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Urban Greening

I surprised myself when I managed to struggle through the science "O" Level for non-scientists – “Physical Science”, it used to be called. It did teach me to respect proper science though, and I reserve a special admiration for scientists at the top of their game who can explain what they’re up to without causing the kind of sensory shutdown induced by Mr. Ball the biology teacher*. I heard one on Wednesday, at the RHS’s first annual John McLeod lecture.

Diane Pataki is a sparky Californian academic whose interest is urban greenspace. She’s based in LA, which has its own peculiarities (not least folk nicking her monitoring kit), but some of her work has very practical application for us in the UK. Perhaps the best reason for planting a million trees in LA, which is what they’re doing, is that there is a really nice correlation between the percentage of tree canopy cover and daytime temperatures. Trees shade and reflect and absorb radiation. Diane reckons that they will be able to reduce daytime temperatures by a whopping 5 degrees (Fahrenheit), which will mean a 15% drop in electricity usage as everyone switches their aircon off. 15%! She’s also worked out which trees to plant too. Among other considerations, different species have vastly different transpiration rates and low water using species like the Honey locust (Gleditsia triacanthos) and Jacaranda trees don’t necessarily grow the slowest. In LA, of course, water use is a real issue, and very much an ecosystem cost rather than a service – so this kind of work is important in mitigating it. In the UK it’s more likely that too much water – and arriving too quickly – is a concern. Urban green spaces can certainly moderate stormwater run-off and absorb groundwater. I hadn’t thought clearly about other societal benefits, which Diane grouped under Provisioning, Regulating and Cultural. This might be old hat to some of you, but it was thought provoking for eco-newbies. Local varieties of fruit trees, for example, tick all sorts of boxes in all three areas of “ecosystem services”; food (provisioning), climate/water (regulation) and aesthetic/sense of place/heritage/etc. (cultural) One area where the message was more equivocal was carbon sequestration. It turns out you would have to plant gazillions of trees to make a minor dent in a major city’s carbon emissions. What was encouraging, though, were calculations showing the difference between playing fields and wildflower meadows.

I’d assumed the emissions from the kit needed to manicure lawns meant they were less friendly. Well yes, but there are other problems with them too. First off there’s the fact that undisturbed earth absorbs much more carbon dioxide. Secondly – and most importantly – is the Nitrous Oxide (struggles to interpret notes at this point) produced by fertilisers, which is a particularly pernicious greenhouse gas. These are the fertilisers which are good for lawns and bad for meadows. Hurrah! We like this. Diane didn’t talk much about habitat, which to be honest is a given.“Woodland edge” (in the UK often approximating to “suburban garden”) is top for a wide range of fauna. Hats off to the RHS for putting the lecture on, and next year’s deserves wider airplay than a 2 minute slot in the Today programme (N.B. John Humphries, this isn’t “gardening”). It was a shame, too, that the introductory and closing remarks didn’t mention the RHS’s own Urban Greening initiative and sounded more like an appeal for the church roof fund. Mr. Ball would definitely have approved of Diane Pataki, though. I look forward to reading a proper write up in The Garden.

*not entirely fair. Mr. Ball was a very nice bloke.