We Want a Shrubbery

We know trees are good for the urban environment, despite the occasional complaint that they shed too many leaves or eventually break up sidewalks. Now researchers in Europe are promoting what they say is an even better idea: hedges.

Prashant Kumar of Surrey University and others have just published research showing that low hedges trap pollutants from car exhaust—being, as they are, right down there at the level of the tailpipe—much better than the canopies of tall trees do. The study doesn’t recommend replacing trees with hedges, but rather planting both wherever possible. Planting hedges close to the road, where they will trap pollutants before they dissipate, is ideal.

In addition to trees’ other benefits—reducing the heat island effect, removing carbon dioxide from the air, and possibly increasing real estate prices for the properties near them—they also reduce peak flows of stormwater runoff. They hold a certain amount of water in the canopy for a time after it rains, and they also transpire quite a bit of water daily, depending on the type of tree. How good are hedges at doing the same?

Lots of data exists for trees’ performance. This article from the October 2016 issue of Stormwater,  “Give Me the Numbers: How trees and urban forest systems really affect stormwater runoff” by Aarin Teague and Eric Kuehler, offers some details. There are also plenty of estimates out there for intensive and extensive green roofs, and for various types of vegetated cover. For example, this (Table 3-5) and many other sources provide runoff curve numbers for different land uses, including general types of vegetation. What I haven’t been able to find are specific numbers for trees versus low-growing plants—boxwood, privet, oleander, and so on. If you’re aware of research in this area, please leave a comment below.

Just as trees have their critics, though, one complaint about hedges is that they require more frequent maintenance than trees do to keep them looking good. The UK researchers point out that in some areas of the country, cities are now charging more to collect green waste, which might also prompt people to rip out their hedges rather than trim them.