How to rid your garden of Japanese Knotweed

We explain how to spot and deal with this ‘Godzilla of the garden’, which can break through tarmac and concrete, causing untold problems for homeowners.

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Originally brought into Britain for its beauty, Japanese Knotweed is now feared and loathed by UK gardeners and homeowners. It’s not illegal to grow in your garden, but you have to prevent the plant from spreading or face hefty fines. Should Japanese Knotweed encroach onto neighbouring gardens, the local council can also order you to spend thousands of pounds to remove it.

With UK winters getting warmer and wetter, this plant has been springing up across the country. The Environment Agency has labelled Japanese Knotweed as ‘the UK’s most aggressive and destructive invasive plant,’ so if you find it in your garden, you’ll need to take urgent action.

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Why does Japanese Knotweed need to be removed?

While no home has ever fallen down due to Japanese Knotweed, it could have an impact on the saleability of your property. In fact, some lenders will even refuse mortgage applications on the home if it’s found on the mortgage valuation inspection. There have also been cases of homebuyers suing because the weed wasn’t declared on the conveyancing forms.

At its worst, Japanese Knotweed can grow up to twenty centimetres each day. The plant can break through tarmac and concrete, and has no natural UK predators to halt its progress. Reaching heights of over two meters, the weed can smother the growth of all other plants and, having evolved to grow on the sides of volcanoes, the knotweed can even resist fires and some herbicides.

How can you spot Japanese Knotweed?

Japanese Knotweed spreads through rhizomes – specially adapted underground stems. In early spring, you’ll see fleshy, red-tinged shoots sprouting all over the garden. Once it starts rising up, the Knotweed shouldn’t be hard to spot. The plant has lime green bamboo-like cane stems with purple flecks and spade-shaped leaves arranged in a zigzag pattern. In July, these leaves will host clusters of cream flowers.

How to remove Japanese Knotweed from your garden

If you have the time and determination, it is possible to remove Japanese Knotweed yourself. Generally, the best time to do this is in July, when the plant will draw in chemicals. To get rid of the weed, you will need to decapitate it to leave a hollow, foot-high stem above ground. Then you’ll need to pour a strong, glyphosate-based weed killer into the hollow stem. However, be mindful of the potential harmful effects of this herbicide while using it. 

As Japanese Knotweed is classified as ‘controlled waste’ under the Environmental Protection Act, you will have to dispose of it in a licensed landfill site because the weed can still spread through the soil. The whole process may need to be repeated for one or two years to ensure the plant has been successfully eradicated.

If this seems too difficult or you don’t have the time, you can hire professionals to remove the weed. While this can cost between £3,000 and £5,000 for a severe Knotweed problem, a professional service will provide you with a long-term guarantee that the plant will not return. This professional removal may involve using specialist machines to remove the plant and surrounding soil, ensuring the weed is gone for good.

If you’re buying a house, it’s a good idea to get a RICS HomeBuyer Report to check there is no Japanese Knotweed on the property. If it’s discovered before you buy, you can negotiate the cost of professional removal from the asking price, avoiding an expensive problem in the future.

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