What is Japanese knotweed?

Japanese knotweed is an invasive non-native plant that originates from East Asia which was introduced to the UK as an ornamental garden plant in the early 19th century. The characteristic structure and leaf shape made Japanese knotweed popular, and it soon spread around the UK. Unaware of how invasive it is, it was put on sale and lauded as ‘a capital plant for the small town garden’ – it was even taken to Royal Botanic Gardens, Kew!

These days we are aware of how invasive this species is, so it is now a legally controlled plant. First, the Wildlife and Countryside Act 1981 made it an offence to cause Japanese knotweed to grow in the wild. Then, under the Environmental Protection Act 1990, it was classed as ‘controlled waste’. Despite this, Japanese knotweed now flourishes in every six-mile radius in the UK. It’s not an offence to have it in your garden but it is against the law to allow it to spread and, without input, it will spread rapidly.

How do you identify Japanese knotweed?

In spring, reddish-purple fleshy shoots emerge from crimson-pink buds at ground level. These grow rapidly, producing dense stands of tall bamboo-like stems in summer, which grow to 2.1m (7ft) tall. The hollow stems look like canes, with clearly defined segments growing straight out of the ground. These stems have characteristic purple flecks and produce branches from nodes along its length.

The leaves are heart or spade-shaped, with a very distinctive straight edge and pointy tip. These can grow up to 20cm long, growing alternately along the stem – they have a strong zig-zag pattern which is characteristic of the plant.

Bloom in late summer (August and September), creamy-white flower form tassel-like clusters which can reach up to 15cm long. 

In winter the knotweed dies back, taking its energy back into its rhizomes under the ground, leaving behind dried-up stems. In Spring, the Japanese knotweed will again emerge. At first the leaves are rolled and may have red veins – it may look a bit like asparagus shoots, which quickly turn into the bamboo type canes, growing at a rapid rate.

What’s the issue with Japanese knotweed?

Although it rarely sets seed in this country, Japanese knotweed can sprout from very small sections of rhizomes, or 'creeping rootstalk'. The plant then spreads rapidly in the wild by natural means and as a result of spread by humans – this spread is solely by vegetative means, by either fragments of rhizome or stem.

The vigorous growth of this large plant – up to 30 cm per day! – means that it can easily outcompete native plants, leading to a reduction or loss of flora and their dependant fauna. Because the plant dies back in winter, if it’s growing on a stream or riverbank then there won’t be any vegetation to hold the bank firm and bank erosion is more likely to occur, as well as increasing the likelihood of flooding.

Perhaps most worryingly Japanese knotweed can cause significant delays and cost to development, as well as structural damage. Since 2013, home sellers are required to state whether Japanese knotweed is present on their property through a TA6 form (a property information form used for conveyancing). The plants complex root system will take advantage of any crack or hole, which is why our problem with this weed is so widespread. It can grow through any weakness in structures or buildings, and left unchecked can damage a buildings structure, drains or sewers.

An amendment to the Anti-social Behaviour, Crime and Policing Act 2014 also includes Japanese knotweed.

How do we manage Japanese knotweed on our reserves?

Japanese knotweed is near impossible to kill. It can grow back from a tiny fragment of stem or rhizome, and the rhizomes themselves can stretch for 3 metres or more from the visible 'areal growth' of the plant. One positive thing is that, as only the female plant was introduced to the UK, most plants cannot spread by seed (though a few have hybridised with other knotweeds).

• Digging out: This option depends on the location of the knotweed, as the rhizomes need to be dug out too. All of the arisings must be disposed of appropriately, but if a fragment remains it will regrow.
• Chemical treatment: Treating with herbicide each year can be effective to stop Japanese knotweed spreading and remove surface growth after a few years. There are different methods of application, and some are very targeted. However, this does not remove the Japanese knotweed and if the ground is disturbed it is likely to emerge again. After treatment the knotweed will re-emerge, looking more like a shrub that will need to be re-treated.

Japanese knotweed can only be disposed of in a few ways by a licensed professional. It should never be disposed of in garden waste or household waste bin! Other options are available via specialist companies, each with their limitations.

Any areas where knotweed has been removed or treated is monitored annually for any regrowth, as you never know when it may pop back up!