At a glance, the upright sword-shaped leaves of sweet flag make it resemble cattails or irises. Like them, sweet flag also lives in wet soils. But the flower heads are distinctive, and details of the leaves set them apart, too.
Sweet flag is an upright, herbaceous perennial that grows from stout 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 the leaves of cattails. Unlike cattails, however, 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. Finally, sweet flag leaves, when crushed, have a strong, sweet odor that is rather spicy or citrusy.
The flowers of sweet flag are tiny and crowded onto a greenish-yellow, fingerlike column called a spadix 1-4 inches long. The spadix is either upright or slightly inclined at an angle. At first, 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. As the flower heads stop blooming, they turn dry and brown. The fruits are not fertile.
Two Kinds of Sweet Flags
There has been confusion about whether A. calamus is native to North America or not, and botanists debate the taxonomy of this genus. So far, there seem to be two kinds of sweet flags 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. Some botanists group these two varieties of a single species, while others consider them different species:
- Sweet flag, A. calamus (described above), 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 a sterile triploid plant (it has three sets of chromosomes), so it cannot reproduce by seed. Its dry seeds appear sunken in or shriveled and do not mature. It readily reproduces asexually, however, as the rhizomes spread; a small section of a rhizome, broken away from the parent plant, can easily start a new colony somewhere else. For botanists who group the North American sweet flags together as a single species with two varieties, this is 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 midrib). Also, it is diploid, so its seeds are viable and will germinate. For botanists who group the North American sweet flags together as a single species with two varieties, this is called A. calamus var. americanus.
- Grassy-leaved sweet flag (A. gramineus) is the other species in the genus. 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 generally resemble sweet flags and are more familiar to most people. Use the description above to identify by the foliage. Also note that the flowers of sweet flag are perfect, with both stamens and pistils together in each tiny flower. This is a big difference from cattails, whose flowers are unisexual and whose flower stalks are divided into separate sections for female flowers (the brown, sausage-like section) and male flowers (the 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 usually clearly 3-ranked, bending outward from the main stem from each side of the triangle after clasping the main stem for a distance at their bases, while the leaves of sweet flag diverge at the base in a general whorl.