Japanese sweet flag plant

Japanese Sweet Flag Grass, Acorus gramineus: “Water’s Sedge”

Japanese sweet flag grass is one of those plants with a proclivity for water which also happen to resemble flowing water. This makes it much admired and appreciated, at least in my book. When I design around a water feature, whether it’s a tumbling waterfall, a bubbling urn or a meandering dry creek bed, I most always employ Acorus gramineus because this evergreen grass-like plant mimics the way water flows, cascades, and spreads out in a carefree way.

Japanese sweet flag grass, which belongs to the Acorus genus of wetlands grass, is a friendly species for a home garden. Its sibling common sweet flag (Acorus calamus) is a taller species which is useful at the edge of a pond or to stabilize marshy soil.

To learn whether your water feature or other garden areas should partner with Japanese sweet flag grass, please keep reading:

Above: Japanese sweet flag grass (Acorus gramineus) flows down a slope at San Francisco Botanical Garden. Photograph by Daderot via Wikimedia.

Native to Japan (where Acorus occurs in wetlands and shallow waters), this narrow-leaf, densely clumping perennial erupts from rhizomes that lie below the surface. The “sweet” part in the common name comes from the pleasant aroma the leaves emit when broken. Acorus is frequently used around the edges of water gardens and ponds; it is happiest when allowed to drink water without too many restrictions as it gracefully softens the look of rocks.

Above: When landscape architect William Dangar designed a garden for his own family (near Sydney, Australia), he incorporated a small backyard pond fringed by Japanese sweet flag grass. See more in Downsizing a House to Expand the Garden: At Home with Landscape Architect William Dangar. Photograph by Prue Ruscoe, courtesy of William Dangar.

Side note: I once planted A. gramineus ‘Ogon’ near a pond’s edge and watched the sedge slowly and quietly creep closer to the water. It decided to become one with the pond and to magically float and expand on the surface. Eventually, of course, I had to remove large sections of the grass-like plant after it overtook the pond and “naturalized,” but I admired its tenacity.

Above: Australia-based landscape architect William Dangar also uses Japanese sweet flag grass as a front-of-the-border ground cover. Here, in his own garden, it creates a gentle wave. Photograph by Prue Ruscoe, courtesy of William Dangar.

Indoors, Acorus can be kept as a houseplant, living in medium to direct light with high humidity and high water (the pot may even be allowed to stand in a shallow saucer of water). Because Japanese sweet flag grass is a marsh plant, water it plentifully to ensure the potting mixture is always thoroughly moist.

Above: On Washington’s Bainbridge Island, a path is edged with lime green Acorus gramineus ‘Ogon’ grass. In the foreground, yellow Sedum ‘Angelina’ and tall spires of Acanthus mollis create visual interest. See more of this garden in Water’s Edge: A Saltwater Courtyard Garden on Bainbridge Island. Photograph courtesy of Wittman Estes.

Best Acorus Varieties:

  • Acorus gramineus ‘Ogon’ is a dwarf cultivar of A. gramineus which grows to 10 inches tall and spreads slowly. It has vibrant fans of glossy pale green and yellow-striped leaves to produce an overall lemony yellow effect.
  • Acorus gramineus ‘Variegatus’ is a variegated-leaf form with attractive creamy white stripes on its green leaves. It looks lovely paired with silver-leaf plants and white blooming perennials. It grows to 10 inches tall and 6 inches wide, while slowly spreading.
  • Acorus gramineus ‘Pusillus’ is a petite grass-like perennial, growing typically 2 to 3 inches tall, but perhaps a bit taller with rich soil and regular irrigation. Though spreading very slowly, it’s considered to have more of a clumping habit. Plant ‘Pusillus’ in full coastal sun to light shade and irrigate regularly or keep in shallow water. Also, because of its low stature it acts as an excellent plant between stepping stones in moist areas.
  • Acorus gramineus ‘Pusillus Minimus Aureus’ features tufts of yellow grassy-like leaf blades and is a wonderful ground cover, forming a stunning golden carpet. Consider planting this between pavers in a partly shaded garden. Grows to 3 to 4 inches tall.

Cheat Sheet

Above: ‘Ogon’ has a clumping form. A 6-inch plant in a 3-inch pot is $9.88 from 9EzTropical via Etsy.

  • Japanese sweet flag grass is a beautiful accent plant or massed ground cover near water gardens, along streams or ponds, or in moist, open woodland gardens.
  • The variegated varieties of Japanese sweet flag grass brighten partly shaded to fully shaded areas.
  • Acorus be grown as a houseplant and as a companion in containers with other moisture-loving plants such as cannas, coral bells, and ferns.
  • Deer leave this plant alone, thankfully.

Above: For wholesale purchases only, see Hoffman Nursery for Golden Variegated Sweet Flag.

Keep It Alive

  • Plant Japanese sweet flag grass in full sun to full shade, but be aware that more sun equals higher water needs.
  • Acorus grows well in both boggy conditions, including very shallow water and consistently moist garden soils. Scorched leaf tips will occur if the soil is allowed to dry out, subtly letting you know that you are depriving it.
  • Trim blades to the ground if a Japanese sweet flag grass is looking tattered or unhappy.
  • Propagate this perennial by separating overcrowded clumps in spring or summer. Carefully pull apart the clumps with your fingers, making sure that a medium-sized piece of rhizome is attached to each section and treat each divided clump as a plant.

For more growing tips, see Sweet Flag Grass: A Field Guide to Planting, Care & Design in our curated guides to Garden Design 101. Read more about how to use grasses in a landscape:

  • Hardscaping 101: Ground Covers to Plant Between Pavers
  • Grasses: A Field Guide to Planting, Care & Design
  • Architect Visit: A Hazy Landscape of Grasses in Santa Monica

Sweet Flag Care: Tips For Growing Sweet Flag Grass

Japanese sweet flag (Acorus gramineus) is a striking little aquatic plant that tops out at about 12 inches. The plant may not be statuesque, but the golden-yellow grass provides plenty of bright color in soggy garden spots, along streams or pond edges, in semi-shady woodland gardens – or nearly any area where the plant’s moisture requirements are met. It is a good choice for stabilizing the soil in damp, erosion-prone soil. Read on for more information about Japanese sweet flag.

Arorus Sweet Flag Info

Japanese sweet flag, also known as Calamus, is native to Japan and China. It is a cooperative, slow-spreading plant that attains a width of 2 feet in about five years. Miniature greenish-yellow blooms appear on spikes in spring and early summer, followed by tiny red berries. The grassy leaves emit a sweet, rather spicy aroma when crushed or stepped on.

Sweet flag is hardy to USDA plant hardiness zones 6 through 9, although some Acorus sweet flag info indicates the plant is tough enough for zones 5 through 11.

Sweet Flag Care

It doesn’t take much effort when growing sweet flag grass. Sweet flag plants tolerate light shade or full sun, although the plant benefits from afternoon shade in hot climates. However, full sun is best if the soil is extremely boggy.

Average soil is fine, but be sure the soil is consistently moist, as sweet flag doesn’t tolerate bone dry soil and may scorch. Similarly, the leaf tips may turn brown in periods of extreme cold.

To grow sweet flag in a pond or other standing water, place the plant in a container and set it in water less than 4 inches deep.

Sweet flag plant benefits from division in spring every three or four years. Plant the small divisions in pots and let them mature before transplanting them into their permanent locations. Otherwise, growing sweet flag grass is nearly effortless.

Sweet Flag

Sweet Flag

Sweet flag is a grasslike, low-maintenance perennial. It grows well in moist soil or several inches of standing water, making it an excellent choice when used as an accent plant in water gardens or moist, marshy areas along shorelines. Sweet flag spreads slowly over time via rhizomes and forms a dense groundcover, but it is not considered invasive. The foliage of sweet flag has a light, sweet scent when crushed.

genus name
  • Acorus
light
  • Part Sun,
  • Sun
plant type
  • Perennial,
  • Water Plant
height
  • 6 to 12 inches,
  • 1 to 3 feet,
  • 3 to 8 feet
width
  • From 6 to 24 inches
foliage color
  • Blue/Green,
  • Chartreuse/Gold
problem solvers
  • Deer Resistant,
  • Groundcover
special features
  • Low Maintenance,
  • Fragrance,
  • Good for Containers
zones
  • 4,
  • 5,
  • 6,
  • 7,
  • 8,
  • 9,
  • 10,
  • 11
propagation
  • Division

Colorful Combinations

Commonly, sweet flag is a grass green color. While it isn’t the showiest of plants, it does add a textural component to a garden or container. Along with the common green types, there are several other varieties of sweet flag in assorted and variegated colors. Some are variegated with a golden stripe on one side and a green stripe on the other. Most varieties are a vibrant chartreuse yellow, which contrasts with rich-colored but not showy flowers. You can also find white variegated varieties as well. The tiny, insignificant yellow-green flowers bloom on lateral flower spikes; you will see these blossoms from spring to early summer, but they typically are only develop when growing in water.

Everything you need to know about ornamental grasses.

Alternative Uses

The subtle, sweet smell and flavor of this grasslike plant has long been prized for making a soothing, sweet tea. The slightly bitter roots can also be used in cooking and have been known to be a substitute for spices such as cinnamon and ginger. Just be careful to use them in moderation as the rootstocks can have a stimulating effect and mild psychoactive properties. These plants also have insecticidal characteristics, and the leaves used to be harvested and placed in closets to impart a slightly sweet scent to repel insect pests.

See more insect-repelling plants for your garden here.

Sweet Flag Care Must-Knows

Sweet flag will grow easily in medium to wet soil and boggy areas, and does well in anything from full sun to part shade. The soil should never be allowed to dry out; if your plant has scorched leaf tips and withering leaves, it is too dry and needs to be watered. Sweet flag really thrives in water gardens. Plant it in containers and allow enough water to cover the crown of the plant, or place sweet flag in soil at the water’s edge. Sweet flag appreciates relief from hot summer sun with either afternoon shade or filtered sun. Because of its slow-growing nature by rhizomes, these plants are easily divided. This also helps to encourage a nice new flush of growth and can be done regularly every few years to rejuvenate them.

Try sweet flag and these other plants in a water garden.

More Varieties of Sweet Flag

Dwarf golden sweet flag

Acorus gramineus ‘Minimus Aureus’ reaches only 4 inches tall but forms a clump up to 1 foot wide. Zones 6-11.

Golden grassy-leafed sweet flag

Acorus gramineus ‘Ogon’ makes a 1-foot tuft of grassy golden leaves striped with green. Its foliage is evergreen in warm climates. Zones 6-11.

Variegated sweet flag

Acorus calamus ‘Variegatus’ forms an upright plant with striking green-and-white striped foliage stretching 3 to 5 feet tall. Zones 4-11.

Plant Sweet Flag With:

Hibiscus flowers might be the most dramatic in the garden and can bloom as large as a child’s head in gorgeous colors. The hibiscus plant itself is large and dramatic, and it needs plenty of space to show off. Although the huge funnel-shape flowers seldom last more than a day, they are abundant and the plant blooms over several weeks. The large leaves tend to draw Japanese beetles. Hibiscus needs plenty of water, so grow it in rich, loose, well-drained soil where you can water it easily and regularly during dry spells.

The curious corkscrew rush loves wet or boggy conditions. It makes a fascinating architectural accent in planters, beds, and moist borders. It’s technically leafless, with green cylindrical stems that are pointed at the tip. Plant rush alongside streams and ponds, though it will tolerate dryer conditions elsewhere. It’s excellent in container gardens.

Pitcher plants are one of those cool carnivorous plants; they can devour insects. But don’t let this amazing fact overshadow their inherent beauty. They produce fascinating pendant chartreuse or purple flowers in spring. Pitcher plants are fascinating to grow, and adapt well to containers where the plants can be observed up close. In mild regions, they can also be grown in acid bog gardens. They do not need a diet of insects — the insects are attracted by nectar at the base of the pitchers and slide down and drown in collected liquid at the base. The tall pitchers of some species are cut and dried for indoor arrangements, but only remove a few to retain the vitality of the plants.

Acorus Calamus-Sweet Flag

Acorus Calamus-Sweet Flag

This forever popular marginal plant also known as ‘Scented Rush’ or ‘Sweet Flag’, is one of my favourite plants to propagate as the rhizomes give off an addictive aniseed scented aroma when they are cut or broken, however, the leaves give off a less attractive scent when brushed. This marginal plant produces the freshest, tidiest, lime green leaves, but also has very interesting cone shaped cream coloured flowers. Acorus have been grown for centuries and have been used for many medicinal purposes and culinary uses throughout history. The oils from their rhizomes have been used in other cultures for perfumes and flavourings and the leaves in the past, have been used for thatching. Also the leaves can be traditionally stacked in piles to create a nice aroma in churches and many other religious and cultural buildings and are used in ceremonies.

Acorus Calamus is a marginal plant that will thrive in full sun or partial shade and grow quite happily submerged up to a water depth of 30cm, but will be just as happy in damp or boggy soil therefore making it versatile, and suited to a vast amount of different environments where it will establish itself and colonise around lakes, ponds, rivers, streams, ditches, bog-gardens, ornamental ponds and swimming ponds. Nutrient hungry, Acorus Calamus is widely used throughout the filtration industry and is often used for natural habitat replacement schemes as it offers excellent cover for wild-fowl. It is also a favourite breeding ground for Amphibians and emerging Dragonflies, so, what more can I say, apart from its an all rounder and an excellent choice for any water garden from the biggest lake, to the smallest garden pond.

Last but not least is Acorus Calamus Variegata which is an ornamental cultivar. This plant grows more compact and less tall up to a height of 60cm.

Acorus americanus

More Info & Photos of Sweet Flag

Non-Herbicide Management Options

1. Physical Management Options

Blue Flag can be cut, and the rhizomes can be dug up but physical control is difficult.

2. Biological Management Options

There is no known biological control for Blue Flag; although goats are known to forage on many types of emergent vegetation.

Herbicide Control Options

Always read the product label for directions and precautions, as the label is the law. Click on the name of the product to see the label. Read the label for specific water use restrictions.

The active ingredients that have been successful in treating Sweet Flag include:

  • Diquat (Rated: Good)
  • Glyphosate (Rated: Excellent)

These ratings are based upon the U.S. Army Corps of Engineers aquatic herbicide trials.

1) Diquat:

Diquat is a contact algaecide and herbicide. Contact herbicides act quickly and kill all plant cells they come in contact with.

Common trade or product names include but are not limited to:

2) Glyphosate

Liquid glyphosate formulations have been effective on sweet flag above the water line, but ineffective on plants in the water. They are broad spectrum, systemic herbicides. Systemic herbicides are absorbed and move within the plant to the site of action. Systemic herbicides tend to act more slowly than contact herbicides. An aquatically registered surfactant (see the label) will have to be added to the glyphosate solution for good results.

Common trade or product names include but are not limited to:

Precautions

One danger with any chemical control method is the chance of an oxygen depletion after the treatment caused by the decomposition of the dead plant material. Oxygen depletion can kill fish in the pond. If the pond is heavily infested with weeds, it may be possible (depending on the herbicide chosen) to treat the pond in sections and let each section decompose for about two weeks before treating another section. Aeration, particularly at night, for several days after treatment may help control the oxygen depletion.

One common problem in using aquatic herbicides is determining area and/or volume of the pond or area to be treated. To assist you with these determinations see SRAC #103 Calculating Area and Volume of Ponds and Tanks.

Many aquatically registered herbicides have water use restrictions (See General Water Use Restrictions).

To see the labels for these products click on the name. Always read and follow all label directions. Check label for specific water use restrictions.

Cultivation Options

Sweet flag can be propagated through rhizomes kept in constantly moist soil. After emergence, the rhizomes can then be cut into small pieces and planted. Sprouted rhizome pieces are planted 11 inches apart and a depth of 1.5 inches in May to August.

Questions?

If you need assistance, contact the Ag & Natural Resources agent in your county or hire a professional.

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