Japanese knotweed (Polygonum cuspidatum)—sometimes known as Japanese bamboo or more ruefully as Godzilla weed—is one of the world's most invasive plants. If you've ever attempted to get rid of Japanese knotweed, you already know of its monster-like qualities. Japanese knotweed is a shrublike, semi-woody perennial with bamboo-like stems that can grow up to 10 feet tall. It is so tenacious that it has been known to grow through solid masonry foundations, and its roots can penetrate up to 6 feet deep and spread as much as 65 feet.
You can treat Japanese knotweed yourself, and there are several approaches you can use. It is possible to get rid of Japanese knotweed naturally via smothering, cutting, and digging. But there's a good chance you'll need an herbicide for Japanese knotweed, especially if the plant has become established. Plus, it often requires multiple attacks to kill Japanese knotweed permanently.
Here's how to get rid of Japanese knotweed from your garden.
What Is Japanese Knotweed?
Japanese knotweed is an herbaceous perennial plant, meaning it dies back into the ground for the winter before sprouting anew in the spring. It can grow between 3 and 10 feet tall on average and has a bushy appearance. Its leaves are a medium green color, and it sports small white-green flowers in the late summer.
When to Remove Japanese Knotweed
The proper timing for removing Japanese knotweed depends on which strategy you employ. Severe infestations will require repeated attacks throughout the year.
- Smothering: Spring
- Cutting: Throughout the summer
- Digging: Anytime, especially just before using the smothering technique
- Herbicide: Summer or early fall
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Before Getting Started
Japanese knotweed is a member of the buckwheat family. It prefers sunny, moist areas, including riverbanks, roadsides, lawns, and gardens. The plant arrived from Japan to the U.K. and then to North America in the 19th century as a landscaping ornamental. The Westerners who first planted it might have been drawn to its masses of flowers, its heart-shaped leaves, and its bamboo-like canes. But the weed soon spread like wildfire.
There is one piece of good news: Japanese knotweed typically only takes advantage of areas disturbed by humans—areas affording not only ample sunlight but also friable (or crumbly) soil for its invasive roots. It is commonly found along streams and rivers and in low-lying areas.
Using multiple eradication methods right from the start will increase your chances of successfully removing Japanese knotweed from your yard. For instance, you might keep a tarp over the bulk of the problem area during the warm months while cutting or using herbicide along the perimeter as necessary. Then, in late autumn and/or early spring, dig up as many of the rhizomes as you can. Place the tarps back on for winter, so they're already set for the next growing season.
Killing Japanese knotweed permanently might require several seasons. The key is to stick with your project. This weed can be completely eradicated with a dedicated effort on your part.
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Equipment / Tools
- Tarps or black sheet plastic
- Rocks or other weights
- Garden sprayer
- Rubber gloves and protective clothing
- Wood chips or mulch
- Plastic garbage bags
How to Smother Japanese Knotweed With Tarps
Covering Japanese knotweed with tarps suppresses the plant's growth and ultimately kills it. This method is best to do in the spring to catch the plant at the start of its growing season. Be aware that while this is a way to get rid of Japanese knotweed naturally and with relatively low effort, it can take several years.
Prepare the Area
Prepare the area by using pruners to cut mature weed canes (the tall stems) down to the ground and removing any debris. Bag all the debris to prevent it from taking root.
Then, cover the entire area with a thick layer of cushiony material, such as mulch, leaves, or grass clippings. The canes have sharp edges that can easily puncture plastic sheeting or tarps.
Cover the Area
Cover the plant area completely with thick-grade black sheet plastic or heavy non-canvas tarps. If you must overlap pieces, make sure to overlap them by at least 2 feet. The plastic should extend 5 to 10 feet outside the boundary area of the knotweed growth
Use rocks or other heavy materials to weigh down the tarps, so they don't move or blow away. This covering will need to remain in place for a long time, so you can put wood chips or mulch over it for a nicer appearance.
Trample Any New Shoots
As new shoots emerge over time, they might push up the tarps. However, you can easily trample them by walking over the tarps. What growth does occur under the tarps won't amount to much because it lacks sunlight.
Remove the Covering and Replant
After roughly five years, the smothered knotweed and roots should be completely dead. You can now remove the covering and replant the area with whatever groundcover, shrubs, or garden plantings you want.
How to Remove Japanese Knotweed by Cutting
Japanese knotweed can be suppressed, though usually not fully eradicated, by cutting it back. This process often must be used in conjunction with other methods to get rid of Japanese knotweed completely.
Cut the Plant Down
Use pruners to cut the plant down to the ground throughout the growing season, so it's not able to photosynthesize efficiently. Because the cuttings can easily sprout new roots and take hold in the soil, make sure you gather them all up and bag them for disposal.
Monitor and Repeat
Inspect the area weekly, and clip off any new shoots that appear. This is an essential step, as cutting tends to stimulate Japanese knotweed into new growth. So you should not cut Japanese knotweed unless you stay on top of the new shoots, or you might end up with a more serious infestation than you started with.
After initially cutting the stems, you can use a lawnmower set at a low height to trim off new growth as it appears. It will likely require weekly cutting over many months to completely eradicate Japanese knotweed.
How to Remove Japanese Knotweed by Digging Up the Roots
Another way to get rid of Japanese knotweed naturally is to dig up the ground where the weed shoots come up most vigorously. This is typically used concurrently with other methods.
Find the Rhizome Clumps
Locate and dig up the plant's rhizome clumps (underground stems that send up shoots). In mature plants, these rhizome clumps are often very woody and can easily reach widths of a foot or more.
Bag Rhizomes for Disposal
Try to get as much of the rhizomes as possible, and bag them for disposal. Even the tiniest piece left behind can sprout a new plant.
How to Remove Japanese Knotweed Using Herbicide
Some gardeners resort to applying weed killer to kill Japanese knotweed permanently. The best herbicide for Japanese knotweed is one that contains glyphosate. Follow label instructions carefully, including all safety warnings. Late summer is the best time for using a herbicide on Japanese knotweed.
Select a Weed Killer
Select a weed killer appropriate for Japanese knotweed, and read its label instructions and warnings. You'll likely want to wear rubber gloves and protective clothing when using the herbicide to avoid skin contact.
Prune the Knotweed
Cut the Japanese knotweed to roughly 3 to 4 inches above the ground. This will stimulate tender new growth that will readily absorb the herbicide.
Apply the Weed Killer
Once new growth has sprouted from the cut stems (within one to two weeks), carefully saturate the growth with herbicide. Glyphosate works when it is absorbed by foliage to inhibit plant enzymes necessary for growth. Foliage that turns yellow and dies indicates that the plant is dying—roots and all.
Repeat as Needed
Watch the area carefully over the next few months, and apply additional weed killer to any new growth that appears. This herbicide method generally will kill Japanese knotweed permanently over time.
Other Methods of Japanese Knotweed Removal
Some people have tried killing Japanese knotweed with other methods, including with diesel, bleach, lime, and salt. However, these methods are not effective. Killing Japanese knotweed with diesel might seem promising, as the diesel will damage the foliage. But the plant will regrow just as healthy as ever, and you'll have polluted your yard in the process. You'll have the same issue when attempting to kill Japanese knotweed with bleach or lime. Moreover, Japanese knotweed has a high tolerance for salt, so that won't be able to get rid of it either.
When to Call a Professional
If your attempts to get rid of Japanese knotweed have been unsuccessful, it's likely time to call in a professional. They often have more substantial tools and products that can get the job done. Japanese knotweed removal costs can vary widely, depending on how widespread and stubborn your plants are. A lawn service should be able to give you a quote for your specific area. The good news is reputable lawn services often have a money-back guarantee that they can resolve the problem.
Invasive Plant Profile: Japanese Knotweed. National Park Service.
Shin J, Lim N, Roh S. Severe chemical burns related to dermal exposure to herbicide containing glyphosate and glufosinate with surfactant in Korea. Ann Occup Environ Med., vol. 32, 2020. doi:10.35371/aoem.2020.32.e28
Managing Japanese Knotweed. Vegetation Management Department of Horticulture College of Agricultural Sciences, Penn State University.
Weed Control Handbook. Center for Invasive Species and Ecosystem Health.
Invasive knotweeds are highly tolerant to salt stress. National Library of Medicine.