How to Kill Japanese Knotweed

Introduced to the UK & Ireland in 1825, Japanese Knotweed was originally grown in gardens for it's attractive & exotic appearance. It wasn’t until some years later that it came into focus that it was becoming an invasive plant and was threatening our countryside & waterways. Now Japanese Knotweed is number one on the list of most problematic plants in the country.

Japanese knotweed is such an issue that it has been known to effect house prices. If you find Japanese knotweed in your garden it will mostly likely effect the future sale of your house and eradication of the weed can cost up to €30,000. Banks have been known to refuse mortgages on land due to the presence of this weed. If the plant is found growing within 7m of your home it can effect household insurance. It cost UK Olympic Committee 70,000,000£ to eradicate Japanese Knot Weed from just 10 acres of land.

Both Giant Knotweed & Japanese Knotweed (Fallopia Japonica) are native to Japan, China & Taiwan. It is very recognisable as a tall herbaceous plant. It can grow 3m a year and it's underground rhizomes can spread 7m across and penetrate anywhere from 2 to 7m deep. This clearly makes the plant almost impossible to eradicate. Tall red stems grow at a rapid rate and produce large heart shaped leaves with flat base. Plumes of cream flowers appear in Summer. By Winter the foliage dies back to leave tall, bare, hollow stems.

Today Japanese Knotweed can be found across all of the UK and over most parts of Ireland. There are a number of steps in controlling this plant & all which every method your choose to apply, you must do so thoroughly

Chemical Control of Japanese Knotweed

• Knotweed can be controlled with routine & persistent spraying
• (Alternatively you can inject the chemical directly into each individual stem)
• The best time to spray is in August or September
• This is ideal as this is when nutrients are returning down into the plant’s root system for Winter storage
• If you spray at this time, the weed killer will work it's way right down to the deepest roots
• As Knotweed is susceptible to frost, you will need to spray before frost has a chance to kill off the growth
• Apply round-up or similar products following specific guidelines found on the weed killer’s label
• It takes anywhere from 2 - 6 weeks for the weed killer to take effect
• Remove growth once it has died back
• Spray again to hit leaves that have not died or those missed due to shading
• Several more applications will be necessary to completely kill off the plant & root system
• This can take several years and site must be monitored for at least 3 years after to ensure no re-emergence occurs
• An application of 20ml of round-up per litre of water is recommended for effective control

Physical Removal of Japanese Knotweed

• It is not advised to attempt digging out Japanese Knotweed
• Doing so can make the situation worse, as the weed can easily spread from the smallest of fragments
• For effective control excavation of soil needs to be 5m (Yes - 5 Metres Deep!)
• Extreme care is required to prevent contamination of surrounding areas
• All soil needs to be screened to remove all traces of knotweed roots
• All parts of Japanese Knot Weed must be burned on site
• The area needs to be observed for 2 years & ensure no regrowth occurs