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 acorus top out at about 39-inches tall, 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. 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. 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. Acorus works well as a ground cover in boggy areas, similar to liriope plants.
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
Acorus is tolerant of a variety of soil types, including heavy clay, but it prefers fertile soil that stays moist at all times.
Water
This is an aquatic plant, so constant or near-constant contact with water is essential. If the plants are not growing at the 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, acorus can tolerate the heat and humidity in any U.S. region; dryness is another matter. But as long as the plant gets enough water, it will do fine. Protection from the hot afternoon sun can be beneficial in very hot weather. Leaves can brown at the edges in cold winter weather.
Fertilizer
Feed acorus 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.
Varieties
’Ogon’ features variegated leaves with a green stripe running along one side and a yellow stripe along the other.‘Variegatus’ grows to about 30 inches in height.‘Pusilus Minimus Arueus’ has gold-colored foliage and spreads to form a dense mat, it’s a diminutive variety.
Pruning
Acorus rarely needs any pruning. If it starts to look unkempt, you can tidy it up with some clean garden shears.
Propagating Acorus
Divide acorus 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.
Potting and Repotting Acorus
Acorus is a whimsical addition to your container garden. Choose a container where your sweet flag grass will live for several years, so 4-inches larger than your plant’s root balls, giving it room to grow. Use a high-quality potting medium, and be sure your container has adequate drainage holes. Remove your acorus from the container it comes in and gently loosen the roots with your fingers. Add some gravel to the bottom of your container, then cover with potting medium. Place the plant on the soil and fill the pot with potting medium to 1 inch from the top of the container. Water well, until it drips through the drainage holes.
