How To Kill Japanese Knotweed

Also known as Polygonum cuspidatum or Fallopia japonica, Japanese knotweed is one of the most invasive plants in the UK. It can grow to more than 2 meters in height, and the roots could reach up to 4 meters in depth, making it extremely challenging to remove.

Even worse, this ornamental plant can spread quickly and thrive in any conditions. This means you should have a multifaceted approach to get rid of it from your garden and lawn. Don’t worry, however, because you can achieve this task with a little patience, effort, and time.

Here are a few common techniques and useful tips on how to kill Japanese knotweed.

How Japanese knotweed grows and spreads

First introduced to the UK from Japan in the 19th century, Japanese knotweed belongs to the buckwheat family and can be used as an ornamental plant. It features white, small flowers, bamboo-like canes, and heart-shaped leaves.

The ideal habitat for Japanese knotweed is moist, sunny spots, such as gardens, lawns, roadsides, and riverbanks. It usually spreads via landfill from a fragment of root, hidden in a pile of topsoil or fill. This weed can grow exponentially and dominate the living space of other plants in the area.

For this reason, it is necessary to get rid of them as soon as possible to ensure the habitat for native species.

Four common methods to kill Japanese knotweed

1. Suffocate with tarps

During spring, you can cover a Japanese knotweed area with tarps to prevent its early growth, thus making it simpler to eradicate. You should use poly or plastic units which are spacious enough to cover the whole site.

Before starting, you should prepare by cutting down old canes to the ground and getting rid of loose materials because these components sharp edges which would easily puncture your tarp.

Consider overlapping multiple tarps to ensure that no sunlight would go through. Make sure to weight them down to prevent the wind from blowing away or lifting. When the plants emerge, they can push up your tarps. However, it is simple to suppress them by walking over the tarps.

Leave these tarps in place long enough to suffocate the Japanese knotweed. Meanwhile, you could use the above area for gardening.

For example, you can create a raised-bed garden or apply mulch for a container garden. These tarps will work as a protective layer against the invasion of the weed, so you don’t need to worry about the safety of your above-ground garden.

2. Apply herbicides

Another method to eliminate Japanese knotweed in your garden is to use a weed killer. While there is a variety of products on the market, it is recommended to choose glyphosate-based herbicides. They are often mixed with water and sprayed to the leaves. However, it is possible to inject these substances to the canes.

As a rule of thumb, early autumn or late summer is the best time to apply herbicides. During this time, the plants are flowering and their foliages are transmitting most of the nutrients to reserve food for the rhizome.

However, you can still spray these substances constantly during the growing season to prevent Japanese knotweed from gaining much height.

Glyphosate is a popular non-selective herbicide for homeowners, landscapers, and farmers. This means it can kill any type of plant in your garden. Thus, make sure to consider carefully before choosing this method, especially when you want to protect your ornamental beds or lawns.

Also, avoid spraying these substances on an area that can be used for growing veggies in the future.

This clip will show you how to apply herbicide safely and effectively:

3. Cut

You can also suppress Japanese knotweed by cutting back the plants during the summer (you will need hand shears like these). This method will reduce the efficiency of photosynthesis, thus minimising the growth of these plants.

As the cuttings can spout new roots quickly and take hold in the ground, you need to pick up all of the cuttings and put them in a bag for disposal.

Do not rely on this method alone because it will not eliminate the weed completely. Instead, you should use it as a complementary approach to the application of weed killers for better results.

4. Dig up

The last method you can try is to dig up the ground where the shoots emerge most strongly. In those areas, you will find the rhizome-clumps from which their shoots and roots spring. All you need to do is removing these rhizomes and putting them in heavy duty garden bags for disposal.

It is important to know that you can’t get immediate results with this technique. No matter how thorough you try, some rhizome roots would snap off and eventually sprout quickly in the future. But keep in mind that Japanese knotweed removal is a long-term project.

Thus, this can be a good method to prepare the area for other techniques mentioned above.

A multifaceted approach

These control and elimination techniques aren’t mutually exclusive. Indeed, you can combine or alternate them to increase the success rate of eradicating Japanese knotweed from your garden.

For example, you might start with smothering the areas with tarps during the summer, then dig up the rhizomes in late autumn or early spring. By doing this, you might increase the effectiveness and reduce your workloads with each method.

After all, eradicating Japanese knotweed might require a lot of time. Depending on your location and area of weed, there will be different approaches for removal. The key is to be patient and stick to your plant.

How to dispose of Japanese knotweed

Japanese knotweed is basically classified as “controlled waste” by the UK government. This means you are required to dispose of it in licensed landfill sites. Make sure to do research and look for the local regulations concerning Japanese knotweed before disposing.

You could reduce the removed amount by drying out the canes and burning in a fire pit. After that, fill the remaining in trash bags and call a professional carrier to bring it to licensed landfill sites. Avoid using dead branches for compost because the knotweed would keep sprouting and spreading.


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