This article is the first in a series about invasive plants that are changing our landscape and harming native species of plants. Don’t miss more from this series: Garlic Mustard Nutritious Invasive or English Ivy, A Pretty Pest!
Japanese Knotweed is a highly invasive species that was once sold in the U.S. as an ornamental. It has taken over large areas along creeks and rivers and along highways and roads and wherever the ground has been disturbed. Now found in 42 out of 50 states, 8 Canadian provinces, and much of Europe, it is one of our worst invasives.
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Japanese Knotweed on the other side of our creek 
Japanese knotweed showing oblate leaves and flowers. Jan Samanec, State Phytosanitary Administration, Bugwood.org From nyis.info website.
Japanese Knotweed and related species are aggressive perennials that form dense thickets of stalks that shade and crowd out native plant species leaving a monoculture. The plant is tolerant of heat, full shade, salinity, drought and flooding. Knotweed spreads by its underground root system, especially when pieces break off and are carried down stream. In winter, the dead stalks that are generally still standing do nothing to prevent erosion creating more problems for native plants and wildlife habitat too.
Knotweed is able to push up stems through roadways and can damage public property as well as homes. In some areas of the United Kingdom, a home buyer may not be able to get a mortgage for a home that has knotweed growing within 7 meters (about 23 feet) of the house!
I’m grateful there is no knotweed on our land! But just across the creek on our neighbors property it is growing and spreading. Thankfully, I have permission to pull it out and have done so for many years. I got an early start this year and plan to stay on top of it, so I expect better results.
Stop the Invasion, Know Knotweed
To prevent an invasion of knotweed where you live, it’s important to be able to identify the plant. Stalks can grow from 3 feet to as high as 15 feet. The leaves look somewhat triangular or heart shaped, rounded at the bottom and pointed at the tip. New leaves start curled as in the picture below right before opening fully and darkening to a deeper green. Stalks are hollow and reminiscent of bamboo with rings where the leaves emerged. In late summer, sprays of small white, cream or greenish flowers grow in clusters. A seed grows in each achene, a small brown, triangular, dry fruit.
A bush sized clump of Japanese Knotweed is taller than me! 
New Knotweed sprouts where I had pulled stalks just 9 day before! 
Knotweed Leaves
Several species and names exist for knotweed which is in the buckwheat family, Polygonaceae.
- Japanese Knotweed or Mexican Bamboo – Polygonum cuspidatum, also Fallopia japonica, or Reynoutria japonica
- Giant Knotweed – Polygonum sachalinense, also Fallopia schalinense
- Bohemian Knotweed – Polygonum x bohemicum
- Himalayan Knotweed – Persicaria wallichii
If you find knotweed on your property, don’t delay addressing it. The longer it stays, the more established it becomes with rhizomes growing out from the plant as much as 65 feet and as deep as 10 feet!
Roots jut out over a foot on either side of these stalks. 
Japanese Knotweed appears to have 2 root systems 
Roots for individual stalk of Knotweed 
A new knotweed plant emerges from the roots spreading underground. 
Japanese Knotweed stalks look a lot like bamboo
Stop the Invasion, Remove Knotweed
There are a few methods you can use to stop the invasion and remove knotweed. You can cut the stalks near the base or dig out as much of the roots as you can pull. Even though you will simply never get all the roots, this can weaken the plant. Either method is best done 2 to 3 times a year.
After cutting or pulling, place a thick, opaque tarp over the area to cut off light, further weakening the plant. Cut stalks can be sharp and rip through many materials, so keep a watch on your tarp. You will notice the tarp lifting where new shoots are emerging. You can trample these down by walking on the tarp to further weaken the plant. If you need to use multiple covers, be sure to overlap well so no light gets in. Ideally the tarp should extend 10 feet from the largest group of stalks.
Please do not compost or dump this plant! I won’t even put it in trash that goes to landfill. Knotweed is considered a noxious weed in some countries and U.S. state. Therefore, it is not allowed in general landfill where it can continue to grow and spread. I suggest checking your local regulations for disposal. After I remove as much of the plant and roots as I’m able, I lay them out in the driveway to completely dry out and die. Only then will I consider bagging them in thick black plastic bags and put them in the trash. My preference is to burn them!
Years ago, I had seen recommendations to mow knotweed to keep it stop the invasion and keep it under control. Experts no longer recommend mowing because it leaves behind plant pieces that will just grow more plants within the original stand of knotweed or spread further by water, wind, people, or animals.
Stop the Invasion, Kill Knotweed
Many experts use herbicides to kill knotweed, but I can’t recommend using them. Knotweed often grows in areas near waterways where herbicides should never be used. Treatment with herbicides is best done by spraying into individual cut stems which is labor intensive and uses even more herbicide than a topical application. Because the primary herbicide, glyphoste, is non-selective, it will kill any and all plants it touches. Additionally, you will need to apply it repeatedly to weaken the roots. I feel strongly that herbicides are not worth the risks they bring to you or the environment.
A greener alternative is to use steam on the plants and inside the stalks. This can be done by renting a machine created for that purpose or hiring a company to do the work for you. Hot water pressure washers run thousands of dollars, but I don’t know if temperatures are high enough to kill weeds. Amazon carries weed torches and a steam weed killer that are priced much more reasonably as seen below. Weed torches can only be used where there is no flammable litter on the ground which may require raking a lot of dead leaves and sticks away in areas where knotweed tends to grow. This year I experimented by pouring boiling hot water over what remained after digging and pulling out a small stand. I’ll be monitoring what effect it has and expand the treatment if it’s effective.
Letting the Knotweed dry out and die 
Knotweed emerges from either side of plywood I put on it. 
I poured boiling water over the largest area of knotweed I pulled
Stop the Invasion, Prevent Knotweed
Once you have cleared the majority of knotweed from an area, it’s a good idea to plant it heavily with perennial native plants. They will help prevent erosion and, with your help, just might get a fighting chance against the knotweed. You can find a list of native plants for your location at https://www.audubon.org/native-plants/ or https://www.nwf.org/NativePlantFinder/Plants
If you see a stand of knotweed away from your home, consider contacting your town and asking them to keep it under control. If you see it while hiking, I think the rules about not removing any vegetation can be overlooked!
Though we will never eliminate the majority of the knotweed in our states, with a bit of effort a few times a year, we can keep local outbreaks under control and prevent them from spreading.
Please share your experience with knotweed or other invasive plants!
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