FAQ on Japanese Knotweed

Japanese Knotweed Questions and Answers 

Of all the invasive weeds we treat at Wise knotweed Solutions, it will come as no surprise that Japanese knotweed is the number one cause for concern out there. It is also the weed we get the most questions about. Below are some of the most common questions asked regarding Japanese Knotweed.

Click to reveal the answers.

Q1: How Does Japanese Knotweed Spread?

Q2: How To Get Rid Of Japanese Knotweed

Q3: How To Identify Japanese Knotweed

Q4: How To Kill Japanese Knotweed

Q5: Japanese Knotweed Removal Cost

Q6: Plants Mistaken For Japanese Knotweed

Q7: Cost Of Removing Japanese Knotweed

Q8: How To Dispose Of Japanese Knotweed

Q9: How To Get Rid Of Japanese Knotweed Naturally

Q10: How To Recognise Japanese Knotweed

Q11: How To Report Japanese Knotweed

Q12: Japanese Knotweed Rhizome

Q13: What Does Japanese Knotweed Look Like In Winter?

Q14: When Does Japanese Knotweed Flower?

Q15: Why Is Japanese Knotweed So Bad?

Q16: How Long Does It Take To Eradicate Japanese Knotweed?

Q17: Japanese Knotweed Harmful To Humans

Q18: Should You Cut Japanese Knotweed?

Q19: What Does Japanese Knotweed Look Like In Spring?

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q1: How does Japanese knotweed spread?

Unlike other invasive weeds, Japanese Knotweed does not spread through seed dispersion. Instead, Japanese Knotweed typically spreads through deliberate or unintentional movements of the plants chopped stems or fragments of rhizomes (roots).

Even the smallest part of the rhizomes or cut stem (a finger nail size) can start a new growth of Japanese Knotweed. It is also worth pointing out that Japanese Knotweed can spread more aggressively if the plant is disturbed.

q2: How to get rid of Japanese knotweed

Getting rid of Japanese Knotweed typically involves the use of chemical treatment over a 5-year period. Alternatively, a quicker but expensive option is to excavation.

q3: How to identify Japanese knotweed

Japanese Knotweed is an attractive plant with it’s an eye-catching lushest’ green leaves and tall bamboo-like stems. However, Japanese Knotweed changes its appearance with the seasons. Click here to view images of what Japanese Knotweed looks like throughout the year.

q4: How to kill Japanese knotweed

Killing Japanese Knotweed is typically achieved through 2 methods:

Chemical Treatment:

Injecting chemicals straight into the plants stem will help weaken the roots system underground. However, this process needs to be repeated up 5 to years for the roots to be completely killed.

Excavation:

If 5 years is too long to wait for complete eradication of Japanese Knotweed then excavation, which involves digging the plant and its roots from out the ground, is an option. Although this will remove Japanese Knotweed much quicker than chemical treatment, it is the more expensive option. 

q5: Japanese knotweed removal cost

The cost associated with Japanese Knotweed removal is typically determined by the area size of the infestation and how accessible the plant is. This is usually established through a Japanese Knotweed survey.

To find out more about a Japanese Knotweed survey and what is involved, click here.

q6: Plants mistaken for Japanese knotweed

There are at least 7 plants that are most commonly mistaken as Japanese Knotweed. Bindweed, Russian Vine, Houttuynia, Lilac, Dogwood, Poplar and Red Bistort. Visit our dedicated page on ‘Plants that look like Japanese Knotweed’ for images and more information about these plants.

q7: Cost of removing Japanese knotweed

Japanese Knotweed removal costs vary depending on the area size of the infestation. Treatment costs are usually established after a survey of the property has been conducted.

To find about our Wise survey process or to book a Japanese knotweed survey today, click here.

q8: How to dispose of Japanese knotweed

The environmental agency published guide on how to dispose of Japanese Knotweed includes;

1. Burning

Japanese Knotweed is less likely to survive if it is burnt and it means there is less to bury or dispose of. If you are considering burning Japanese Knotweed, then local laws must be taken into account as well as the potential to cause pollution.

2. Burying

Japanese Knotweed should be buried at least 5m deep within impenetrable barriers or plastic sheeting. However, before bury Japanese Knotweed on a development site, you will need to consult with the Environmental Agency to ensure the quality of groundwater will not be affected in any way.

3. Approved Landfill Disposal

Japanese Knotweed can only be disposed of within licenced landfill sites. It is recommended that you contact the landfill site in advance so they can prepare a suitable area for the Japanese Knotweed to be disposed of.

q9: How to get rid of Japanese knotweed naturally

Currently, there are no natural enemies to Japanese knotweed in the UK. Getting rid of Japanese Knotweed is typically achieved through chemical treatment or excavation.

However, an experiment is underway in Wales where scientists have released insects, imported from Japan, within 6 different locations that have a Japanese Knotweed presence. The hope is these insects will stunt the knotweed plants and allow native species to flourish again.

q10: How to recognise Japanese knotweed

Japanese Knotweed is an attractive looking plant that stands out right away with its lushest green, shield-shaped, leaves. The bamboo-like stem of a Japanese Knotweed plant is another tell-tale sign. Japanese Knotweed will grow to around 7 feet tall, with leaves up to 14cm. In the late summer, early Autumn, the plant will start producing its creamy white flowers.

q11: How to report Japanese knotweed

You can report any sightings of Japanese Knotweed to SEPA (Scottish Environmental Protection Agency) whose main role is to protect and improve the environment.

q12: Japanese knotweed rhizome

The Japanese knotweed rhizome is the underground part of the weed. It looks knotty with a brown bark. Underneath the bark, it is orange or yellow in colour. If the Japanese knotweed rhizome is fresh you will be able to snap it.
The knots in the rhizome are usually spaced a few centimetres apart and there are often small white roots or buds emerging from them which can grow into new plants if the rhizome is cut up.

q13: What does Japanese knotweed look like in winter?

During late Autumn/Winter, the canes die off and the plant becomes dormant. Shoots may, however, be visible for the new growing season. The canes lose their leaves and turn dark brown/red in colour. The dead canes remain standing and may take up to 3 years to decompose.

q14: When does Japanese knotweed flower?

The creamy-white flowers of a Japanese Knotweed plant are typically visible towards the end of summer/early autumn.
where does Japanese knotweed come from
Japanese knotweed, as you may guess, originated in eastern Asia where it thrived on waste ground and on its natural habitat on the side of volcanoes.

q15: Why is Japanese knotweed so bad?

At its most prolific, Japanese Knotweed can grow up to 20cm per day. The roots can grow 3 metres deep into the ground and spreads 7 metres in all directions, which can lead to structural problems within properties.

Mortgages have been declined, with lenders refusing to lend against a property that has Japanese Knotweed. There have also been lots of stories in the media where homeowners are seeing their property value slashed due to Japanese Knotweed being present.

q16: How long does it take to eradicate Japanese knotweed?

It can take up to 5 years to completely remove Japanese knotweed through chemical treatments. However, it can be eradicated immediately through excavation, which involves digging it up out the ground. Although this is a more expensive alternative to the more commonly used chemical treatment.

q17: Japanese knotweed harmful to humans

No, Japanese Knotweed is not harmful to humans and is actually edible. Here are some Japanese knotweed recipes that may be of interest.

q18: Should you cut Japanese knotweed?

Cutting back Japanese knotweed not solve the problem as Japanese Knotweed can grow up to 20cm during the summer months. Attempting to dig the plant out or disturbing it in any way can increase the density of the stem. If you are going to cut Japanese Knotweed, then it’s important to acknowledge that as little as 10mm of the rhizome (roots) or tiny amounts of cut stem or the crown can regenerate into another plant.

q19: What does Japanese knotweed look like in spring?

When trying to identify Japanese Knotweed in spring, look out for the following:

• New shoots are red/purple in colour
• Early Spring, Knotweed shoots begin to appear and look not unlike asparagus spears
• Fast growing canes with leaves that begin to unroll as the plant turns green
• Late spring - canes that are approx. can reach 3 metres (10 feet) in height
• Bamboo like canes that are hollow and have a characteristic pattern of purple speckles
• Leaves with a distinctive zigzag pattern on the stems

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