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Gardeners' World - - Contents -

A knotty prob­lem

Does Ja­panese knotweed de­serve its bad rep­u­ta­tion as an in­va­sive weed ca­pa­ble of de­stroy­ing build­ings? In the past, the weed’s pres­ence has af­fected prop­erty prices and those buy­ing prop­erty with Ja­panese knotweed have strug­gled to get mort­gages, but new re­search sug­gests that the prob­lem is not that bad. Stud­ies by the Univer­sity of Leeds and in­fras­truc­tural ser­vices firm AECOM found that there is “no ev­i­dence that Ja­panese knotweed causes sig­nif­i­cant struc­tural dam­age.” Ja­panese knotweed is be­lieved to cause prob­lems to struc­tures that are within seven me­tres of the above-ground part of the plant – which is known as the ‘7m rule’. It can re­grow from very small bits of its un­der­ground roots or rhi­zomes, which makes it dif­fi­cult to dig out. Yet the univer­sity’s re­search showed, in a sur­vey of 81 ex­ca­va­tions of the weed, that “rhi­zome spread was gen­er­ally less than 2.5m.” Dr Mark Fen­nell, prin­ci­pal ecol­o­gist at AECOM said, “We found noth­ing to sug­gest that Ja­panese knotweed causes sig­nif­i­cant dam­age to build­ings and cer­tainly no more dam­age than other species that are not sub­ject to such strict con­trols.” Legally, since 2013, a seller is re­quired to state whether Ja­panese knotweed is present on their prop­erty. A re­port fol­low­ing a July 2018 YouGov poll on the weed said that it can cost £166 a year to treat the in­va­sive weed. But Dr Karen Ba­con, from the Univer­sity of Leeds said: “Ja­panese knotweed is ca­pa­ble of dam­ag­ing built struc­tures, but where this oc­curs, it is usu­ally be­cause an ex­ist­ing weak­ness or de­fect has been ex­ac­er­bated. The im­pact of Ja­panese knotweed on fac­tors such as bio­di­ver­sity and flood­ing re­mains a con­cern, but this plant poses less of a risk to build­ings than many woody species, par­tic­u­larly trees.”

Ja­panese knotweed poses less of a risk to build­ings than trees

Ja­panese knotweed could be less of a worry to home­own­ers than pre­vi­ously thought

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