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Japanese Knotweed: Plant ID, Uses, Invasiveness & Removal

Japanese Knotweed: Plant ID, Uses, Invasiveness & Removal

Some plants have been known to spark controversy, and Japanese knotweed is one of them for sure.

Of the people who are quite familiar with the plant, you won’t find two people who totally agree on whether it’s a good plant or not. Most people have mixed opinions about it while some hate it outright.

What is Japanese knotweed? Japanese knotweed (Reynoutria japonica) is a herbaceous perennial plant native to East Asia. It has hollow stems and raised nodes like bamboo with broad leaves and small cream or white flowers that attract bees. It’s considered invasive in many areas and can be quite destructive to buildings and concrete foundations.

Whether you want to plant Japanese knotweed in your garden or you just want to identify it to avoid its perils, this article is for you. Read more to learn about its uses and how to safely remove it.

Japanese Knotweed Identification

The problem with Japanese knotweed is that it looks just like many other plants, such as dogwood, Hymalian balsam, and lilac.

You need to know how to identify Japanese knotweed to safely remove it from your property.

Japanese Knotweed Size

In its natural habitat, a mature Japanese knotweed plant can grow to 10 or 13 feet tall.

However, the plants grown in North America and Europe won’t grow above 7 feet due to regular cutting and adverse growing conditions.

Japanese Knotweed Stems

The stems of Japanese knotweed are the main reason the plant grows to such heights. Each stem is hollow with raised nodes. This is what makes many people mistake Japanese knotweed for bamboo.

The stems start growing in the summer and reach their full height before the fall.

New growths emerge from the nodes on older stems, which in turn become stems that carry nodes and leaves. They often have purple speckles all over them.

Japanese Knotweed Leaves

The leaves of Japanese knotweed are broad with a narrow base that makes them look like shovels.

The mature leaf grows between 3 and 5.5 inches long and between 2 and 4.5 inches wide. The new leaves are red and turn dark green with red veins as they mature.

Japanese Knotweed Flowers

The flowers of Japanese knotweed are small compared to the broad leaves. They’re usually cream or white and open in late summer or early fall.

The flowers are full of nectar that attracts bees and other pollinating insects to the garden.

A close look at Japanese knotweed's leaves, stems, and flowers.

Japanese Knotweed Roots

The roots of Japanese knotweed plants in addition to their invasiveness are the main reasons the plant is so problematic in North America and Europe.

The roots are well developed, grow in a 20-foot radius, and are quite aggressive in their growth — so much so that they can damage the roots of buildings, flood-defense systems, concrete formations, and even roads. 

The roots of the plant are such a big problem that insurance companies often refuse to cover properties with Japanese knotweed growing in the vicinity.

Does Japanese Knotweed Have Thorns?

Japanese knotweed stems are green and smooth with purple speckles, but they don’t have any thorns.

Young Japanese Knotweed

If the mature Japanese knotweed looks like bamboo, the young plant looks like asparagus. The stems are often green with purple speckles. The leaves shoot out from the nodes on the stems in a zigzag shape.

As it grows, the plant develops into a thicket with aggressive roots that smother all types of vegetation around it. 

Where Is Japanese Knotweed Found?

Japanese knotweed is hardy in USDA zones 4 to 10, so it can be found growing just about anywhere. It grows near roads or flood-defense systems. It grows in unattended gardens, yards, and vacant lots.

It can even emerge from the cracks of the curbside and concrete slabs!

If you find Japanese knotweed on your property, you need to remove it quickly as its presence can cause the property to lose value.

Japanese Knotweed Look-Alikes

Japanese knotweed poses a threat not just to buildings and roads but also to plants whose only misfortune is that they look like the invasive Japanese knotweed. These plants include the following:

  • Houttuynia: When young, houttuynia (chameleon plant) looks like Japanese knotweed both in stems and leaves, but since it only grows to about 12 inches tall, the mature plants look nothing like each other.
  • Red Bistort: One of the many ornamental bistorts, also known as Mountain Fleece,  that look like Japanese knotweed. The flowers help distinguish the two plants since bistorts have clusters of flowers that are either pink or white. The stems of bistorts are thinner than Japanese knotweed stems.
  • Himalayan Balsam: This is an invasive plant that reaches the same mature height as Japanese knotweed and has hollow stems as well, but the leaves of Himalayan balsam are longer and thinner with a red midrib. The flowers are pink and larger in general.
  • Broadleaved Dock: Although it looks like Japanese knotweed, the stems of the broadleaved dock are fluted, filled with foam, and only grow to about 3 feet tall. The leaves form a rosette shape.

Japanese Knotweed Growth Rate

The Japanese knotweed has a fast growth rate, and during the summer it can grow at a staggering rate of 4 inches per day.

The roots are even more aggressive in their growth and would strangle or push aside anything that comes their way.

The roots grow to about 20 feet in every direction and cause problems to buildings and plants alike. 

Is Japanese Knotweed Invasive?

Japanese knotweed is an invasive plant both in Europe and North America. Its hardiness and ability to withstand adverse growing conditions such as poor soil and drought allow it to grow just about anywhere.

It spreads easily and aggressively. If you plan to grow Japanese knotweed, you should consult your County Extension Service for policies regarding this invasive plant. 

Is Japanese Knotweed Edible?

In Japan, Korea, and other Asian countries, Japanese knotweed is considered to be quite edible. The new shoots and leaves can be eaten raw or cooked. They have a tarty flavor, and the shoots are crunchy.

It can be added to many desserts. Some dishes that are based on Japanese knotweed include Japanese knotweed muffins, Japanese knotweed crumble, and Japanese knotweed fool.

Japanese Knotweed Benefits

For all its invasiveness and destruction, Japanese knotweed has its benefits. Here are some of them:

  • The flowers are packed with nectar that attracts bees, butterflies, hummingbirds, and other pollinators. 
  • The leaves and young shoots are edible with a tart taste and go into many desserts.
  • Eating the edible parts of the plant might improve cognitive abilities.
  • Eating it regularly could prevent cardiovascular diseases.
  • Studies show a link between eating Japanese knotweed and improved blood pressure.
  • It has medicinal properties including relief from gastrointestinal discomfort and pain.

Why Is Japanese Knotweed Bad?

Some people hate Japanese knotweed on sight, and they are not to blame.

The plant has a destructive habit and fast growth rates that make it difficult to manage or control it. Its roots run deep and extend way beyond the crown of the plant.

Japanese Knotweed Damage

Japanese knotweed causes a lot of damage to buildings and structures within a 20-foot radius.

The aggressive roots can damage buildings, clog underground tunnels, and even cause pipes underground to crack.

Even roads are not safe from damage as the roots explore the fault lines in roads and concrete slabs.

A property with Japanese knotweed growing on it loses value over time, and insurance companies often refuse to cover such properties.

Can Japanese Knotweed Kill Trees?

The aggressive roots of Japanese knotweed are quite competitive when it comes to finding moisture and nutrients in the soil. This can impact trees and shrubs growing in the same area.

It also towers over many trees and cuts off the sunlight, which causes them to wither and die.

Is Japanese Knotweed Poisonous?

Japanese knotweed is not poisonous. The edible parts are tart in taste but cause no harm to those who eat them.

How Does Japanese Knotweed Spread?

Japanese knotweed spreads fast through broken stems or parts of the rhizomes.

It’s not clear whether the stems are broken by accident due to strong wind or the plant propagates itself by breaking the stems so that they land on another spot and start a whole new Japanese knotweed. 

Does Cutting Japanese Knotweed Make It Spread?

When you cut Japanese knotweed, there’s a risk the broken stems and parts of the roots will spread elsewhere and start new plants on their own. Cutting the plant is not the ideal way to get rid of it.

Japanese Knotweed Removal

  1. Use pruning shears to cut the canes and stems of the mature plant down to the ground.
  2. Cover the area where the thickets grow with a thick tarp. Place cinder blocks at the corners and edges of the tarp to weigh it down.
  3. When new shoots emerge and push against the tarp, trample them with your feet.
  4. Keep the tarp on for about 5 years to make sure the roots and all new shoots are dead.
  5. Replant the area with a good ground cover.

Conclusion

Japanese knotweed is an invasive plant that causes damage to buildings, foundations, roads, and flood systems. However, it also has some benefits as the leaves and new shoots are edible either raw or cooked.