At a glance, the upright sword-shaped leaves of sweet flag make it resemble cattails. Like them, sweet flag lives in wet soils. But its flower heads are distinctive, and details of the leaves set it apart, too.
Sweet flag is an upright, herbaceous perennial that grows from stout, rootlike rhizomes. As the rhizome grows horizontally under the soil surface, new whorls of leaves arise in clusters.
The leaves may be 4–8 feet long and ½–1 inch wide, are pinkish at the base, and somewhat resemble cattail leaves. But unlike cattails, sweet flag leaves have a prominent midrib, which is usually slightly off-center. Mature sweet flag leaves usually have one side that is somewhat wavy, or even crimped-looking like crisped bacon. When crushed, the leaves have a strong, sweet odor that is spicy or citrusy.
The flowers of sweet flag are tiny and crowded onto a greenish-yellow, fingerlike column (spadix) that is 1–4 inches long. At first glance, the spadix appears to be attached about midway up a leaf, but it is actually at the tip of a stalk that is triangular in cross-section, with a long, leaflike extension (spathe) that is also attached at the stalk tip. Blooms May–July.
The fruits are not fertile. As the flower heads stop blooming, they turn dry and brown.
Similar Species
- Grassy-leaved sweet flag (A. gramineus) is a native of east Asia. It does not occur in the wild in Missouri, but it is grown as a landscaping plant as a ground cover or in rain gardens. You may see it for sale at garden centers.
- Cattails (Typha spp.) generally resemble sweet flags and are more familiar to most people. Use the description above to separate them based on foliage. Also note that the flowers are very different. Cattails have separate female and male flowers: The female flowers are clustered in a brown, sausage-like section; the male flowers are in a yellowish, fluffy-looking section above the female flowers.
- Sedges (in family Cyperaceae) have triangular stems, which makes them somewhat similar to sweet flag, whose flower-cluster stalks are also triangular. The leaves of sedges, however, are 3-ranked (arising from 3 sides of the stem). Each leaf bends away from the triangular stem after clasping the main stem for a distance at its base (the leaves of sweet flag diverge at the base in a general whorl).
Height: to 4–8 feet.
Scattered statewide.
Habitat and Conservation
Occurs in various aquatic and semiaquatic habitats, including ditches, fens, prairie swales, spring branches, pond margins, and sloughs. Usually grows as an emergent aquatic plant.
People may cultivate sweet flag and its close relatives as pond-edge plants, in rain gardens, water gardens, and other places where the ground doesn’t dry out.
The variety of sweet flag that occurs in Missouri’s natural settings was introduced to North America in the 1600s. Like the common dandelion and the daylily, it has become naturalized throughout much of North America.
Status
Introduced, naturalized emergent aquatic shoreline plant. Cultivated as a pond-edge plant.
Taxonomy: There appear to be only three species of sweet flags in the world, but botanists are still working to understand them.
So far, not counting the nonnative grassy-leaved sweet flag (A. gramineus) sold in garden centers, there seem to be two kinds of sweet flags that grow wild in North America: one that is not native to North America but was introduced long ago and is now widespread, and another, which is native to our continent:
- Sweet flag, A. calamus (the subject of this page), is native to Eurasia and was introduced to North America in the 1600s. It was spread by waterfowl, European settlers, and apparently by Native Americans. It seems to be the only sweet flag that is present in Missouri. It is genetically sterile and cannot reproduce by seed. Its dry seeds appear sunken in or shriveled and do not mature. It readily spreads when a small section of rootstock, broken away from the parent plant, begins a new colony somewhere else. This species is sometimes called A. calamus var. calamus.
- American sweet flag, A. americanus, is native to Canada and the northern United States. It has been collected in Iowa and Nebraska, so it could potentially be found in our state. It is identified by its leaves having 2 to several prominent midveins (not just a single midvein). Also, it produces viable seeds, which can germinate. This species is sometimes called A. calamus var. americanus.
Human Connections
Sweet flag is a hardy and attractive plant for water gardening, and it’s been widely cultivated as a pondside or bog plant. It is visually interesting, unaggressive, and low enough to be fished over. It seems to compete well with cattails and could be planted in areas where cattails are not desirable. As interest in rain gardening grows, sweet flag is one of the plants to consider. If the ground dries out, sweet flag’s leaves will develop scorched-looking tips. It can tolerate shade and erosion. Some varieties of sweet flags have been cultivated with pale-streaked (variegated) foliage.
The dried rhizomes of sweet flag, called “calamus root,” and derivative extracts have a long history of use as medicine, perfume additives, and food and beverage flavoring. The species was probably introduced to America for use as a medicinal plant. In the past, people candied the rootstocks by boiling them in sugar. Calamus root, however, contains toxic substances and has been banned as a food additive by the U.S. Food and Drug Administration. We do not recommend ingesting this plant.
With its nearly worldwide distribution, sweet flag has many other uses besides food and medicine. People value it for its aromatic scent. In medieval England and pioneer America, people scattered it on floors to give a pleasant aroma when it was walked on. Sacred uses by Native Americans might be tied to its aromatic qualities.
If you’ve studied American literature, the species name, calamus, might ring a bell. The 19th-century American poet Walt Whitman, in his famous collection Leaves of Grass, celebrates the glories of nature and the material world that common people experience every day. The book has one section of poems, symbolically titled “Calamus,” that was controversial in the 1800s because of its candid celebration of human sensuality.
Ecosystem Connections
Some insects are closely associated with sweet flag. One is a black and brown, nondescript but subtly iridescent type of aquatic leaf beetle, Plateumaris shoemakeri. Most of its close relatives eat sedges, so that species’ taste for sweet flag is remarkable. The larvae of the various aquatic leaf beetles live in water and attach themselves to submerged stems of their host plants, from which they obtain both food and oxygen. Certain aphids and mealybugs feed on sweet flag, too.
Sweet flag and other rooted shoreline plants are important for holding soils in place and preventing erosion.
The maze created by the stalks and leaves of this and other shoreline plants provides valuable cover and habitat. A host of aquatic, semiaquatic, and water-loving land animals live in and around these plants, including fish, aquatic invertebrates, frogs, salamanders, muskrats, birds, and more. Aquatic dragonfly nymphs can creep up the stalks, split their skins, and emerge as winged adults. Later, they may perch gracefully on the leaf tips, looking for meals, mates, and rivals.
Taxonomically, sweet flags (Acorus spp.) are the only genus in their family, the Acoraceae. They used to be placed in the arum family (Araceae) because of their similar flower structures. You are probably familiar with several famous members of the arum family, such as jack-in-the-pulpit, calla lily, peace lily, skunk cabbages, elephant ears, and philodendrons. Unlike arums, sweet flags do not have the distinctively protective leafy spathe that shields the column-shaped spadix of florets. Using genetic evidence, botanists now understand that the sweet flag family represents the only surviving members of unique a group of plants.
