Removal Options For Japanese Knotweed
by Cormac Reynolds
February 27, 2020

Japanese knotweed is an extremely damaging and invasive weed that can cause significant issues for homes and buildings. If you end up being infested, here are your options.

Non-Chemical Removal Options

It is possible to dig it out, but regrowth will generally occur because of the depths of the rhizomes according to Environet. There is also the issue of disposal as Japanese knotweed is considered controlled waste according to the Environmental Protection Act 1990. This means it has to be disposed of at a licensed landfill site. Hiring a specialist Japanese knotweed contractor can reduce the issues with this as they are usually licensed to safely remove it from the site. Of course, you will need to check this before you hire anyone.

It is also possible for the knotweed to be destroyed on site. It will need to be dried out before it is burned. Japanese knotweed should never be placed in normal household waste or put into the green collection bins.

If you are digging it out, you have to remove as much of the root as you can and repeatedly destroy any regrowth. When you do this, the energy reserves in the underground parts will be exhausted, but this can take several seasons to occur.

Biological Removal Options

A biological control that has been released in the UK can control Japanese knotweed. The plant sucker, psyllid, has been released to a handful of trial sites and is not available to the general public. However, if the trials are successful, it will be released more widely and should become available to the public within the next 5 to 10 years.

Chemical Removal Options

The simplest and most effective way of getting rid of Japanese knotweed is to use a glyphosate-based weed killer. The best to use is Scotts Roundup Tree Stump and Rootkiller. This has a label recommendation for Japanese knotweed and the instructions state it should be applied to cut canes. There are other weed killers that have label control as well for this weed. There are other strong formulations of glyphosate that can also be tried.

Glyphosate is generally applied to the foliage of the weed and spreads through the plant to the roots and other underground parts. Cutting away the old stems can be useful and provide good access. The best time for using this removal method is during the flowering stage during the late summer. The problem is that Japanese knotweed can be hard to spray at these times before it can stand 2.1m or 7 feet tall.

For a more practical approach, you should allow the weed to grow to 90cm or 3 feet. This will generally happen in May and then spray it. There can be some regrowth and a second application in mid-summer may be required. You should check in September and if there is any new growth, you will need to spray it again. You need to check it the following spring and repeat the process if there is growth.

When you spray the weed, you need to avoid contact with other garden plants. Knotweed that has been treated will produce small-leaved and bushy regrowth of 50 to 90cm or 20 inches to 3 feet the next spring. This is a very different appearance, but the regrowth will need to be treated. It can take 3 to 4 seasons to completely eradicate Japanese knotweed using chemicals. Of course, a professional contractor will have access to more powerful weed killers which can reduce this timeframe by half.

Residual Control

There are residual weed killers that come in soluble sachets. These products can provide some control in the growth of the plan, but they are unlikely to eradicate the weed.

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