How to grow a healthy city
Plants that fight urban pollution
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 email@example.com
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