Triffids they’re not – but these plants are dangerous invaders of our countryside

Written by Tony Millett on .

You’re driving along Wiltshire’s country roads and the hedgerows are still green and lush.  Then all of a sudden you notice a patch of brown and wilting foliage – and in the middle of it a small white notice.

You’ve just passed a patch of Japanese knotweed that’s been treated by Wiltshire Council.  It may look dead, but underneath it may still be alive.  The Council will be back to see if the treatment’s worked.

Japanese knotweed is one of the most invasive and destructive plants at large in Britain.  It has thick bamboo-like stems, grows into large clumps, has bright green leaves and pretty, fronds of wispy flowers. But under the ground its rhizomes (or underground stems) spread widely producing new plants which can lift up asphalt and damage buildings.

These plants smother all other native plants and spread very quickly.  Tiny fragments of a stem can be carried away by mowers, strimmers and flails, and start new plants.  It is a perennial and although it dies away in winter, it comes back to life in spring with renewed vigour.

Under an act of parliament it is illegal to plant or otherwise cause Japanese knotweed to spread into the wild.  And you can be taken to court if it spreads from your land to a neighbour’s.

Wiltshire Council has had 416 Japanese knotweed sites reported over the last four years and last year its contractors treated 181 sites.  By the end of 2010’s growing season 58 sites were still thought to be live and were treated again this spring.  Most reports come from council contractors as they carry out the twice yearly cutting of verges.

Wessex Tree Care are contracted to treat the county’s Japanese knotweed sites and they no longer have to use dangerous sprays.  Instead they inject the stems with a commercial pesticide called Tordon. A pigment in this pesticide turns the plants blue for a couple of hours as the liquid is carried own the stems to the rhizomes – which it then kills.

But it’s very resilient.  Wiltshire Council’s expert on knotweed is Graeme Hay.  He calls it ‘a remarkably effective plant’ and told Marlborough News Online: “You never give up on knotweed – you always have to go back and look again.”

The Council’s work to eradicate Japanese knotweed from the county is important. South Wales provides the lesson of what it can do when left to its own devices. There it has taken over abandoned industrial and mining sites in the valleys and around major towns – and has got into the water courses where it flourishes.  On that scale it is very hard indeed to overcome.

The other plant that is giving local authorities a headache is common ragwort.  As a defence, this wild plant has evolved strong toxins that can damage the livers of grazing animals.  It can be lethal to horses – especially when included in hay.  Humans are advised to wear gloves when handling it.

Unlike knotweed there is a positive side to ragwort. It has a key role in the wild environment, supporting many insects – especially the cinnabar moth whose orange and black striped caterpillars cover the plants in late summer.

Each ragwort plant can produce up to 30,000 seeds and though they won’t be carried much more than five metres away, the plant spreads through fields at an alarming rate.  Graeme Hay says that it’s impossible to eradicate ragwort, you just have to keep it to a minimum.  His aim is to keep it out of the county’s grass verges and so prevent it infesting farmers’ fields.

Control of ragwort comes under two acts of parliament – the Weeds Act (1959) and the Control of Ragwort Act (2003.)  These put responsibility for control squarely onto the owners of land where it grows.

The last government published a code of practice on how to prevent the spread of ragwort. This will probably end up on the coalition government’s bonfire of red tape. Last summer the agriculture minister, Jim Paice, announced that “Tackling common ragwort can be a practical example of the Big Society in action.”

So if you see the yellow heads of ragwort thriving in fields or on verges, you’ll know the Big Society hasn’t reached them – yet?

For further information on Japanese knotweed  click here to visit the DEFRA information section on Japanese Knotweed 

You can report knotweed sites in Wiltshire via CLARENCE (Customer Lighting & Roads Enquiry Centre) from landlines within Wiltshire call 0800 232323. From mobiles or outside the county call 01225 777234. Or email This email address is being protected from spambots. You need JavaScript enabled to view it.