How to kill Japanese knotweed – the 4 key things you MUST know

Timelapse shows alarming rate Japanese Knotweed grows at

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Japanese knotweed is a blight for any homeowner, with the ability for the plant to spread rapidly, cause structural damage to walls and buildings and kill off other plants. Summertime is when Japanese knotweed grows quickly, with deep rhizomes spreading underground.

The plant causes damage as it is known to target weak points of structures, like cracks in masonry, and tries to grow through them.

Due to the destructive nature of Japanese knotweed, Schedule 9 of the Wildlife and Countryside Act 1981, states it is an offence to cause the plant to grow in the wild.

And for any homeowners with Japanese knotweed in the garden, since 2013 any seller is required to state whether Japanese knotweed (Fallopia japonica) is present via a TA6 form – the property information form used for conveyancing.

While it is not illegal to have Japanese knotweed in your garden, anyone with the invasive weed present should try and control it.

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How to identify Japanese knotweed

Japanese knotweed has reddy-purple fleshy shoots in the spring, which transform in the summer to dense bamboo-like canes which can grow to 7ft (2.1m) tall.

These bamboo-like growths have purple flecks and can grow branches along them.

According to the Royal Horticultural Society (RHS) the leaves are “heart or shovel-shaped and up to 14cm (5½in) in length and borne alternately (in a zig zag pattern) along the stems”.

In late summer and early autumn, the plant produces creamy-white flower tassels, which grow up to six inches (15cm).

How to kill Japanese knotweed

1. Hire a professional

There are specific services you can call on to remove Japanese knotweed for you, with a combination of strong weedkillers and extraction.

They can also dispose of the plant for you, as Japanese knotweed is classed as ‘controlled waste’ under the Environmental Protection Act 1990.

The RHS advises: “Specialist Japanese knotweed contractors must be registered waste carriers to safely remove the weed from site but check first before employing their services”.

If you are tackling the plant by yourself, there are several key things you need to bear in mind.

If you opt to tackle Japanese knotweed yourself, you will not get an insurance backed guarantee without using a professional company.

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2. DIY methods

While you may save money on not hiring a professional, in the long run an insurance backed guarantee may be more valuable when it comes to selling your home or if a neighbour is threatening litigation.

The RHS says the best weedkiller for Japanese knotweed is “a glyphosate-based weedkiller such as Roundup Tree Stump Weedkiller.”

The gardening experts explain: “This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes or a foliar spray. SBM Job done Tough Weedkiller (soluble sachet only) and Roundup Ultra also have label control for this weed”.

There are alternatives to this which are also tough formulations of glyphosate – like SBM Job done Tough Tree Stump Killer (soluble sachet only), Doff Weedout Extra Tough Weedkiller or Westland Resolva Pro Xtra Tough Concentrate.

3. ALWAYS treat regrowth

If using a glyphosate treatment, you will need to treat the regrowth which will appear the next spring.

This regrowth will be 50-90cm (20in-3ft) and appear bushy with small leaves.

If you don’t treat it, the plant could return with a vengeance in the summer months. 

However, the RHS warns: “Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS.

“It is a list of products currently available to the home gardener.”

4. Dispose of any cuttings properly

There are strict regulations about the disposal of Japanese knotweed.

These are laid out on the Government’s website and are as follows

  • You must use a registered waste carrier and an authorised landfill site or suitable disposal site.
  • You must follow the law if you’ve been employed to transfer goods or material by road and you’re disposing of any waste that has or might have Japanese knotweed in it.
  • You must dispose of Japanese knotweed waste off-site by transferring it to a disposal facility that’s permitted, such as a landfill site that has the right environmental permit.

You must not:

  • dispose of Japanese knotweed with other surplus soil
  • sell soil contaminated with Japanese knotweed as topsoil

You can only reuse knotweed-contaminated soils after treatment, on the site where they were produced.

You cannot get a waste licensing exemption for the use of Japanese knotweed.

Before you transfer Japanese knotweed waste you must:

  • check with the waste site in advance to make sure it’s got a permit to accept material containing invasive plants – the waste site may also need time to prepare
  • tell the waste site that you’re transferring Japanese knotweed waste

When you transfer the Japanese knotweed waste you must cover or enclose it in the vehicle so that no waste can escape.

Read more on disposing of Japanese Knotweed on the Government’s website here.

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