While it is now against the law to buy or sell Japanese knotweed, this plant can still be found all over North America. If you are unfortunate enough to have the plant enter your yard, here are the steps you can take to kill it.
How to kill Japanese knotweed? Japanese knotweed has an incredibly tough system of roots called rhizomes, which makes it hard to kill the plant. You can cut it back throughout the growing season or try to dig it out of your yard. You can also smother the plant, but this can take a few years to be successful.
What is Japanese knotweed?
Japanese knotweed looks a little like bamboo, thanks to its tall, hollow stems. It then has branches of leaves as well as cream-colored flowers that stand up.
While Japanese knotweed is native to East Asia, in particular Japan, Korea, and China, it can also be found in North America. However, while it is part of a natural ecosystem in East Asia, it is considered a noxious weed in North America.
A noxious weed disrupts its surrounding ecosystem and causes damage to agricultural or horticultural crops. Japanese knotweed was introduced to North America with the best of intentions but it now takes over any area it grows in and is incredibly resilient at attempts to control it.
The best ways to get rid of Japanese knotweed?
There are four ways you can get rid of Japanese knotweed. These include smothering the plant, cutting the plant back, digging the plant up, and applying herbicide.
Unfortunately, you may end up employing multiple methods to get rid of Japanese knotweed as it is so hard to get rid of. We’ll go over how to use these methods in the next section.
Steps on how to kill Japanese knotweed
While not effective on its own, you can regularly cut your Japanese knotweed down to suppress its growth. Cutting is often used in conjunction with other methods of removal.
Use sharp pruners and wear gloves to protect you from the sharp canes. You will need to start cutting down the plant in the spring and then continue to do so as the growing season progresses.
Unfortunately, Japanese knotweed grows quickly and you need to stay on top of its new growth. This includes cutting back any new growth once a week. Eventually, you may be able to use a weed eater to cut back new growth if it is low enough to the ground.
One way to kill Japanese knotweed is by cutting off access to oxygen and light. The process is pretty slow but can ultimately be successful at getting rid of this invasive species.
The best time to smother Japanese knotweed is in early spring before it starts to grow again. You can start by clearing away any of the larger canes of the plant, right down to the ground.
Any canes that poke out of the ground will be sharp, so add leaves or mulch to the base to prevent damage to your smothering material.
Then, lay a heavy-duty tarp or sheet of black plastic over the affected area, ensuring the cover reaches more than 5 feet further than the plant’s growth. This process can take up to five years, so check regularly if the plant has poked through the tarp and be sure to patch up any holes.
Those that have a bit of extra strength may want to tackle Japanese knotweed head-on by digging up the roots. It’s important, however, to understand that the roots are called rhizomes and their construction makes them difficult to dig up completely.
Rhizomes spread out underground and send up shoots along the way. It’s not enough to dig up a root ball like you would with a tree. Instead, you need to dig up all the rhizomes as they will want to propagate.
Start by finding the main clump of rhizomes and dig this up. Then, look for any offshoots making their way through the soil and remove those.
You may end up digging an area that is over 10 feet wide. Be sure to dispose of these rhizome pieces as any bits left out will create a new plant.
For the most part, herbicides are not recommended when combatting plants. There are other organic methods that require just a bit of elbow grease to get the job done.
With Japanese knotweed, however, you might have to resort to chemicals to remove the plant properly. There are a few herbicide options available, but try to find one with glyphosate as it will be the most effective.
Thoroughly read the instructions of any herbicide you purchase and notify your neighbors you will be using one so they can take important precautions. Be sure to wear protective clothing such as rubber gloves and don’t let children or pets access the area.
Cut the Japanese knotweed back so it is only 4 inches tall. Then, wait for a week so new growth appears, which is more tender and better able to absorb the herbicide.
Check on the knotweed area every few weeks to see if the herbicide is taking effect. You will probably have to reapply the herbicide every month or two but over time, the effects will be permanent.
Why is it so hard to get rid of Japanese knotweed?
Japanese knotweed has a rhizome system of roots which means it has a vast underground network that facilitates its growth. Even if you cut off part of the rhizome, the rest will still be alive.
This is why when you try to remove Japanese knotweed you need to place all the pieces of the plant in a bag and ensure it reaches the garbage. The rhizomes are so pervasive, Japanese knotweed has been known to reach through cement cracks and rock walls as the underground branches look for a way up toward the sunlight.
Unfortunately, there are many invasive species that are hard to get rid of, including Japanese knotweed. While you can try to cut it back, smother it, or dig it out, there’s a good chance you will need to resort to a herbicide to remove it altogether.