Japanese knotweed (Polygonum cuspidatum)—sometimes known Japanese bamboo, or more ruefully, as Godzilla weed—is one of the world's most invasive plants. If you've ever attempted to eradicate this weed, you already know of its monster-like qualities. Japanese knotweed is a shrublike, semi-woody perennial with bamboo-like stems that can grow to as much as 9 feet tall. It is so tenacious that it has been known to grow through solid masonry foundations. It is an incredibly stubborn plant, with roots that can penetrate up to 6 feet deep and spread as much as 65 feet. There are several approaches you can use to get rid of this plant, and it often requires multiple attacks for complete eradication.
What Is Japanese Knotweed?
Japanese knotweed is a herbaceous perennial plant, meaning it dies back into the ground for winter before sprouting anew in the spring. It can grow between 3 to 9 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 fighting Japanese knotweed depends on which strategy you employ. Severe infestations will require repeated attacks throughout the year.
- Smothering: Spring
- Cutting: Throughout the summer
- Digging: Any time, especially just before using the smothering technique
- Herbicide: Summer or early fall
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 doesn't tend to invade forested areas. Instead, it typically takes advantage of areas disturbed by humans—areas affording not only ample sunlight but also friable (or crumbly) soil for its invasive roots.
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 weed killer 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.
Getting rid of Japanese knotweed 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.
Equipment / Tools
- Tarps or black sheet plastic
- Rocks or other weights
- Garden sprayer
- Plastic garbage bags
- Wood chips or mulch
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 this method, while non-toxic and relatively easy, takes years.
Prepare the Area
Prepare the area by cutting mature weed canes (the tall stems) down to the ground and removing any debris. 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 or tarp.
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 cover the area with wood chips or mulch.
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 will not amount to much because it lacks sunlight.
Remove the Covering and Replant
After five years, the smothered knotweed and roots will generally 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.
Cut the Plant Down
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 simple cutting tends to stimulate Japanese knotweed into new growth, and unless you stay on top of new shoots, you may end up with a more serious infestation than before. Once you have cut off the stems initially, 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 option, which also is typically used concurrently with other methods, is to dig up the ground where the weed shoots come up most vigorously.
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 eradicate Japanese knotweed. 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 follow its label instructions. The recommendation from many university Extension Service experts is to use a glyphosate-based herbicide, such as Roundup, mixed with water in a 1:1 ratio. This is a more concentrated ratio than is normally used with glyphosate.
Pay special attention to the safety warnings. While glyphosate is regarded as safer than many herbicides, handling does come with risks, and you should make sure to avoid skin contact or breathing the mist.
Cut Off Plants
Cut off all Japanese knotweed growth to a length of 3 to 4 inches above the ground. This will stimulate new growth that will readily absorb the herbicide.
Apply the Weed Killer
When new plants sprout from the cut-off stems (within one to two weeks), carefully saturate the new 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 herbicide to any new growth that appears. This method generally will eradicate Japanese knotweed completely over time.