Japanese Knotweed is a pesky weed that can cause an enormous amount of damage to your lawn. This invasive plant has been found in every state in the continental United States and it’s no wonder why.
It spreads by rhizomes, which are underground stems with nodes on them. The nodes produce new plants when they reach the surface and create contact with air or water.
Japanese knotweed also reproduces through its seeds, which are dispersed freely by wind, animals, human activity and people who move infested materials from place to place. This means it can spread very rapidly and it is very hard to stop once it has rooted in your yard.
Luckily there are ways to get rid of this pesky and harmful weed! In this article we’ll help you identify if you have Japanese Knotweed in your lawn or garden, and detail how to get rid of it.
What is Japanese Knotweed?
To better understand how to kill Japanese Knotweed, first you need to know what it is. Japanese Knotweed (Fallopia Japonica) is an invasive plant species that was introduced to North America in the 1800’s as an ornamental garden plant. It is part of the buckwheat family polygonaceae, and it can grow very quickly, reaching heights of up to 10 feet! Japanese Knotweed grows into dense thickets that crowd out other plants, reducing biodiversity. It spreads by rhizomes which grow 1 to 2 inches per day and sends up new shoots. It can even grow up to 4 inches a day in the height of summer! After 10 years you will never see your beautiful lawn again if this weed is left unchecked.
How Can You Identify Japanese Knotweed?
So, how do you know if this invasive weed has taken root in your garden? Japanese Knotweed’s leaves are green on top and purplish underneath, divided into two leaflets with wavy edges. The stems are reddish-purple, hollow with nodes that produce new plants when they reach the surface or come in contact with water. The stems look like bamboo, and if you break one off the inside will be hollow.
In the fall, Japanese Knotweed produces tiny green-white flowers with seeds will be dispersed by wind. In the summer you can find white or pinkish spindle shaped fruits about 1/3 inch long.
However, the most distinguishing characteristic of Japanese Knotweed is its size and impressive growth rate. Again, it can potentially grow between 1 and 4 inches per day!
Laws Around Japanese Knotweed
There are many laws that govern the growing and disposal of Japanese Knotweed. If you suspect that this weed has invaded your property, be sure to either remove it yourself very carefully or have a professional do it, because the rhizomes can send up new shoots many feet from the original plant and can take over an area in no time.
Also, never dispose of Knotweed by dumping it in a landfill or throwing it on the side of the road. This weed is so prolific that if you contribute to its spread it can become an even larger problem for you and your neighbors.
In fact, in the U.K. Japanese Knotweed is classified as a ‘controlled waste’ substance under the Environmental Protection act of 1990. This means you cannot add it to compost or garden waste bins. You are only permitted to burn the weed or bring it to a verified licensed landfill site.
If you’re unsure, contact your local authorities about any laws that pertain to Japanese Knotweed.
How Do You Remove Japanese Knotweed?
Fortunately, getting rid of Japanese Knotweed is actually pretty easy once you have the proper tools and materials. First, get yourself some glyphosate-based herbicide (RoundUp is one example). Just be careful when applying to make sure it doesn’t splash on your skin or in your eyes. Always read the labels carefully before spraying any pesticide!
Removal by Organic Methods
If you don’t want to use pesticides, you can try digging out the Knotweed with a spade or hoe. It may take quite some time and effort, but it is completely doable if you are dedicated! This should be your first method you try, as it has the best success rate and is relatively simple.
Another option is trying to smother the weed by covering it with black plastic for about 2 weeks at a time. You can also try to remove all of the leaves from the plant to stop it from photosynthesizing. This method will take a lot longer, however, and will require much more diligence for months or even years.
Another organic option is planting as many ground-covering plants as possible to shade out and prevent Knotweed from growing back. This is where herbaceous perennials like Virginia Bluebells (Mertensia Virginica) and Creeping Phlox (Phlox Stolonifera) come into play.
Additionally, these smaller plants can also help to prevent your other perennials from being smothered out by the Knotweed overgrowth!
It is also possible to use another invasive weed as a ‘living mulch’ to overpower and take over the area where Knotweed is growing. This should be more of a last resort option, however, rather than a first choice.
One weed that could work best is the flowering plant called ‘Prickly Pear Cactus’ (Opuntia ficus-indica or O. robusta). Prickly Pear can cover quite a large area in one summer and smother out Knotweed. If you plant the cactus in the fall, it will overwinter and return for a second season. However, be sure that you have a plan to get rid of this plant before resorting to it, as it can be invasive as well!
Removal with Chemical Methods
If you want a surefire way to kill off any Knotweed on your property, the best method is using chemical herbicides.
It is vital that you apply any herbicides in exact accordance to their instructions for dosage and frequency of application.
Take care not to spray the weed when it is windy outside as this increases risk of drift which can damage non-target plants! Spray in calm conditions, and on a sunny day so that the leaves can dry quickly and stay in control of the pesticide.
You will need to apply the herbicide multiple times, 2 weeks apart each time, before you see any results. This is especially true for plants that are larger than 1 foot tall; if they’re smaller, it may take less time for them. Also, make sure you treat early in the season when it’s easier to pull out any survivors!
An herbicide like RoundUp will kill Japanese knotweed very effectively. In fact, you can use any herbicide that contains the active ingredient glyphosate. This will be listed on the product label, or if not, it will be clearly stated whether or not the product is ‘non-selective’ (meaning it will kill all plants, including other desirable ones).
There are plenty of RoundUp brand products available in hardware stores and nurseries. Just pick up one that has the highest percentage of glyphosate in its active ingredients! You can also find these at garden supply stores online.
One of the most frustrating invasive plants out there, Japanese Knotweed is a tough plant to get rid of. If you have it in your yard or garden and don’t know how to take care of it, we can help! We’ve compiled everything you need to know about getting rid of this pesky plant including how to identify it, and both organic and chemical removal methods. If you have any more questions, please don’t hesitate to contact us!