How to grow a healthy city

Plants that fight ur­ban pol­lu­tion

The Daily Telegraph - Gardening - - Front Page - Ken Thomp­son

Ev­ery year around this time, the RHS hosts its an­nual John MacLeod Lec­ture.

The lec­ture, named af­ter a for­mer pro­fes­sor of hor­ti­cul­ture at the RHS, was cre­ated to high­light im­por­tant and in­spir­ing top­ics in hor­ti­cul­tural science. This year’s lec­ture, on Novem­ber 10 at the RHS Lind­ley Hall in Lon­don, will be given by Dr Ross Cameron from the Univer­sity of Sh­effield. The lec­ture is en­ti­tled “Re­pair­ing the rift – putting hu­mans back in touch with their own ecol­ogy”, and in it Ross will de­scribe the many ben­e­fits hor­ti­cul­ture (and plants in gen­eral) pro­vide to those of us who live in towns and cities.

This is a big sub­ject, much of it need­ing no in­tro­duc­tion to keen gar­den­ers; for ex­am­ple, we don’t need telling that gar­den­ing keeps you fit and makes you feel bet­ter (at least most of the time). How­ever, I want here to fo­cus on some­thing slightly dif­fer­ent.

Specif­i­cally (to get the jar­gon out of the way at the start), the way in which “green in­fra­struc­ture” (i.e. plants) pro­vides “ecosys­tem ser­vices” such as reg­u­la­tion of air tem­per­a­ture, noise and pol­lu­tion, re­duc­tion of run-off and flood­ing, and ther­mal in­su­la­tion for build­ings.

Of course, we’ve known for some time that plants do all these use­ful things.

But the ob­vi­ous fact that any plant is vastly su­pe­rior to tar­mac, con­crete or bare soil has rather blinded us to the fact that some plants are so much bet­ter than oth­ers. Re­cent re­search in this area is re­ported in a pa­per in the jour­nal An­nals of Botany by Ross and his co-au­thor Ti­jana Blanusa.

More fre­quent heat­waves are just one of the un­pleas­ant con­se­quences of cli­mate change and, even in Bri­tain, cities can be­come dan­ger­ously hot in sum­mer. Al­though plants can help a lot, it helps to un­der­stand that they do so in at least three dif­fer­ent ways: by shad­ing, by evap­o­ra­tive cool­ing, and by re­flect­ing sun­light. It’s tough to find plants that do all three well, but re­search shows that in the UK evap­o­ra­tive cool­ing may be most im­por­tant, and is max­imised by fast-grow­ing trees with spread­ing, mul­ti­lay­ered canopies. At the op­po­site tem­per­a­ture ex­treme, build­ings shel­tered by trees and shrubs lose less heat in the win­ter. One re­cent study com­pared cherry lau­rel and Co­toneaster franchetii. The for­mer pro­vided bet­ter in­su­la­tion, but over­all the lat­ter was a bet­ter choice – at least for a sunny wall – since its smaller leaves and less dense canopy still pro­vided some in­su­la­tion, but also al­lowed more sun­light to pen­e­trate dur­ing the day.

An­other cli­matic hazard, much in the news re­cently, is flood­ing. All trees mas­sively re­duce the amount and ve­loc­ity of wa­ter reach­ing the ground but, as usual, some are bet­ter than oth­ers. A large canopy with ex­ten­sive branch­ing helps, but so does the “fine­tex­tured” canopy of many evergreen conifers. Even bark tex­ture turns out to be im­por­tant; the rough, grooved bark of red oak re­tains two-and-a-half times more wa­ter than the smooth bark of Be­tula lenta.

Trees and shrubs also in­ter­cept dust, pol­lu­tion and noise – im­por­tant if you live near a busy road. One Ital­ian study found that lime and plane were bet­ter at cap­tur­ing the small­est (and most dan­ger­ous) par­ti­cles from diesel en­gines, while oaks were bet­ter at cap­tur­ing larger ones. A UK study found that Cor­si­can pine was very ef­fec­tive at cap­tur­ing par­ti­cles of all sizes.

Dense plant­ings of shrubs, ideally a few me­tres taller than the re­ceiver of the noise, are best for re­duc­ing noise. Species with dense fo­liage and low fork­ing branches work best; one study found that bam­boos fit­ted the bill per­fectly. Stud­ies also show that at least some of the ef­fect of a noise-re­duc­ing shel­ter belt are psy­cho­log­i­cal; noise doesn’t seem so bad if you can’t see where it’s com­ing from.

It’s still early days for fig­ur­ing out which plants are best for im­prov­ing the ur­ban en­vi­ron­ment, and es­pe­cially find­ing those re­ally use­ful plants that can pro­vide two or three dif­fer­ent ser­vices. But the ba­sic point is one I’ve made be­fore: when it comes to sav­ing the planet, don’t make the mis­take of over­look­ing the im­por­tance of gar­den­ing.

Tick­ets for the MacLeod Lec­ture are avail­able to RHS mem­bers only, email sci­encead­min@rhs.org.uk

Rough, grooved bark holds two-and-a-half times more wa­ter than smooth bark

Plant pro­tec­tion: the tow­er­ing plane trees in Wellington Square, Chelsea, are both beau­ti­ful and use­ful; re­cent re­search shows that Co­toneaster franchetii is a great plant for in­su­la­tion

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