Japanese Sweet Flag Shrub: Growing, Planting, and Care for Sweet Flag Grass

Image Credit: Daderot¹⁶

Japanese Sweet Flag Grass (Acorus gramineus)

Acorus gramineus is an aquatic plant that is native to Japan, South Korea, and other regions in East Asia.² This plant can grow in swamplands, bogs, and other wetlands.

It grows optimally in at least four inches of water, very moist soil, and when partially submerged in water. This Japanese variant of Sweet Flag propagates its existence mainly through clumps of it, or its rhizome being cut and divided to grow as separate sections.

A rhizome is a modified version of a plant stem that grows horizontally underground while also growing nodes, shoots that grow leaves, and roots.³ Rhizomes grow out horizontally and create sprawling plant ground cover above ground as they grow.

Growing a Japanese Sweet Flag From a Seed, Cutting, or Seedling

Acorus gramineus is a sterile species of plant that does not grow seeds. The optimal way to start growing the Japanese variant of Sweet Flag is to get a rhizome cutting or to get a smaller portion that was cut away from a larger portion.

This Japanese variant of the Sweet Flag should be divided every 24 to 36 months because as perennial, it will die.

Additionally, as it grows it will thin out and begin dying off in the center.

(Image: cultivar413¹⁹)

You can use almost any soil medium in any condition with any pH level. Acorus gramineus is not picky about such things.

You can use heavy or light clay, loamy soil, chalk, gravel, or coconut coir as a soil medium. Use a large and wide bucket or a large aquarium tank to grow Acorus gramineus in.

The rhizome and soil medium of this plant should be submerged under at least four inches of water to grow optimally. The soil medium should be always soaked and sightly submerged at all times.

While you might be able to grow this Japanese variant of Sweet Flag via a rhizome cutting, it could take months or over a year before you notice any growth. You can get a patch of rhizome clumps or better yet a divided smaller section from a larger section to grow.

Do yourself a favor, and get yourself a smaller section that was cut from a larger patch of the plant. This is the easiest way to get started, especially if you are looking for low-maintenance plants to take care of.

If you have a clean pond on your property or landscaping stream, you could just put the Acorus gramineus division patch directly in the pond or on the bank of the stream. If you opt to put your small patch in a container, make sure that you use a large and wide flower pot, bucket, or aquarium tank without the fish.

Gently loosen the rhizome roots before planting. The hole in the soil medium should be just as deep as the rhizome stem mass depth and a little wider than its width.

You don’t have to use a wide patch either; the patch of Japanese Sweet Flag you plant can be a few inches wide and long.

Best Growing Conditions for Japanese Sweet Flag

Here are some planting tips for the Japanese Sweet Flag and the best maintenance advice for growing conditions.

Sunlight Needs

So, how much sunlight does Japanese Sweet Flag need each day? This plant needs at least 4 to six hours of direct sunlight.

However, this Japanese variant of the Sweet Flag can also scorch easily in direct sunlight. Moreover, while it will do fine in humid heat, it could go dormant or die in dry heat.

This plant will do well in indirect sunlight, but if you grow it indoors, try to expose it to direct sunlight for a few hours per day.

As for when to plant Japanese Sweet Flag for the best yield, you can plant it outside anytime from spring to summer to experience optimal growth.

Watering Needs

What are the watering needs for Japanese Sweet Flag plants? If you put your Japanese grass patch in a pond on plant it on the side of a landscape stream, it will basically grow without much input from you.

If you opt to grow it in a container then you should keep it submerged in at least four inches of water at all times.

You could also create your own makeshift water dispenser for your plant. Take a water bottle cap and carefully poke a hole in it wide enough to narrowly insert a cotton swab in.

Insert a cotton swab and make sure that it is tightly wedged in the hole with enough room for minute trickles of water to pass through. Fill the water bottle with water and screw on the cap.

Then, tape a chopstick to the water bottle. Insert the chopstick at an angle so that it rests on the side of the bucket, pot, or aquarium tank.

Your makeshift water dispenser will now slowly drip water off the cotton swab onto the soil. Otherwise, you should remember to add half an inch to an inch of water to the container every week.

As long as the soil is lightly submerged in water and remains soaking wet constantly, it will grow. It will take more vigilance to water a tree in a container.

Spacing Requirements

Wondering how far apart to plant Japanese Sweet Flag for optimal growth? That is up to you.

A small patch of sweet grass will grow slowly via incremental sprawl over 24 to 36 months, relative to the size of your growing container. If you are growing a patch of Japanese grass in a large bucket, pot, or aquarium, then this is not an issue.

If you don’t care about separate patches touching each other, you can grow them as close together as you wish. However, if you plan to plant outdoors and want to give each underground rhizome system time and space to grow and sprawl horizontally, then separate each separate plant patch by at least a foot or more.

Pest Concerns

If you are wondering how to stop Japanese Sweet Flag disease then you don’t have to worry about it. There are not many common pests of the Japanese Sweet Flag.

In fact, Japanese grass is known to be a deer deterrent. So, there is no need to enact natural pest control for Japanese Sweet Flag.

Even if your plant is afflicted with rust or Anthracnose, for example, which are common plant fungal diseases, Acorus gramineus is known to recover quickly. You are more likely to invite disease or pests if your plant does not have adequate air circulation, grows in excessive shade or darkness, or is fertilized too much or often.

So, there is no need to initiate any Japanese Sweet Flag disease prevention tactics.

Japanese Sweet Flag Growing Zone

This plant will grow in USDA Hardiness Zones 4,5,6,7,8,9, 10, and 11. The plant tends to go dormant in cold weather, so if you live in a frosty region, then grow it in a container indoors.

(Image: Krzysztof Ziarnek, Kenraiz²⁰)

Be mindful of ambient heat around your plant. Japanese grass cannot tolerate dry heat for extended periods but will be fine in humid heat if you’re worried about planting zones.

You don’t necessarily have to worry about growing zones for Japanese Sweet Flag or where to grow if you follow the basic instructions in this comprehensive guide.

Japanese Sweet Flag Growth Rate

Japanese grass is a plant that grows very slowly and on its own time. It can sprawl and grow horizontally for about two feet within five years.¹²

It should grow vertically a few inches per year. For context, have you ever wondered how long does it take for a tree to grow?

That may sound like a long time, but it could take you years or even a decade to grow a tree depending on the types of trees and species.

What Are Some Companion Plants For Growing Japanese Sweet Flag?

Any companion plant that you grow about a foot from your Japanese variants of the Sweet Flag should be able to survive in submerged water or constantly soaked soil. Such plant species include monkshod, bushy aster, witch hazel, Coleus, Philodendron, Begonia, arrowhead, and Japanese primrose.

If you are interested in extremely low-maintenance gardening, then you can do no better than plant a small patch of ornamental Japanese grass in a container. Visit a local nursery today and get some Japanese Sweet Flag to start your low-maintenance gardening hobby.

How To Identify Japanese Sweet Flag

This Japanese variant of the Acorus genus can easily be mistaken for grass except for the exacting ambient conditions it requires to grow optimally. This plant closely resembles traditional grass but needs to be submerged in water or grow in a consistently moist soil medium.

(Image: F. D. Richards¹⁷)

The biggest telltale sign for recognizing Acorus gramineus is the pleasantly sweet smell it emits when its leaves, which look like elongated narrow blades of grass, are stepped on or broken. The smell is sweet with aromatic hints of citrus.

The leaves can grow anywhere between a height of six inches to a foot. A patch of Acorus gramineus can be a few inches to over two feet long or wide.

Since it is a perennial, you may notice that this plant grows in patches as the rhizome grows horizontally and sprawls.

Japanese Sweet Flag Leaves

The leaves of this plant usually grow anywhere from six to 12 inches long, although it might even grow as long as a foot and a half. Some leaves of this species in the wild can grow as long as three feet or more.

The narrow leaves are sword-shaped and about a quarter of an inch wide. The leaves resemble traditional grass blades and can be green, dark green, greenish yellow, or golden yellow in color depending on the cultivar.

The leaves are known for swaying and arching in the wind like a narrow flag, a signature aesthetic action that may have contributed to the designated namesake.

Japanese Sweet Flag Flower

Barring any known cultivars of this plant, Acorus gramineus does not bloom flowers. This species is sterile and does not grow seeds or bloom flowers.

While some experts do believe that this plant does bloom flowers, they are usually too insignificant and non-viable to be noticed. Additionally, this Japanese variant of Acorus must be submerged for sustained amounts of time for inefficient flowering to occur, which usually does not.

This flower, when it grows, is usually a small and non-viable spadix that grows more prominently on sibling species like Acorus calamus and Acorus americanus.¹¹

Japanese Sweet Flag Seeds

Acorus gramineus is a sterile plant and does not produce seeds. It propagates itself via the division of its leaf patches or manual division or cutting of the rhizome.

Facts About Japanese Grass

Here are several Japanese Sweet Flag facts to consider. The plant gets its name from the fact that the narrow leaves emit a pleasing and sweet smell whenever they are broken or crushed.

Its rhizome is supposedly edible if chopped finely and soaked in water several times, though bitter to the taste. The rhizome can be eaten either raw or cooked.

Acorus gramineus is the Japanese sibling of the Acorus calamus, which is known as the Common Sweet Flag or Muskrat Root.⁴ Japanese and common Sweet Flag are not the same thing even though they share the same scientific family and genus.

A Common Sweet Flag produces a spadix, a cone of flowers that looks like a corn cob and it produces seeds. Besides that, the Common and Japanese Acorus variants look similar.

(Image: F. D. Richards¹⁵)

Sweet Flag is an ancient plant that has been used by humanity since ancient times. There are references to Calamus, or Sweet Flag, in the Christian Bible.⁵

Ancient Romans, Greeks, Indians, and other cultures used Sweet Flags to create traditional medicines, foods, and even purification agents. Some experts believe that the Sweet Flag was used by the Mongol hordes as a purifying agent to clean stagnant water.⁵

Plants in the Acorus genus, like Acorus gramanieus, are true descendants of ancient Acorus plants, according to science. Plants in the Acorus genus are descendants of the first monocots, or grass-like plants, that ever existed in the world.⁶

It cannot be understated that this Japanese variant of the Sweet Flag is a member of an ancient plant that humanity depended on for millennia. The Common Sweet Flag was exported to Europe and the Americas in the 1600s, although some experts believe that it was native already and might have been imported much earlier.⁷

Sweet Flag was used as an industrial ingredient in medicine, products, and food, and was even processed to make candied rhizome flag roots in the early 20th century.⁷ Acorus calamus was banned by the American government in 1968 and deemed to be toxic and a carcinogen.⁸

The Common Sweet Flag cannot be used in commercial foods and its rhizome is not recommended for consumption whether raw or cooked. It was also deemed to have psychoactive properties as well.

Modern research has yet to thoroughly prove whether common sweet grass can manifest all of the medicinal benefits that ancient humans used it for since antiquity.⁴ It must be stressed that even though Acorus gramineus is a sibling of Acorus calamus and share the same scientific genus and family, Acorus gramineus has not been scientifically proven to be toxic or dangerous to humans.⁹

Modern science believes that the Japanese variant of the Sweet Flag could offer medicinal benefits that the Common Sweet Flag can’t. Acorus gramineus is currently being experimented on to create anti-allergy medications.¹⁰

Some experts even believe that you can use the rhizome of Acorus gramineus as an alternative to ginger when cooking.⁹ The point is that if you design to divided segment of a Japanese Sweet Flag or a rhizome from a nursery, make sure that you request Acorus gramineus.

Where Did the “Flag” in the Plant’s Name Come From?

No one knows for sure. Some experts believe the term comes from the fact the narrow leaves droop and sway in the wind like a flag.

Others believe that since some Sweet Flag species were exported to Europe in the 1600s, the plant was named after the Middle English word “flagge” which roughly translates to “reed.”¹⁴ Otherwise, no one knows for sure the Japanese Sweet Flag symbolism connected to the term “flag.”

Japanese Sweet Flag for Your Home Garden

The Japanese Sweet Flag is actually an aquatic plant that looks like a patchy swath of grass with long blades. It propagates mainly via rhizome growth and grows out horizontally, like a sprawling ground cover of grass.

It can be grown in swamps, bogs, marshlands, and on the sides of lakes. Many homeowners like to use it as an ornamental gardening aesthetic to visually accent strategic parts of their landscapes.

(Image: cultivar413¹⁸)

For example, a patch of Japanese Sweet Flag can be used on a small pond or stream on a luxury landscape to draw attention to the lawn, flower garden, and gravel walkway. If you’re interested in growing a larger patch of Japanese Sweet Flag in a home garden or a more flamboyant home and lawn landscape, this plant is a great buffer medium to stop soil erosion.

You can divide some of the rhizome undergrowth or divide a patch of the plant into two parts or a quadrant and then just continue growing it. Japanese Sweet Flag will accommodate any gardener who is interested in gardening in a basic easy mode.

Many home gardeners prefer plants that can grow in gravel because gravel-based gardens require 80 percent less work, maintenance, and vigilance than a traditional garden. The second most popular type of gardening in the country is urban gardening, where city gardeners make the most of gardening in limited spaces.¹

Still, if you are more interested in low-maintenance home gardening, you can grow a Japanese Sweet Flag in a large and wide bucket, in a big aquarium tank minus the fish, or in heavy clay, loamy soil, chalk, gravel, or any kind of soil with any type of pH.

Frequently Asked Questions About Japanese Sweet Flag