Japanese knotweed (Polygonum cuspidatum)—nicknamed 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 Godzilla-like qualities. Japanese knotweed is so tenacious that it has been known to grow through solid masonry foundations. There are several approaches you can use to get rid of this plant, and it sometimes 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 8 feet tall on average with a bushy appearance. Its leaves are a medium green color, and it sports small white-green flowers in the late summer.
Identifying Japanese Knotweed
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.
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
Equipment / Tools
- Rocks or other weights
- Garden sprayer
- Plastic garbage bags
Using Tarps to Smother Japanese Knotweed
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.
Prepare the Area
Prepare the area by cutting mature weed canes (the tall stems) down to the ground and removing any debris. The canes have sharp edges that can easily puncture a tarp.
Cover the Area With Tarps
Cover the plant area completely with one or more tarps depending on its size. Overlap the tarps, so no sunlight can penetrate the seams. Use rocks or other heavy materials to weigh down the tarps, so they don't move or blow away.
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.
Leave the Tarps
Leave the tarps in place for as long as it takes for shoots to stop emerging and the existing plants to die. In the meantime, you can use the tarped area for above-ground container gardening.
Cutting Japanese Knotweed
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.
Gather the Cuttings
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.
Repeat With New Shoots
Repeat the process as new shoots emerge.
Digging Up Japanese Knotweed
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.
Using Herbicide to Kill Japanese Knotweed
Some gardeners resort to applying weed killer to eradicate Japanese knotweed. However, many of these products aren't safe for humans, pets, or the environment. And they might kill nearby garden plants that you want to keep.
Select a Weed Killer
Select a weed killer appropriate for Japanese knotweed, and follow its label instructions. Pay special attention to the safety warnings.
Carefully Distribute the Weed Killer
Take care to have gloves or a mask on as you distribute the weed killer and to properly store the excess.
Tips for Removing Japanese Knotweed
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 eradicated with a dedicated effort on your part.