Gardens

How to get rid of Japanese knotweed

Japanese knotweed can spread like wildfire. Here’s how to identify it and banish it from your garden forever

Japanese knotweed
(Image credit: Alamy)

A favorite back in Victorian times, Japanese knotweed was highly prized for its rapid growth. Who could have known then that it would get the upper hand, proving invasive over swathes of the US and Britain? 

Not only does knotweed grow at an astonishing speed, it spreads fast, far and wide, and can cunningly regrow from the tiniest portion of its underground root system. 

Invasive Japanese knotweed can find its way into the smallest cracks in paving and paths, doing damage as its rhizomes expand, especially if close to walls and buildings. In the UK, disposal of Japanese knotweed is restricted, and if you’re selling your UK home, any Japanese knotweed in your garden must be declared.  

This shows how serious a problem it is; unlike getting rid of weeds generally, it can't be tackled by herbicides permanently – removing it once and for all should be left to professionals who are experienced with the plant named 'Godzilla weed'.

How do I identify Japanese Knotweed?

Come the spring, when the garden starts to wake up, so does Japanese knotweed, (Fallopia japonica). 

Look out for fleshy reddish-purple shoots sprouting from ground-level crimson pink buds. These will grow into purple-flecked canes, in some cases reaching over 7ft (213cm) tall. 

Along the length of each cane, in a zigzag fashion, heart-shaped leaves, up to 5½in (14cm) long, will sprout. In summer, the plant produces long tassels of cream-coloured flowers. 

Like other garden perennials, Japanese knotweed dies back over winter, though you can spot it by the dead brown canes while it’s dormant. 

Other plants such as Russian vine, Himalayan honeysuckle and even lilac sometimes get mistaken for Japanese knotweed. 

If you’re in any doubt, look up the plant’s credentials or take a photo of it and get a specialist removal firm to identify it.

How difficult is it to get rid of knotweed? 

Getting rid of Japanese knotweed takes true grit so steel yourself. Once you’ve identified the knotweed, the instinct is to get out in the garden and start ripping it out, but let’s take a step back, here. 

Although it’s not illegal to have the plant in your garden and it can be zapped with weed killer, it will just sprout back up, so the laborious task will have to be repeated over several years. 

And in the UK, you can’t dispose of any part of the plant in the usual way but will have to contact your local council for advice. 

And even if that's not the law where you live – in the US and beyond – there is a good reason for it that you should apply to your own disposal method: knotweed can regrow from the tiniest of roots, so if you pull it out and dump it somewhere, it will take root elsewhere – and unless you have removed every last bit of the plant, under and overground, it will just reappear.

Burning the plant is a bad idea. This is not permitted by many local councils and won’t necessarily destroy the rhizomes. 

Don’t be tempted to dig out the plant either – by breaking up its underground rhizomes, the weed can spread even further. Once you’ve bagged up the plants, seek advice from your local authority on safe disposal at a licensed landfill site. 

How to get rid of knotweed

If you haven’t been put off treating the problem yourself and want to go ahead, choose a strong glyphosate-based weed killer. Dress up in protective clothing, keep children and pets well away, and apply the weed killer following the instructions on the label, avoiding spraying any of it onto other plants. 

Leave to die off and cover it up with a tarp or old carpet (see below) to complete the job. Bear in mind that with this system, the knotweed may well return next season.

How to get rid of knotweed naturally

One way to kill knotweed naturally is to smother it with tarps, old carpet, even plastic sheeting. Essentially, you want to limit its access to light, air and water. This is not a quick method and is best done as growing season starts. 

First, cut down tall stems and trample them down with some tough boots or a roller. Next, cover the entire area with the tarp or carpet and weigh it down with bricks. Leave for as long as it takes to stop new shoots pushing up – and when they do, trample them back down again. 

How to get rid of Japanese knotweed permanently

Though it can prove costly, by employing a professional firm you can heave a sigh of relief, as the problem will be taken care of for you. What’s more, after the work is complete, you’ll get a guarantee, proof that the knotweed problem has been solved, pertinent if you intend to sell your home. 

A registered firm will perform a site assessment, draw up a plan, spray the plants and sometimes inject a strong, approved herbicide to kill them. The waste will be disposed of at a licensed site. 

To get rid of Japanese knotweed completely, the treatment might have to be repeated for up to four growing seasons. Other treatments such as excavating the soil or installing a root barrier to prevent the weed getting into neighboring yards can also be considered. 

If neighboring gardens are affected a joint management plan can be put in place, where neighbors will club together to share the costs.

How dangerous is Japanese knotweed? 

Japanese knotweed is not dangerous to people or pets, but it does pose a danger to buildings. It is so invasive that it can, over time, penetrate concrete, so will damage foundations and walls if allowed to spread unchecked. In the UK, having knotweed in your garden can affect your chances of getting a mortgage secured, which shows how much of a problem it can be.