Japanese Knotweed is an Infestation. Here’s How to Remove It.

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Japanese knotweed strikes fear in the hearts of many homeowners, even more so than the common dandelion. Originally sold as an erosion-control plant in the 1800s, this resilient weed is nearly impossible to kill and spreads like, well, a weed. And despite some of the rumors about this plant being untrue or overblown, property values are still affected by it.

Japanese Knotweed Identification

Japanese knotweed looks similar to bamboo, except it has wide and ovate purplish leaves. It produces small, white flowers in August or September and can grow six to 15 feet tall. It’s shrubbery in appearance, and you can find the plant in the U.S., Canada, Europe and Asia.

In fact, Japanese knotweed is native to Eastern Asia, including Korea, China and Japan. It was introduced to the U.S. as an ornamental plant in the late 1800s and was originally used for erosion control. By the 1930s, it became a problematic species. Its erosion-resistant properties have been debunked, with a Canadian study finding that the plant increases erosion.

Why is Japanese Knotweed Bad?

Most invasive species wreak havoc on local ecosystems, and Japanese knotweed is no exception. However, this plant is a massive problem for humans, too.

Here’s why knotweed is bad:

• Grows quickly: It can grow 10 centimeters, or almost 4 inches, a day during its growing season. This makes it harder for homeowners to keep up with trims. Because it grows so quickly, the leaves can choke out surrounding flora, as it’s a dense plant.
• Biodiversity loss: When native plants are displaced or killed by Japanese knotweed, insects, birds and other wildlife are left to find food elsewhere.
• Erosion risk: This plant increases erosion along riverbanks rather than preventing it.
• Legal risks: In places like the UK, landowners must prevent Japanese knotweed from spreading beyond their property. Failing to manage its spread could lead to legal consequences.
• Difficult to remove: Japanese knotweed is incredibly resilient. It can take up to 5 years to remove it using smothering, and even longer for the ground to be plantable again. Incorrect removal could cause the plant to grow more aggressively or in another place. It’s so resilient that its roots can survive cold Ontario winters underground.
• Expensive to remove: Because knotweed is difficult to remove, the cost adds up. An expert should also do herbicide applications and excavation to avoid another infestation. This may make your home harder to sell in the future.
• Devalued property: Though it’s unclear how much Japanese knotweed can reduce a property’s value, most property professionals in the UK agree it has a negative impact on value. It’s unclear if knotweed can affect property value in the U.S. However, if a potential homeowner knows the property has the plant, it could deter them.
• Property damage: While Japanese knotweed doesn’t cause property damage itself, it will take advantage of issues in the foundation to break down the structure.

Knotweed is a pest that can cause serious issues, but knowing how it spreads can lessen the stress. Unfortunately, this plant is incredibly easy to sprout.

How Does Japanese Knotweed Spread?

In most cases, Japanese knotweed spreads through the dispersal of its stems, crown and rhizome fragments. Its fragments usually spread when water or soil migrates elsewhere. Since its seeds rarely germinate, it’s uncommon for knotweed to grow this way.

The transportation and sale of knotweed is illegal or prohibited in multiple countries, including Canada. Despite this, the plant has no problem taking over. This is because each Japanese knotweed plant is supported by an underground system that fuels its growth. This is why killing weeds down to the root is important to stop an infestation.

While it’s difficult to spread the weed through cross-breeding, contaminated boots and animal droppings can move the plant to another location. Removing it entirely from any country may be impossible, but for some homeowners, it’s a matter of saving money.

What Kills Japanese Knotweed?

Killing Japanese knotweed will be very difficult, so calling a professional is best. But if you’re considering handling the removal process yourself, here is what to do.

Get the Right Tools

Death by smothering, herbicide, digging up the roots and cutting it back are the four common methods of destroying knotweed. Depending on how you kill the weed, you may need pruners, a rake, a tarp, rocks, a shovel, a garden sprayer, rubber gloves, wood chips, plastic garbage bags and herbicide. For the herbicide, you may need a specific type of chemical.

The Smothering Method

The smothering method requires pruners, a bag, wood chips and a tarp:

• Prepare the area by cutting the tall stems and bagging all the debris
• The area should be low enough to cover it with wood chips
• Above the wood chips, place a heavy non-canvas tarp to block out the sun.
• When new shoots grow, walk over the tarp and stomp on them until crushed.

This method can take many years, but you’ll know you’re done when the plant stops growing.

The Cutting Method

While this method doesn’t completely get rid of Japanese knotweed, it can contain it while you use other means to destroy the plant. All you’ll need is pruners and a garbage bag. Every week, prune the weeds back until they are short enough to walk over. Always put the weeds in a bag and remove them from the area when done. Do not use a lawn mower to cut the stems.

Digging Up The Roots Method

This method complements cutting the weed, as the only true way to destroy the plant is by targeting its roots. You’ll need a shovel and garbage bags. To start, break up the ground with a shovel and locate the plant’s rhizome clumps, which are thick, underground stems. Bag the rhizomes carefully for disposal. Any clump left behind could grow into another plant.

The Herbicide Method

Herbicides can be dangerous, so wear protective clothing and read each chemical’s instructions carefully before use. Here is how you could treat Japanese knotweed with herbicide:

• Purchase a 25% triclopyr or glyphosate solution. Imazapyr and picloram also work.
• Prune the knotweed three to four inches above the ground.
• Once new growth sprouts, saturate it with herbicide. 
• When the foliage turns yellow and limp, remove it with a shovel.

You may need to repeat the process several times until the weed stops growing.

Japanese Knotweed is Complex, but Possible to Remove

Knotweed is an invasive weed that poses environmental, structural, legal and monetary risks to homeowners and governments. Getting rid of the plant is usually a good idea, especially if your government mandates it. Fortunately, there are many ways to remove Japanese knotweed, even though these methods can take a while. But for property values to increase, it’s necessary.