Don’t panic about an over­hyped in­va­sion

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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.

PT, Tad­worth

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?

RW, Bar­net

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

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