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How to deal with Japanese knotweed in your garden

Japanese knotweed, is it as bad as you think?

Who would have thought that the presence of a humble plant could affect house prices and sales? When it comes to Japanese knotweed, it seems that everyone fears this beast. But is its reputation well-deserved or are we over-reacting?

Japanese knotweed pernicious weed Japanese knotweed. These are two words that strike fear into the hearts of homeowners!
The botanical name

Fallopia japonica is the plant which is commonly known as Japanese knotweed. It has red, green and brown hollow stems, green, heart-shaped leaves and pretty clusters of creamy white flowers in summer.  Unfortunately, the perennial has become a common sight here in the UK. It was introduced during the 19th century as an ornamental plant that could have potential benefits as an animal feed. Back in the day, it was actually named as the ‘most interesting new ornamental plant of the year’ by the Society of Agriculture and Horticulture in Holland.

The main problem with this perennial is that it grows so quickly, with each plant being capable of growing 20cm every day during the spring and summer. It has therefore turned into a rather destructive  weed that spreads rapidly. It tends to suppress the growth of other plants, meaning that it can prevent native plants from thriving nearby. There are many plants that practice allelopathy in this way, including the black walnut, Juglans nigra, many pine trees, azaleas and even sunflowers.

the black walnut tree with fruits and leaves The black walnut tree also emits a substance called hydrojuglone which inhibits the growth of many other plants.
The root of the problem

Japanese knotweed is a tall perennial with bamboo-like stems that shoot up from underground rhizomes which can often be several metres deep. It grows to a height of over two metres and quickly forms a dense thicket which increases in size very quickly. Roots tend to work their way into cracks in concrete and it’s therefore not unusual to see shoots appearing within tarmac and hard surfacing. Removal is difficult by hand because the rhizomes are safely tucked away from the gardener’s fork. In winter, the plant behaves like any other perennial as it dies back to ground level.

Japanese knotweed white flowers The flowers of Japanese knotweed occur in mid-summer
Warning to homeowners

Under an amendment to the Anti-social Behaviour, Crime and Policing Act 2014, there is legislation in place that covers the control of non-native, invasive plant pests. For the last six years there has been an obligation for a house seller to declare the presence of Japanese knotweed on their property on a form called TA6. It is the seller’s responsibility to check the garden for evidence of Fallopia japonica, bearing in mind that during winter the plant is barely visible. Where the plant is present, it is necessary to provide a management plan for its eradication and this needs to be provided by a professional company.

Buyers will therefore be aware that they are purchasing a property which has evidence of Japanese knotweed within the garden. The mortgage lender will require confirmation that the plant is to be eradicated, or they are at liberty to withhold funds. The lender generally asks to see a copy of the eradication plan, provided by a professional company and backed by a guarantee.

Of course, there are many cases of disputes, mainly revolving around identification. Some homeowners will fail to disclose the presence of the plant because they either have no knowledge that it is there, or they claim that it is something else.

Fresh growing tips of Japanese knotweed Fresh growing tips of Japanese knotweed
What can you do?

As a house buyer, you would be wise to inspect the garden personally for evidence of this plant. It’s worth bearing in mind that it’s not illegal to have Japanese knotweed growing in the garden. You should aim to control it, however,  in order to stop the spread. Owners can be prosecuted if the weed isn’t kept under control. It might also be worthwhile to check over the fence into neighbouring gardens to make sure it’s not heading your way.

If you have noticed this plant in your own garden, don’t despair! Provided it’s spotted early,  you can control it, despite what people think about the problem.

How to deal with it

Firstly, you can dig deep and remove as much of it as possible. Then keep on removing it every time it re-appears above ground. It’s vital to dispose of the pieces safely. They should never be included in your green waste council bins; within your household waste, or on your compost heap. It is classed as ‘controlled waste’ under the Environmental Protection Act 1990 and requires disposal at licensed landfill sites. It can also be destroyed on site by allowing all the waste to dry thoroughly, then burning it.

chemical sprayer bottle Sometimes weedkiller is the best way, even if you prefer to garden organically.

You can also use weed killer on the live plant. But bear in mind that it generally takes at least three to four seasons to completely eradicate the pest. A glyphosate-based weed killer such as Roundup Tree Stump & Rootkiller, also Roundup Ultra, have proved to be effective. It’s vital to treat the re-growth because it will return if you don’t re-visit the process a few times.

The truth of the matter is…

Japanese knotweed probably isn’t any more of a threat than many other plants that haven’t been named and blamed. Some organisations believe that the UK’s approach is ‘overly cautious’. However, there is no doubt that it is a ‘pest’ and need to be kept in check. In any event, information is key. Make sure you know what this plant looks like. If you have got it, keep on top of the growth and never allow it to get out of your control.  You’ll be likely to unleash the wrath of your neighbours and the authorities if it escapes!

cartoon man screaming Don't allow the rascals to escape!