Japanese knotweed (Fallopia japonica) was first introduced into Britain in 1825 and at the time was declared to be an outstanding garden plant. Today this herbaceous perennial is better known as a thug, a resilient and pernicious weed that is virtually impossible to eradicate.
Its resistance to traditional weed killers and lack of natural enemies in this country allow Japanese Knotweed to out-compete most plants, threatening our native bio-diversity. With super-strength, it can damage buildings, structures and roads plus also block drains and waterways leading to flood damage.
By law, it is an offence to plant or otherwise cause Japanese knotweed to grow in the wild in the UK. And even dumping contaminated soil and garden waste at your local tip is a serious risk, so beware!
Whilst it is not an offence to have Japanese knotweed or any other invasive weed (Giant Hogweed, Himalayan Balsam etc.) growing in your garden, legislation (the Anti-social Behaviour, Crime and Policing Act 2014) could be used to enforce its control by local authority or police.
Be aware too, that having Japanese knotweed growing in your garden can knock thousands of pounds off the value of your home and make buying, selling and re-mortgaging a nightmare, as mortgage lenders are reluctant to lend on properties with potential structural problems.
Why it’s a thug!
The shoots emerge each spring and are capable of bursting through Tarmac and concrete. Its powerful zigzag reddish stems can grow up to 3m and over 1.5m in a month! They produce shield-shaped leaves along their length and creamy coloured flowers in September and October, before they die back for the winter.
Underground there is even more trouble awaiting as the thick woody rhizomes or creeping stems spread in all directions and up to 2m deep to create an ever-expanding and mobile clump.
Getting rid of the weed is virtually impossible as even a tiny fragment of root left in the soil can grow again. On a small scale, the best approach to prevent it getting out of hand is to dig the plants out and cut back the emerging growth every 2-3 weeks over several years to weaken them. Then cover the ‘cleaned’ soil with a landscape mulching fabric to weaken the new shoots, which will struggle to survive due to lack of light. Dig the plants out and allow them to dry out before burning them. This approach is a relentless task, as you will need to cut back the emerging growth every 2-3 weeks over several years.
Eradicating the problem is a job for professional and will cost several thousand pounds. Only licensed agents are permitted to use herbicides that are strong enough to control and eliminate invasive weeds like Japanese knotweed and dispose of the plant waste. These professionals offer a warranty that will cover the cost of further treatment and repairs to property in the unlikely event that re-infestation occurs.
When selling your property in future, you’ll find that mortgage lenders will insist on a specialist survey to determine the extent of the problem and will normally require evidence of treatment that will eradicate the plant as a condition of lending.
It’s a fact!
If there is a good side to the plant, it has nectar-rich blooms attract bees and the impenetrable thicket of hollow stems creates valuable habitats for wildlife.
The green-purple asparagus or bamboo-like stems are edible, tasting similar to rhubarb, and are a good source of vitamin A.