How To Get Rid of Japanese Knotweed Effectively

Have you spotted Japanese knotweed in your garden or on your property? If so, it’s essential to take action as soon as possible. This weed is highly damaging and difficult to eliminate.

In this blog, we’ll delve into how to identify Japanese knotweed and the impact it has on its surroundings. You’ll learn various methods to control Japanese knotweed and discover what you absolutely shouldn’t do.

Japanese knotweed: a danger

Japanese knotweed (Fallopia japonica or Reynoutria japonica) belongs to the knotweed family (Polygonaceae). It is an invasive species increasingly common in the UK. This fast-growing plant has a significant impact on local ecosystems and can cause considerable damage to buildings and infrastructure.

Its extensive root system can grow through concrete, asphalt, and foundations, leading to cracks and damage. Identifying and controlling Japanese knotweed is therefore crucial for both homeowners and green industry professionals.

Recognising Japanese knotweed

Recognising Japanese knotweed is crucial for effectively controlling this invasive plant. It has distinct characteristics that set it apart from other species.

Stems and leaves
Japanese knotweed is a striking plant. It can grow to a height of 1.5 to 3.5 metres. Its bamboo-like stems are hollow and upright, with distinctive reddish-brown spots.

The leaves are leathery, thick, and sturdy, with a broad, egg-shaped form. Leaf length varies between 10 and 18 centimetres. In spring, the leaves are fresh green with red veins and alternate along the stem, giving the plant a recognisable structure.

Flowers
Japanese knotweed blooms from July through September, producing small cream-white to pale pink flowers that grow in clusters, lending the plant a decorative appearance. But don’t be deceived: the damage it causes is substantial. We’ll return to this later.

Root system
Though not directly visible, the root system plays a major role in the growth and spread of the plant. The roots can grow very deep—often up to two metres and sometimes even three. This deep root system makes it difficult to eradicate the plant and contributes to its invasive nature.

The roots and stems thicken each year, making the plant increasingly robust. Even a tiny piece of root can grow into a fully-fledged plant.

Difference between knotweed and Japanese knotweed

In addition to Japanese knotweed, other invasive species are also found in the UK:

• Hybrid knotweed
• Giant knotweed
• Himalayan knotweed

There is also a common knotweed, a popular border plant with red flowers in narrow spikes. There are cultivars of common knotweed with pink and even white flowers.

Japanese knotweed (which also bears white flowers) is generally larger, has bigger leaves, and causes significantly more damage.

Damage caused by Japanese knotweed

Japanese knotweed has a severe impact on the natural environment. This invasive species poses a serious threat to biodiversity and disrupts ecosystems.

Displacement of native species

Japanese knotweed can establish itself on various soil types and in diverse habitats. Through its rapid growth and formation of dense foliage, it overgrows other plants, leading to the displacement of native plant species, which poses a direct threat to local flora.

This invasive weed doesn’t only affect plants. It also leads to a reduction in invertebrates, such as wood ants, butterflies, and insects, in areas where it dominates, negatively impacting overall biodiversity.

Ecosystem disruption

Its presence disrupts ecological balance. The plant affects its habitat by capturing organic material and nutrients, altering soil composition and impacting other organisms reliant on specific soil conditions.

Long-term effects

The long-term effects of Japanese knotweed are concerning. Although it hasn’t yet directly caused species extinction, there are indications that it may only be a matter of time.

The economic impact is also considerable. It causes damage to infrastructure and buildings, leading to high repair and maintenance costs. Additionally, controlling the plant incurs extra costs for monitoring, public information, and aftercare.

Controlling Japanese knotweed

Various methods are available to control this pernicious weed. The control process is lengthy, requiring patience and persistence. It is crucial to act immediately if you spot a shoot.

Digging out and covering

A useful method for the home gardener is digging out the Japanese knotweed. Begin by cutting back all the large stems and work from the outside in, starting at least 50 cm away from the plant.

Preferably use a fork instead of a spade to avoid breaking the roots. It’s vital to remove as many roots as possible, as even small pieces can grow into new plants.

After digging, it’s advisable to leave the area open for a few weeks to spot any remaining roots. For larger areas, covering with a strong, impermeable tarp can be an option, though this requires patience as the tarp needs to remain in place for three to five years to suffocate the plant completely.

Repeated mowing

Although mowing alone is insufficient to remove Japanese knotweed, it can help to weaken the plant. It’s worth noting that this involves cutting rather than regular (mechanised) mowing, as standard mowing can increase the risk of spreading.

Cut the plant regularly, ideally every two weeks during the growing season. This process can take years before the last shoot disappears. Be sure to collect and dispose of cuttings carefully to prevent spreading.

Natural control

There are various natural methods that individuals can try, although their effectiveness varies. Some people use boiling water to douse the plant and roots, but without root removal, this has limited effect.

An intriguing approach involves using grazing pigs, which eat Japanese knotweed. However, this control method may take years. Moreover, allowing pigs into your garden isn’t practical, but for larger areas, the results are promising.

The use of vinegar as a control agent is sometimes mentioned, though its efficacy is unproven scientifically. Additionally, controlling Japanese knotweed with vinegar may have other negative effects. Acetic acid could pose a risk to plants adjacent to the sprayed area and to soil organisms.

It’s important to remember to dispose of all plant material, including roots and stems, carefully. Never place them in the compost bin but dispose of them with general waste to prevent further spread.

Professional control and legislation

For effective Japanese knotweed eradication, it’s often necessary to seek specialised help.

Specialist control methods

Professional companies have access to advanced techniques and products that are unavailable to the public, such as electric shock treatments or injections.

Reporting requirement and enforcement

For works on council-owned land, there is often an inspection and reporting requirement. This means the site must be checked for the presence of Japanese knotweed, and any findings must be reported. These measures aim to limit the spread of the plant and coordinate its control.

Understanding your legal responsibilities in regards to knotweed growing on your land is crucial should you wish to avoid an unexpected date in court or a run-in with your local council.

How to get rid of Japanese knotweed: conclusion

Japanese knotweed is a highly invasive plant that can cause significant damage to buildings, infrastructure, and natural ecosystems. It’s a tenacious plant that is difficult to control, so you may need to enlist professional help. Always remain vigilant after treatment for any new shoots and act immediately if growth is visible.

Looking for more gardening inspiration or advice? View more articles on weed control or read our other articles for tips and advice.

Louis Hooft

Founder & Lawn expert

Introducing Louis Hooft, the founder of MOOWY and your reliable expert. With a profound love for stunning lawns and extensive experience in garden maintenance, Louis is here to assist garden enthusiasts in achieving a greener and livelier outdoors than ever before. Count on Louis for invaluable tips, clever tricks, and top-notch products to make your garden flourish!