Japanese Knotweed Removal: Where to Start

japanese knotweed
(Image credit: getty images)

Japanese knotweed was introduced to the UK from Japan in the 1840s as an ornamental plant, and it is now number one on the list of the UK’s most invasive plant species. The Environment Agency has described it as: ‘Indisputably the UK’s most aggressive, destructive and invasive plant.’

It is particularly rampant along waterways, railways and on many brownfield sites, and its rapid growth – it can grow up to 3m in three months – means that it overshadows native plant species, has a significant impact on wildlife and causes damage to property.

With official records stating that Japanese knotweed is now so prevalent in the UK that there is not a single six-mile-square patch where it is not present – the Orkney islands excepted – it is likely that a good proportion of homeowners, renovators or self builders will come up against it when they come to build. Whether the plant originates on your land or encroaches from a neighbour, your project cannot proceed properly until it is dealt with effectively.

There are hybrid varieties of Japanese knotweed, too — notably giant knotweed, which has larger leaves and generally taller plants. It is not as invasive as Japanese knotweed, but has the same legal status and treatment methods.

Why is it bad?

Why Does Japanese Knotweed Have Such a Bad Reputation?

There are all sorts of myths about Japanese knotweed. It’s not entirely true to say that it will grow through a hard surface, but in its insatiable quest for light and water, it will exploit any weaknesses in the formation and break through cracks in mortar, expansion joints in concrete, splits in drains and joins in paving.

The most common form of property damage is, however, caused by laying a hard surface, such as asphalt, concrete, patio slabs, driveway block paving and the like, over Japanese knotweed-infested ground. Covering over the plant might hide it temporarily, but it won’t solve the problem in the long term.

Underground sewers and drains, and land drains are particularly susceptible to Japanese knotweed. The knotweed rhizome (the root system) will find its way into the smallest hole on a pipe joint to find water. The rhizome will then continue to grow, gradually blocking the drain and finally breaking it apart.

Japanese knotweed can also grow within cavity walls. We have experienced stems and healthy leaves growing out of vents and airbricks positioned 2m above ground level. When knotweed grows in cavity walls, it has the capacity to force the two skins of the wall apart. We even have a recorded incident of knotweed growing within a cavity wall of a single storey building and forcing its way through the flat roof.

In practical terms, what happens is that driveways and pavements become uneven, potentially dangerous, as well as unsightly; pathways lift, walls collapse, drains need replacing, fences are pushed out of line or fall over — all of which are headaches for homeowners to deal with. But unless the underlying cause is dealt with – i.e. the Japanese knotweed is eradicated or correctly controlled – any repair work or rebuilding will all be wasted and the plant will return.

How does it spread?

How Does Japanese Knotweed Spread?

All Japanese knotweed plants in the UK are female, so the seeds in the flower are not pollinated. The plant spreads by vegetative means, with canes arising from the rhizome that grows underground. The rhizome may grow from either an existing crown, where previous growth has taken place, or from a cut stem.

All new outbreaks of knotweed result from fragments of viable rhizomes that may be spread within soils being moved from site to site by careless gardening and DIY projects, by fly-tipping, or by natural processes such as river bank erosion, or by animal movement. This is why it’s imperative to dispose of any fragments of the plant at a licensed waste disposal site if it’s to be taken off site, and why soil surrounding existing plants shouldn’t be spread to other areas of the garden.

How to spot it

How to Spot Japanese Knotweed

Japanese knotweed looks different throughout the year, and it’s sometimes confused with Russian vine, ivy and bindweed. In the early spring, red/purple shoots appear from the ground and grow rapidly forming canes. As the canes grow, the heart-shaped leaves gradually unfurl and turn green. The plants are fully grown by early summer and mature canes are hollow with a distinctive purple speckle and form dense stands up to 3m high.

The plant flowers in late summer and these consist of clusters of spiky stems covered in tiny creamy-white flowers. In most areas of the UK, the plant is still in leaf in October, with pretty white flowers faded, but in a few short weeks the leaves will fall and the stems, although still standing, will appear dark brown and look dead.

Don’t be fooled. The rhizome system beneath the ground is alive and well, just waiting for new shoots to re-emerge next year — bigger and stronger. The rhizome can be identified as knotty with a leathery dark brown bark, and when fresh snaps like a carrot (and is orange beneath the bark).

How to remove it

Japanese Knotweed Removal

Beware of DIY solutions or trying to eradicate the weed yourself. Cutting it down repeatedly, pouring diesel on it, covering it in salt, burning it, burying it and saturating it in over-the-counter weed killers are the most common methods I come across, but they categorically don’t work.

Japanese knotweed can lie dormant for up to 20 years only to strike again when you least expect it.

There are a few different methods for eradicating the plant, and getting a reputable professional in is advisable. For years now, Japanese knotweed treatments in residential situations have largely been carried out between April and October, simply because the industry has relied upon herbicide treatments, which require the plant to be in leaf.

However, if the ground is to be disturbed (i.e. you’re extending or building upon the land), the knotweed-infested soils have to be removed. Herbicide treatment is only suitable when the knotweed is left in situ, undisturbed. If you build over knotweed, herbicide treated or not, do not be surprised when it finds its way growing through your new structures, or new surfaces.

If you intend to build on the land or don’t wish to be restricted to treating it during the spring and summer, you can opt for one of the available soil removal methods. One option we offer is the Resi Dig-Out solution, whereby you’re guaranteed to be Japanese knotweed free in a matter of days, at any time of the year.

It works by removing all viable rhizome from infected soil, allowing the soil to be reused on site. It causes far less disruption than ‘dig and dump’, and uses no chemicals. (Dig and dump involves the physical excavation under expert supervision of all soils containing viable Japanese knotweed rhizome. These are then loaded into a lorry and transported to a licensed landfill site, for disposal. It’s a very expensive method that attracts Landfill Tax.)

It is advisable that any treatment or removal work on Japanese knotweed is guaranteed. Make sure that whoever carries out the work is able to provide full evidence of their treatment plan together with an insurance-backed guarantee. These are required by most lending institutions when you decide to sell the property or seek additional mortgage finance.

Looking to buy or sell a home with Japanese knotweed?

What Happens If I am Looking to Buy or Sell a Home with Japanese Knotweed?

It is more difficult to obtain mortgage finance on properties with Japanese knotweed present, and under the legal updates to the TA6 conveyancing form, any owner looking to sell is obligated to declare the presence of it together with evidence of previous treatment plans and guarantees. Any attempt at concealment is likely to prove an expensive mistake.

Nic Seal is Managing Director at Environet.