Plant Profile: Japanese Sweet Flag Grass (Acorus spp)

  • An Ideal Plant Near Water

    Grass-like leaves of Ogon sweet flag, a variegated plant.
    Sweet flag is treated as if it were an ornamental grass, but it isn't, really. David Beaulieu

    Japanese sweet flag grass is a perennial aquatic plant with sword-like leaves that resemble those of iris. It is often used along the borders of landscape ponds and other water features. Despite its common name, Japanese sweet flag grass is not an ornamental grass, but instead is a perennial that spreads by underground rhizomes. 

    Description

    Japanese sweet flag grass has vertical sword-like leaves that top out at about 39 inches in length, depending on variety.  It is slow-growing, spreading to fill a 2-foot wide area in about five years. The leaves are typically light green to golden-yellow in color and can help brighten dim garden areas. Small greenish-yellow flowers appear on spikes in spring and early summer, followed by red berries. The "sweet" label comes from the pleasant, spicy aroma it produces when crushed. 

    Botanical Information

    Japanese sweet flag grass (Acorus spp.) is native to Japan and China. In the U.S., it can generally be grown in USDA hardiness zones 6 to 9, with a few cultivars appropriate as far north as zones 4 and as far south as zone 11. The plant also goes by the common names sweet flag and calamus. The term flag derives from the Middle English word flagge, meaning reed.

    Although not natively occurring in North America, settlers introduced the plant in the 1600s, and it now has naturalized in many places. 

    Landscape Uses

    This plant is a good choice for stabilizing damp areas prone to erosion, such as the banks along creeks or landscape ponds. The bright color is good for brightening shady areas. It can be also grown in containers within landscape ponds or other water features. Japanese sweet flag grass works well as a ground cover in boggy areas, similar to liriope plants.

    Growing Japanese Sweet Flag Grass

    Japanese sweet flag grass is a nearly effortless landscape plant to grow. Plant it in full sun to partial shade, in a location that is moist to constantly wet. It often performs its best right at the edge of a pond or stream. 

    When growing Japanese sweet flag in a pond or other standing water, first place the plant in a container, then set it in water less than 4 inches deep.

    Divide Japanese sweet flag grass in spring every three or four years. The small divisions can be planted in small pots to mature for a year or so before transplanting them into permanent garden locations. 

    Recommended Varieties

    • Acorus gramineus 'Ogon' has variegated leaves with a green stripe running along one side and a yellow stripe along the other.  It is a dwarf specimen, growing to about 1 foot in height and width at maturity. It can be grown in USDA hardiness zones 5 to 9. 
    • Acorus calamus 'Variegatus' is another variegated type, this one growing to about 30 inches in height. It, too, is ideal in boggy areas, but will also grow in ordinary garden soil kept consistently moist. It can be grown in USDA hardiness zones 4 to 10.

    Problems

    Japanese sweet flag grass is the unusual garden plant that has no serious insect or disease problems. Scorch can occur if the soil is not consistently moist or wet.