‘Tell-tale signs’ you have Japanese knotweed in your garden

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Listed by the World Conservation Unit as one of the world’s worst invasive species, Japanese knotweed grows extremely fast, up to 10cm a day, and can damage buildings and homes. Gardening experts are urging Britons to check for the invasive weed in their garden, taking action as soon as possible if they do come across it outside.

Chris Bonnett from GardeningExpress.co.uk said: “Japanese knotweed can look fairly attractive with its creamy white flowers and heart-shaped leaves but it’s extremely destructive to its surroundings.

“To a lot of novice gardeners, Japanese knotweed will look normal but it can cause a lot of damage to your home by eating its way through your walls, which is why it’s important to be able to recognise it.

“If you’re still unsure on whether or not you have knotweed in your garden, then it’s best to get in touch with an experienced gardener.”

New shoots start to emerge during the spring and they tend to be a red or purple colour, looking a bit like asparagus spears.

The expert said the leaves are normally rolled up and tend to be dark green or red in colour.

Chris added: “As the leaves start to spread out, they’ll become a vibrant green colour and heart/shovel shaped with a point at the tip. Some can be as big as 20cm across and they grow staggered at the stem.

“In late summer and early autumn, small clusters of white flowers will appear. The clusters grow to approximately 0.5cm wide, but up to 10cm long.

“The leaves will still be apparent and along with the flowers, it will create a dense foliage. The stems are mostly hollow and bamboo like, with nodes and purple speckles.

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“The general growth habit has a distinctive zigzag appearance. Stems can also grow up to three metres tall. In the winter, the stems become brittle.”

Gardeners should also take a look at the rhizomes, the underground part of the weed. If they are fresh, they will snap easily, according to the expert.

The outside will be dark brown, but the inside is usually an orange or dark yellow colour.

Chris noted: “The Japanese knotweed rhizome system can grow up to depths of two metres and can extend up to seven metres horizontally from the plant.

“As little as 0.7g of rhizome can give rise to a new plant, which is one of the reasons this weed can become such a nuisance.”

Samantha Jones, gardening expert at MyJobQuote.co.uk, explained that the process of removing the weed involves applying a glyphosate-based weed killer, but requires multiple applications every season to ensure that the plant is completely eradicated.

The pro said: “The cost of this type of Japanese knotweed removal ranges from £1,000 to £2,950. There are other methods including excavation which involves the removal of the plant.

“However, this may cause damage to the rest of the garden and is also a much more expensive option, which costs around £1,750 to £4,950.”

There have also been some cases where homes have almost been completely devalued as a result of severe infestations, although these cases are rare.

It comes as an accountant, Jeremy Henderson, was sued more than £200,000 after Japanese knotweed was found in the garden of his former home, despite never being picked up in his housing survey back in 2015.

According to a handy map provided by Environet, Wales is currently home to some of the worst cases of knotweed, as well as London and the midlands. 

New sightings are added to the map daily to inform homeowners where there are hotspots. There are also some sighted cats in Scotland as well as mild cases in Norwich.

With cases on the rise, experts are warning that homeowners should “act quickly” if they do suspect the weed is growing on their property.

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