Japanese knotweed (Polygonumcuspidatum)—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.
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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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Click Play to Learn How to Get Rid of Japanese Knotweed
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 knotweedfrom 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 mightrequire 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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What You'll Need
Equipment / Tools
- Pruners
- Rake
- Tarps or black sheet plastic
- Rocks or other weights
- Shovel
- Garden sprayer
- Rubber gloves and protective clothing
Materials
- Wood chips or mulch
- Plastic garbage bags
- Herbicide
Instructions
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.
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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.
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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.
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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.
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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.
(Video) How to Control Japanese Knotweed
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
16 Types of Invasive Plants You Should Avoid
FAQs
What kills Japanese knotweed permanently? ›
Chemical controls of removing Japanese knotweed
A glyphosate-based weedkiller is the best option here, though bear in mind it can take several applications, over up to four seasons, to completely eradicate Japanese knotweed. It's best applied to cut canes so the weedkiller can thoroughly penetrate the plant and roots.
Spraying or injecting the stems with chemicals can be an effective treatment to stop knotweeds spreading. You must only use approved herbicides. You'll have to respray. It usually takes at least 3 years to treat Japanese knotweed.
How do you get rid of Japanese knotweed fast? ›The best approach to control is through a combination of cutting and herbicide application. A late spring/early summer treatment followed by an early fall re-treatment is needed. Several years of treatment may be needed for well-established populations.
How do you get rid of Japanese knotweed naturally? ›How can you get rid of Japanese knotweed organically? Get rid of Japanese knotweed organically by either digging it up, burning it or smothering it with a tarpaulin. All these methods do not require the use of any herbicides that could potentially damage nearby plants, however, they each have their drawbacks.
What is the best Japanese knotweed killer? ›Roundup, Gallup, Landmaster, Pondmaster, Ranger, Rodeo, and Touchdown are all herbicides recommended to kill Japanese Knotweed. They are all glyphosate-based herbicides and will kill the troublesome weed. The best time to spray the leaves of Japanese Knotweed with herbicide is late summer or early autumn.
How do you suffocate knotweed? ›- Identify Japanese Knotweed as soon as possible to prevent further growth and damage.
- Cut down and remove the canes. ...
- Apply Glyphosate based Weed killer. ...
- Wait at least 7 days before pulling the weeds. ...
- Mow the plants weekly. ...
- Reapply Glyphosate.
Due to the powerful root and rhizome system, which extends deep into the ground, knotweed is notoriously difficult to treat or remove without professional help – and failure to do so could result in your property being damaged and devalued.
What eats Japanese knotweed? ›Psyllids feed on the sap of the knotweed, diminishing its energy supply and ultimately killing the plant. Researchers found that the Japanese knotweed psyllid's preference is specific to the three targeted knotweeds, and it is not expected to damage any native or related knotweed family plants.
Is it easy to get rid of knotweed? ›It is very recognisable as a tall herbaceous plant. It can grow 3m a year and it's underground rhizomes can spread 7m across and penetrate anywhere from 2 to 7m deep. This clearly makes the plant almost impossible to eradicate. Tall red stems grow at a rapid rate and produce large heart shaped leaves with flat base.
What is the best chemical for knotweed? ›Glyphosate is the herbicide of choice for controlling knotweed.
How deep are Japanese knotweed roots? ›
Japanese Knotweed is a resilient and tough weed. It has roots that can grow to a depth of 2 meters. These roots are known as rhizomes – they're a special sort of root that is also a 'shoot'. These stretch out horizontally and can extend up to 7 meters wide.
How do you get rid of knotweed naturally? ›Cut Down and Remove the Canes
One method is to use sharp pruning shears or loppers to take down the stems as close to the ground as possible, making sure to remove every last cut piece and fragment because as little as half an inch of the root or cut stem can grow into another plant.
The cheapest way to deal with a Japanese knotweed infestation is with herbicide treatment. However, this is generally recognised as being a 'control' rather than a one-off solution, because there's a risk of dormancy and regrowth, especially on land that's disturbed.
How do you stop knotweed from growing? ›Japanese knotweed is best controlled by the application of a suitable herbicide. Glyphosate-based herbicides are commonly used to treat Japanese knotweed. If glyphosate is applied correctly, at the appropriate time of year, it is possible to eradicate it, although it can take two to three years of repeated treatment.
Is Roundup good for Japanese knotweed? ›Glyphosate based products, like Round Up, are effective against japanese knotweed. These herbicides can either be sprayed onto the leaves or injected into the stem of the plant. The plant should be sprayed with weed killer at several stages during its growth.
Is there anything good about Japanese knotweed? ›Herbalists use Japanese knotweed to reduce plaque, gum pain, and gingivitis, claiming it has anti-inflammatory properties. Additionally, emodin is a natural compound in Japanese knotweed roots that serves as a natural laxative.
What happens if you touch knotweed? ›Japanese knotweed is not poisonous and is not harmful to touch, however, always use caution to avoid inadvertently allowing the infestation to spread.
Will grass choke out Japanese knotweed? ›The best way to prevent knotweed from making a return and taking over is to grow healthy turf grass that will choke out any weed attempting to grow.
What happens if you burn knotweed? ›The burning of Japanese knotweed is an effective way to eradicate it from a site, but only when the plant is absent of moisture. Here, you need to ensure that the strands of the plant are fully dried out. Once this has happened, the knotweed can be properly burnt, ensuring there is no possibility of it regrowing.
What plants compete with Japanese knotweed? ›The riparian buffer and prairie cordgrass – Virginia wildrye mixtures were most competitive with Japanese knotweed. Figure 8. The 2008 planting of prairie cordgrass and Virginia wildrye, shows promise to control Japanese knotweed.
Should you burn Japanese knotweed? ›
Can I Burn Japanese Knotweed? You can but you must do this with extreme care. When the knotweed material has been excavated, cut the stems and leaves and leave it to dry before burning it, ideally without contact with the soil.
How long does it take to smother Japanese knotweed? ›Smothering can take upwards of five years. Japanese knotweed can also be effectively controlled with herbicides, but timing is key. The flow of nutrients in Japanese knotweed is in one direction. In the spring and throughout the growing season, sugars and nutrients move upward from the roots to the shoots.
How long does Japanese knotweed live for? ›Because of this, Knotweed rhizomes can stay viably healthy for up to 20 years even after herbicides have been used, highlighting the need for a professional knotweed rhizome removal treatment plan.
Should I worry about Japanese knotweed? ›It will push its way through expansion joints in concrete, cavity walls, weaknesses in the broken mortar between paving slabs or bricks and can also damage drains and sewers. While structural damage is rare in most residential situations, if left unchecked, a mature infestation of Japanese Knotweed can become critical.
How do the Japanese control Japanese knotweed? ›How the imported insects control Japanese knotweed. The imported insects, aphalara itadori, control Japanese knotweed by sucking out the sap from the stem which causes the invasive weed to dry out. The insect does not feed on anything else other than Japanese knotweed and is therefore entirely dependent on the weed.
How far down do knotweed roots go? ›Under the surface of the ground, roots can grow as far as 7 metres horizontally and up to 3 metres deep [2] from each Japanese knotweed shoot. Due to the speed and ease with which Japanese knotweed is capable of spreading, the plant has been labelled as invasive by the UK government.
Who is responsible for Japanese knotweed? ›Japanese Knotweed is classified as an invasive species it is therefore the responsibility of the land owner to prevent the plant spreading to neighbouring land (or into the wild), and removal of plant must be conducted with due care and attention.
What kills knotweed in lawn? ›For best results, a preemergent herbicide should be applied prior to seed germination. There are many herbicides that can control prostrate knotweed. Preemergent active ingredients labeled to control prostrate knotweed include atrazine, dithiopyr, oxadiazon, pendimethalin, prodiamine and trifluralin.
What happens if you touch Japanese knotweed? ›Japanese knotweed is not poisonous and is not harmful to touch, however, always use caution to avoid inadvertently allowing the infestation to spread.
What kills giant knotweed? ›Glyphosate is the herbicide of choice for controlling knotweed. It is effective, has no soil activity, it is readily available, and somewhat inexpensive.
Is Japanese knotweed good for anything? ›
Japanese Knotweed, or Polygonum cuspidatum, is a Traditional Chinese Medicine used for circulation and heart health. It is a very good source of resveratrol, and most benefits of Japanese Knotweed may actually just be benefits of resveratrol.
What time of year is best to spray Japanese knotweed? ›To achieve control of Japanese knotweed, glyphosate must be applied in late summer/autumn after the plant has flowered. Applying glyphosate earlier in the year may stunt growth, but it will not kill the plant.
What happens if you cut down Japanese knotweed? ›Cutting Japanese knotweed will, over time, weaken the plant, but it will not kill the rhizomes (underground root-like stems). It can be used as part of other control practices. You must handle and dispose of cut plant material carefully. Burning may not be sufficient to kill the plant material.
Can you spread Japanese knotweed by walking on it? ›Japanese knotweed can easily be spread by transferring from shoes or clothes, this can happen when people walk through a contaminated area. One of the most common methods of Japanese knotweed spreading is when land is redeveloped or treated in some way, leading to increased human traffic.
Can Japanese knotweed grow through concrete? ›The simple, and definitive, answer to the question of "can Japanese knotweed grow through concrete?" is no, it cannot. No matter how virulent this weed is, it does not have the force to break through brick or concrete.