Don’t panic about an overhyped invasion
would be interested to learn your views on Japanese knotweed. I keep reading that it spreads rapidly, but the clump I am monitoring has not spread at all in 10 years. All I do is treat it like any other weed – pull up the shoots as they appear and sometimes apply weedkiller.
I also read in the newspapers that it can grow through concrete and damage houses, but the same photo seems to be used every time. If the risks from this plant are being exaggerated, then hundreds of properties are being blighted unnecessarily.
agree with you. The threat of Japanese knotweed appears to be grossly exaggerated, chiefly by an industry that has grown up to eradicate it. I was recently working on a development where Japanese knotweed had been identified in the back gardens. The proposed cure was to excavate the whole area – to a depth of three metres if necessary – and cart the soil away. This is known in the trade as “dig and dump”. The costs ran into tens of thousands of pounds.
Eventually common sense prevailed and the weed was sprayed with the common weedkiller glyphosate (sold as “Roundup”). Two or three sprayings over a growing season seem to be enough to keep it in check.
Nevertheless, the surveying profession has become terrified of Japanese knotweed. Mortgages are being refused on properties where it has been spotted, even in a next-door garden. And some homes have become “blighted” and unsaleable because of the hysteria surrounding the issue.
Japanese knotweed is an attractive flowering plant that was first introduced from Japan in the 1850s. Despite sensationalist headlines about this “alien invasion” that can “tear through brickwork and concrete”, it grows without causing problems in many gardens. After a century and a half it has so far failed to take over the country or destroy our homes and roads.
As regards “growing through concrete”, this has all the ingredients of an urban myth. Any plant that roots in a crack and then grows will exert a force that can expand the crack. It will only cause damage if the concrete is of poor quality, such as thin garden paths. More buildings are probably damaged by elder, buddleia and ivy, but nobody has ever had a mortgage refused because of these. In fact, homeowners frequently encourage the latter to climb up their walls.
roof is covered with plain clay tiles. These have a roughened surface that encourages moss growth. Clumps of moss block the gutters, and drop onto the conservatory roof. Apart from erecting scaffolding and either pressure hosing or scraping the tiles, is there any longer-term method of preventing moss growth? I’ve heard that putting copper wire or tape just below the ridge can help. Is this true?
is the most common question that I am asked by Jeff at Property, The Daily Telegraph, 111 Buckingham Palace Road, London SW1W 0DT; email askj[email protected]graph.co.uk. Also visit askjeff.co.uk readers – sometimes as many as four or five times a week.
In the first place, anyone with moss on the roof should rejoice, because it is a mark of how unpolluted our air has become. Before the clean air legislation of the Fifties and Sixties, you would never have seen a tuft of roof moss in your north London suburb.
Does roof moss do any harm? That depends on how soft your tiles are. Some old, underfired clay pantiles might be damaged by button mosses rooting in cracks and fissures. But most postwar tiles are hard enough to withstand a bit of moss growth.
Should you get it cleaned off? That’s entirely up to you. There is an epidemic of cowboy firms cold-calling and leafleting home owners. They spread the fear of imminent roof collapse, and charge thousands of pounds for roof-cleaning and sealing services. This is borderline fraud.
Copper strip or wire will allow copper oxide to be washed down the roof by (slightly acidic) rainwater, which will inhibit moss growth. It can cause staining, and will also corrode anything aluminium in its path, such as guttering and roof windows.
I would learn to ignore it, or else pay a handyman to scrape it off for you every few years. And next time you’re in a supermarket, pick up a copy of Country Life magazine and leaf through the million-pound house adverts at the front. The homes with the most moss on their roofs invariably have the highest price tags.
I wish I knew what a treenail was, but I’m too embarrassed to ask.
A treenail is a large hardwood dowel – usually oak – used to pin together the joints in traditional oak-framed buildings. The frames are best made from fresh “green” oak, which twists as it dries, locking the members tightly together.
Alien invasion: hysteria surrounding Japanese knotweed is the stuff of a Hollywood horror film