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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.


Plant Family
Sweet Flag (Acoraceae)

Garden Location
Historic - not extant


Prime Season
Late Spring - Early Summer flowering



Sweet Flag is an 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.

The leaves are erect and flat and sword-like, bright green, rising fan-like from a pinkish base although some bases may range from white to red in color. The midvein only is prominently raised and usually off-center. Leaves are 1 inch or less wide. Margins are smooth but sometimes wavy. Cut or bruised leaves are sweetly aromatic with a scent resembling tangerine.

The inflorescence occurs on a scape that grows upward from the base of the outer leaves. [A scape is a flowering stem that rises directly from the root structure]. Unlike the leaves, the scape is triangular in cross-section. At the approximate mid-point of the scape, a dense 2 to 4 inch long pointed spadix angles outward from a long erect spathe which extends upward beyond the junction with the spadix. The spathe resembles a large bract or leaf. No other plants, other than the Acorus genus, in North America have such a structure where the inflorescence is borne midway on the scape.

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 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).

Seed: The fruit of the Acorus genus is a berry, full of mucus, but A. calamus is sterile in the North American population and in England and where it does fruit it does so sparingly (Ref.#7).


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 author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.

Comparisons: There are two species of Acorus that are quite similar. A. americanus is a native species which resembles A. calamus in appearance but with these key differences: The leaves have 1 to 5 additional raised veins, not just the mid-vein. The flowers produce fertile fruit, the spadix is much shorter and the leaves are not as wide.

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 patten, opening from the bottom upward. Drawing 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. The spadix angles outward from the scape while the spathe 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 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 and Gardener Ken Avery reported replanting the species in 1965.

Sweet Flag was originally introduced to North America by Europeans and is now found in most of the eastern 2/3 rds of North America and is known on the west coast.

Both A. calamus and A. americanus are found in Minnesota, with A. americanus being the native variety which has very extensive distribution across the state whereas A. calamus is reported to be far less frequent but the DNR no longer does county by county tracking of the species.

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.

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. 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.

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.