How to Grow Japanese Sweet Flag Grass

Grass-like leaves of Ogon sweet flag, a variegated plant.

The Spruce / David Beaulieu

Japanese sweet flag grass (Acorus spp.) 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

Native to Japan and China, Acorus generally grows 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 of sweet flag and calamus. The term flag derives from the Middle English word flagge, meaning "reed."

The tallest Japanese sweet flag grasses top out at about 39 inches in height, while the smallest dwarf cultivars can be as short as 3 inches. The species is slow-growing but often needs to be controlled (usually through division) over time, to prevent excessive spread. 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 the leaves produce when crushed. 

Botanical Name Acorus
Common Name Japanese sweet flag grass, sweet flag, calamus
Plant Type Herbaceous perennial
Mature Size 3 inches to 3 feet tall and 6 inches to 2 feet wide
Sun Exposure Full sun to part shade
Soil Type Moist to wet, variety of types
Soil pH 6.5 to 7.5
Bloom Time Spring
Flower Color Greenish-yellow (not showy)
Hardiness Zones 6 to 9
Native Area Japan, China

Growing Tips

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 directly in a pond or other standing water, first place the plant in a container, then set it in water less than 4 inches deep.

This is one of the rare garden plants that has no serious insect or disease problems. Leaf scorch can occur if the soil is not consistently moist or wet. Although slow-growing, it can be somewhat invasive in ideal conditions. Where this is a concern, growing it in submerged containers can be the best strategy. 

Light

This water-loving plant also likes a fair amount of sun. It grows well in full to part shade, but more sun usually means that the soil needs more water to prevent it from drying out.

Soil

Japanese sweet flag grass is tolerant of a variety of soil types, including heavy clay, but it prefers fertile soil that stays moist at all times. It is not important that the soil is well-drained.

Water

This is an aquatic plant, so constant or near-constant contact with water is essential. If the plants are not growing at water's edge, water as needed to keep the soil moist. Leaf scorch is an indication of too little water.

Temperature and Humidity

Native to hot, humid climates of Asia, Japanese sweet flag grass can tolerate the heat and humidity in any U.S. region; dryness is another matter. But as long as the plant gets enough water, they will do fine. Protection from hot afternoon sun can be beneficial in very hot weather. Leaves can brown at the edges in cold winter weather.

Fertilizer

Feed Japanese sweet flag grass as needed with a slow-release, slow- to medium-rate fertilizer, applied according to the product directions. If the plants are near a pond with fish, be aware of any fertilizer's effect on the water.

Propagation

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. When separating from the mother plant, be sure to get a decent-size rhizome with the separated portion.

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 grows to about 30 inches in height. It, too, is ideal in boggy areas but will also grow in ordinary garden soil if it is kept consistently moist. It can be grown in USDA hardiness zones 4 to 10.
  • Acorus gramineus 'Pusilus Minimus Arueus' is a diminutive variety that grows only to a height of 3 to 4 inches and is commonly used as a ground cover. It has gold-colored foliage and spreads to form a dense mat. It is hardy in zones 5 to 7. 
Acorus gramineus
Acorus gramineus. Dmitrii Anikin / Getty Images  

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.