the common Yellow-flag, which incidentally, while an iris, has the species name of pseudacorus. The species, calamus, is derived from the Greek kalamos, meaning a reed. The species americanus refers to the North American origin.

The author names for the plant classification: For calamus - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy; for americanus - ‘Raf.’ is for Constantine Samuel Rafinesque, (1783-1840), European polymath who traveled in the United States, lived here many years, collected specimens, and published over 6,700 binomial names for plants. He applied to be botanist on the Lewis & Clark expedition but Jefferson turned him down in favor of training Lewis to act as botanist and saving the expense of another person.

See bottom of page for notes on the Garden's planting history, distribution in Minnesota and North America, lore and other references.

Inflorescence Drawing

Above: The spadix flowers are arranged in a diamond pattern, opening from the bottom upward. Drawing of A. calamus courtesy Kurt Stüber's Online Library.

Below: The leaves are erect, sword-like, flat but with a raised central vein that is slightly off-center, smooth but sometimes wavy margins. In the second photo the additional main veins of americanus are visible. 3rd photo - The spadix angles outward from the sympodial leaf which then flattens and continues upward.

full plant leaf detail flowering stem

Below: Each flower has a large green ovary surrounded by the stamens with yellowish anthers and the small brownish tepals. The base of the plant is typically pinkish or reddish tinged.

flower details plant basal area


Eloise Butler planted Sweet Flag several times in the early years of the Garden. In 1908 she brought in plants from Stony Brook Reservation in Massachusetts and in 1910 from Kelsey's Nursery in Salem, Massachusetts. Martha Crone planted it in 1948 from Henderson's Nursery in Indiana and Gardener Ken Avery reported replanting the species in 1965. As none of Butler's or Crone's plants were from local sources, it is not clear which species was planted.

Acorus calamus was originally introduced to North America by Europeans who got it from Asia and is now believed to be found in most of the eastern 2/3 rds of North America and is known on the west coast. A. americanus has less distribution - principally the lower Canadian Provinces and then from the eastern sections of the Dakotas and Nebraska eastward to the east coast, no further south than the northern sections of Indiana, Ohio and Pennsylvania.

Both A. calamus and A. americanus were once thought to be found in Minnesota, but the Minnesota DNA no longer tracks A. calamus and considers most of the population in the state to be A. americanus.

Flora of North America (Ref. W7) makes this important statement: Traditionally, the name Acorus calamus has been applied to all populations of Acorus in North America without regard for the biological species involved. Other authors (e.g., E. T. Browne and R. Athey 1992; K. A. Wilson 1960) have adopted A. americanus as the "correct" name for all Acorus in North America, including populations in regions where only A. calamus occurs or is the predominant species. The use of either name in the literature requires further study to determine which species is being cited.

When we start to look at the many native uses of the plant, which must pre-date European settlement, the Flora has this to say: Although this plant is cited in the ethnographic and ethnobotanical literature as A. calamus, the distribution of the tribes reported to use Acorus corresponds to the range of the native species (S. A. Thompson 1995).

Medicinal Use: The medicinal parts of Sweet Flag come from the rhizome which produces the drug "calamus", derived from the volatile oil of the root, used primarily for digestive purposes as a tonic and stimulant, and was listed in the U.S. Pharmacopoeia. The official preparation was a fluid extract. The rhizome while having a pleasant aroma has a bitter pungent taste. "Calamus Oil" is also derived and used in perfumery. Mrs. Grieve (Ref. #7) has much detail on European usage. Hutchins (Ref.#12) reports similar use among the North American Native population as a stimulant and tonic, adding the detail that a tea was made from an infusion of 1 oz. of cut or granulated root per pint of water. The use of the tea several times a day or chewing the root was a treatment for stomach discomfort.
Densmore (Ref. #5) in her study of the Minnesota Chippewa reports use of the plant for treating fever and cough using the root. She also notes the root was combined with that of Wild Sarsaparilla, Aralia nudicaulis and made into a decoction which had two uses as a charm - one to put it on fish nets to lure fish and one used as a snake repellant.
Fernald (Ref.#6) writes that candied sweet flag roots were made in New England, sold by the Shakers and others. As the raw root has a soapy or peppery taste, the confection was too strong for eating too much but made "a tempting nibble."

Common Use: In old England Sweet Flag, because of the pleasant odor, was strewn on the floors of churches at festivals and in private houses. Into the late 19th Century this was common at Norwich Cathedral during festivals. One of the charges of extravagance against Cardinal Wolsey during his time as Lord Chancellor for Henry VIII, was his common use of fresh rushes on his floors. See Mrs. Grieve for far more detail (Ref. #7). Edwin Way Teale in his book "Springtime in Britain", 1970, makes similar references where in the area of the old Great Fen, people were occupied with gathering the leaves of Sweet Flag for the floors of monasteries and priories.

Meriwether Lewis noted on July 19th, 1804 in The History of the Lewis and Clark Expedition, that one of the food hunters of the group brought back to camp great quantities of Sweet-flag. [they were near the present site of Nebraska City, Nebraska.]

References and site links

References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.

graphicIdentification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.