Knotweed engulfs a public footpath in SwanseaCredit:Mark Griffiths The only thing it has to do is grow, which it does, up to four inches a day in summer Her husband, Bill, a butcher, 69, discovered knotweed on a piece of land he owned near their home in Stourbridge, West Midlands, earlier this year. ‘He’d had the land for 30 years and then it [knotweed] just suddenly appeared. He didn’t recognise it immediately. But then he went on the internet. ‘Bill’s biggest worry was the financial aspect,’ she continues. ‘That if the knotweed encroached on neighbouring land, he would be liable. Towards the end he was desperate. He thought we’d have nothing left.’An alarming invasionThe couple had raised four children (now aged between 20 and 30) and had bought their house in the 1990s. On February 13 this year, he went to work. Later that day, he was found at home having attempted suicide. He died the following day in hospital. It’s likely the suicide was an expression of a deeper problem. Helen says her husband had ‘occasionally’ suffered from depression before. But she is convinced the precipitating event was the knotweed. ‘Bill was a very strong character. But this was something he couldn’t cope with.This was something he didn’t have an answer to. He couldn’t sleep. He could hardly eat. He just spiralled downwards.’ Japanese knotweed first arrived in Britain in a box of 40 Chinese and Japanese plants delivered to the Royal Botanic Gardens at Kew, west London, on August 9, 1850. ‘Plant number 34’ was a simple shrub with reddish, hollow canes and heart-shaped leaves on a bowing stem. Fallopia japonica was disseminated throughout the UK by the fashion for ‘wild’ gardens – a departure from the Victorian craze for regimented carpet bedding. There are two methods of killing knotweed: poison (it can take five years of repeated applications, costs around £2,000, and shoots can still rise from a plant you thought you’d killed years earlier); and digging it out completely. This means excavating to a depth of at least two metres and taking the resulting earth to a specially designated landfill. The remaining soil may have to be lined with a heavy-duty plastic membrane.This can cost upwards of £10,000, even for a domestic property. A source of excitement in recent years has been biological control – using natural enemies to keep knotweed in check. The key was to find an insect that only fed on knotweed and didn’t attack any of the UK’s native plant species. Dr Shaw and his team at Cabi tested nine insects and whittled it down to one – the psyllid . Releases have been made annually since 2010. But the bug, a sap-sucker, has not adjusted well to the UK. ‘It has got all the characteristics of a successful agent: it lasts a long time; it produces hundreds of eggs; but it’s not taking off and we don’t know why,’ Shaw explains. ‘It might be because it’s been 140 years under a Japanese summer and then it’s suddenly thrown out in Berkshire in the spring.’ The plan is to collect new stock from Japan. ‘Quite often these things lose their mojo when they’re in the lab,’ he says. Shaw is also investigating a second control agent: a leaf-spot fungus. In the meantime, knotweed professionals worry about an impending cataclysm that will result in unimaginable amounts of knotweed, possibly in our own lifetime. Fallopia japonica (a female plant) has already cross-bred with her cousin, the less intrusive giant knotweed. ‘You can get all sorts of hybrids back-crossing with Japanese knotweed and the danger is that we will eventually end up with male Japanese knotweed,’ explains Brian Taylor, who runs The Knotweed Company, a firm specialising in knotweed eradication, based in Daventry, Northamptonshire. The weed’s seeds, which are currently barren, would, effectively, be switched on. In 1823, Philip Franz von Siebold, a Bavarian doctor and ethnologist, was appointed doctor-in-residence for a community of Dutch traders based in Dejima, a man-made island in the harbour of Nagasaki, in Japan. At this time Japan was a closed country with wide-ranging restrictions on the activities of foreigners (for example, learning to read and write Japanese was against the law). But Siebold’s medical skill won him influential contacts and he was granted unprecedented access to the country.For the next five years, Siebold, with a commercial objective, secretly collected specimens (plants, animals, objects). On September 18, 1828, as he prepared to sail for Europe with 89 crates of illicit goods, a storm forced the ship aground. Siebold hid the precious specimens in false-bottomed flower boxes. The ship sailed but Siebold was detained, interrogated and later banished from Japan for life. He moved to Antwerp, in Belgium, and then to the Netherlands, where he tracked down his scattered collection.In 1842, he opened a nursery in Leiden and began to market his collection of ‘exotic’ plants across Europe. And so it was that in 1850 he sent an unsolicited package to Kew Gardens. At the time of Siebold’s death in 1866, his nursery boasted 1,000 different species and varieties of plants. When FW Burbidge and P Barr, two well-known English horticulturalists, visited the nursery in 1883, they found a neglected jungle, overrun by just one plant: Japanese knotweed. Some would go a bit further and put a bit of diesel on it. They’d say, “Whatever I do, it just comes back.”’ Three years ago, he decided to investigate. ‘Some of the infestations had reached the point where the tenants were unable to enjoy their gardens,’ he reveals. One garden was two-thirds knotweed. In 2013, Sawyer decided he needed to spearhead what he describes as a ‘strategic approach’ and ask his board for funding. I wonder how difficult it was to get money for what some might say is just a weed. ‘They’d gone beyond that. They know it’s the dark force,’ he replies. He asked for £90,000 and got £15,000 a year, for a five-year programme of repeated applications of herbicide in 25 properties. ‘What worries me is we’re just tinkering at the margins,’ he admits. ‘Knotweed will develop. We’ve seen how it spreads and it’s very aggressive. We don’t have the budget to deal with the entire estate. We are treating 25 properties. Next year I know there will be 35, and the year after there will be 45. We can probably add 10 every year.It’s almost like trying to resist gravity. It has taken over the valleys. It’s not a battle that we are ever going to win. ‘In the quiet hours of the morning when I can’t sleep and I think of all the roofs leaking, all the central heating I need to put in, I’m now thinking about knotweed and how we have got properties that back on to council-owned land and private land. We are trying to do our bit, but unless every landowner is prepared to do theirs, it’s no good. And we have very little control over private landowners, and almost no working relationship with the council because they say they are almost skint. Meanwhile I am just watching this vast army advance towards us.’ Knotweed leaves absorb the summer sun Credit:Mark Griffiths Taylor goes on to present compelling evidence of knotweed’s growing immunity to herbicides, which is another worry. Taylor is interesting to listen to, if unbelievably alarming. ‘If we’re to get evolution and resistance, what could we do?’ he asks. ‘We are literally just looking at excavation as a control measure.’ I meet Sean Hathaway, an agile, fleece- wearing man, in mid-August – when the knotweed is starting to produce frilly, white blossomy flowers. ‘One of its few benefits – if it has any – is bees love it,’ he says.The battle continuesHathaway works for Swansea council and is known locally as the ‘knotweed officer’, owing to his 20-year battle with the plant. Swansea, Wales’s second city, has for decades now been beset by knotweed. Its history of copper mining and processing, combined with widespread redevelopment in the 1960s and 1970s, when derelict factories were turned into enterprise parks and shopping centres, is, Hathaway suspects, behind the apparently unstoppable infestation. Officially, Swansea has around 250 acres of knotweed. But the last survey was done back in 1998. He describes the battle against knotweed as ‘stable’. Every year he poisons a few thousand square metres, every year it takes hold elsewhere. He takes me on a tour of knotweed sites. This includes a very considerable hillside, rising from the site of an old quarry, not far from the city centre, which is so uniformly green it’s hard to tell from a distance what is knotweed and what is not. Close-up I realise it is all knotweed. Denise Rees in the graveyard at Caersalem Chapel,Swansea: she struggles to find the graves of her parents because of the knotweed infestationCredit:Mark Griffiths The potential cost of trying to eradicate the plant in Britain has been estimated at more than £1.25 billion It’s almost like trying to resist gravity. It has taken over the valleys. It’s not a battle that we are ever going to win In the mid-1800s, when Japanese knotweed was a ‘new’ exotic species just becoming available here, gardeners were urged to consider its benefits. It was reliable, hardy and had ‘great vigour’. It was fodder for cattle (untrue), a reliable screen for the outdoor privy; its underground stems or rhizomes were an effective means of stabilising sand dunes and, especially versatile, its canes could be used to make matches. ‘A capital plant for the small town garden,’ wrote John Wood, who went on to open a nursery in Leeds.Today, Japanese knotweed is Britain’s most destructive invasive plant, costing around £166 million a year to clear and control. It cracks through roads, undermines buildings, eats up property values. It is deeply disgruntling to wildlife: insects can’t feed off it; birds rarely build nests in it. But the animal it most clearly affects is us. Birmingham couple Nasreen and Sajid Akhtar claimed recently that they were unable to sell their home, after an infestation of Japanese knotweed in a neighbour’s garden. Despite 20 viewings with three estate agents, they could not to find a buyer.It was only when they tried to remortgage the terraced house – and were turned down – that they learnt the reason why. The weed was threatening the foundations of their property, meaning that no bank would lend against it. The couple were now ‘in limbo’, according to Nasreen. ‘It is putting my future and my children’s future on hold and it is totally out of control.’ ‘It does seem trivial but for some people it has become a big worry,’ says Helen Jones, 60. ‘It’s choked everything else,’ he says, explaining the harm that would have come to the shrubs and grasses. ‘It is impressive,’ admits Hathaway, who, like many who make a living killing or studying Fallopia japonica, is simultaneously horrified and awed by its power. The council received its first knotweed complaint in 1970. But since 2012, mortgage lenders have started rejecting loans outright if knotweed is found on a property (even an infestation on a neighbouring property can be enough to put them off). Liz Wakeman, a project manager from Bristol, had a great-aunt who lived in Swansea. ‘She was bedridden with quite severe dementia and about 18 months ago, it got to the stage where we had no option but to get her into a nursing home. So we decided to sell her house to raise money for her care,’ she explains. The estate agent went to value the property. ‘He called me,’ Wakeman recalls, ‘and said, “You’ve got a massive problem.”’ After enlightening her about the knotweed, the estate agent estimated the value of the house – not £100,000 or so, as Wakeman had anticipated, but £45,000. ‘Unfortunately, I don’t think people understand the impact not only on property prices but also on what it’s doing to their property. When we got Environet [a firm specialising in the eradication of Japanese knotweed] to do the first treatment and they cut it back, we saw the extent of the damage: there was a wall at the bottom of my great-aunt’s garden and the knotweed had literally pulled it over.’ After treatment (costing £10,000) the house sold for £73,000. Wakeman’s great-aunt died while the sale was going through. ‘Some of the neighbours have got it up to their back door and they don’t seem to care,’ she says. The Reverend Grenville Fisher stands at the graveyard at Mynyddbach Chapel, which has recently been treated for Japanese knotweed Credit:Mark Griffiths Half a mile away, on Llangyfelach Road, is Caersalem Newydd Baptist Chapel. Denise Rees, 75, has been a member of the church since she was a child and she swears knotweed has been in the churchyard for as long. ‘As children we used to go up there, cut off a piece and use the tube as a pea-shooter,’ she says. ‘Many years ago, when we had more members, the youngsters used to go up and try to clear it. But they never got rid of it. And now it’s just got worse and worse.’ The knotweed is so advanced it has smashed up gravestones and tipped them over, like a violent intruder.For the past two years, the congregation of 40 has funded a professional knotweed killer out of the collection money. But the cost – £700 so far – only covers the lower end of work. ‘My parents are buried right at the top and my grandmother’s grave is next door,’ says Rees. ‘We try to keep it clear, so we can visit. My husband sprays it with Roundup [the systemic herbicide], but it’s awful.’ Thirty or so miles north-east of Swansea is the valley town of Merthyr Tydfil, once a centre of iron production and now one of the 10 most deprived areas in Wales.Mark Sawyer works for Merthyr Valley Homes, the social-housing company, and is responsible for 4,200 houses and flats in the area. He’d been receiving knotweed complaints for some years, but, initially, placed the blame on the tenants. ‘When a tenant says, “I’ve got weeds in my garden,” you tend to think, well cut them back then. Or look after your garden, because we don’t provide that service. We do roofs, kitchens, bathrooms. ‘And the tenants would say, “Well I have been cutting it back.” The only thing it has to do is grow, which it does, up to four inches a day in summer. The other problem is the ease with which it spreads – not by seed, the plant is infertile. It reproduces by regenerating its rhizomes, which creep out horizontally deep underground, and sends up shoots. And even a tiny bit of rhizome (just 0.7g, the size of a fingernail) can generate a new infestation. ‘We are a small island with a lot of people and we move soil around a lot – to build roads, develop brownfield sites – and that is how it spreads,’ points out Dr Richard Shaw, regional coordinator for invasive species, at the Centre for Agricultural Bioscience International (Cabi), an intergovernmental research organisation. ‘The main issue is redistribution through human intervention. ‘It’s definitely spreading and the spread is exponential,’ he continues. It is also a problem across Europe and America, but is more extreme in Britain. The potential cost of trying to eradicate the plant in Britain has been estimated at more than £1.25 billion (just clearing it from the 10 acres of the Olympic Park for the London Olympics in 2012, cost more than £70 million). Last year, George Eustice, an environment minister, said there were ‘no plans to attempt a national eradication’ because of the cost. Combatting the enemy‘Our strategy is really focused on trying to stop the next Japanese knotweed from getting into the country and from spreading,’ says Olaf Booy, technical coordinator of the Non-Native Species Secretariat, a team based within the Animal and Plant Health Agency, which works on behalf of the Government. ‘But the Government is investing in a bio-control agent [more about this later] and has been helping to fund local groups that are dealing with Japanese knotweed,’ he adds. Defra (the Department for Environment, Food & Rural Affairs) spent £1.5 million on such groups from 2011 to 2015. It is currently employing a Local Action Groups coordinator to assist with funding bids. ‘We ought not to forget the quick growing ways of the great Japan Knotweeds growing fast and tall,’ wrote Gertrude Jekyll, the influential horticulturalist, in a 1900 edition of Home and Garden. Jekyll thought it was an excellent flanking plant for a woodland walk. And when, inevitably, the weed breached the garden walls, and began to run amok in someone else’s garden, break into someone else’s drains, qualifications began to creep in. Japanese knotweed, warned William Robinson, the Victorian gardener, in 1898, ‘…springs up everywhere’.Nevertheless, it was sold as a fashionable exotic until the 1930s. It was first spotted growing in the wild in Maesteg, a small town in south Wales. Japanese knotweed, noted John Storrie, a curator at Cardiff Museum, in The Flora of Cardiff (1886), was ‘very abundant on the cinder tips’ near the town. It has since colonised just about every corner of the British Isles (with hotspots in London, Wales, Cornwall and the West Country), growing in all sorts of places plants are not supposed to grow: sandy, salty beaches; heavy asphalt; swamps and marshes. The source of its almost supernatural resilience lies in its native habitat – it was dug up from volcanic fumeroles, outcrops of volcanic ash, near Nagasaki, where it thrived amid lava and poisonous gases owing to an extensive network of underground stems (rhizomes) that sucked up the limited nutrients available. But in Japan it has enemies, specifically 186 bugs and about 40 fungi. Here, it luxuriates in being predator-free. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.