Trees do a tremendous amount for our communities in a number of essential ways. Unlike a streetlamp or a water main, their influence is diffuse— they affect everything from air and water quality, to flooding, to temperatures, to property values and incidence of crime. Urban trees are under more pressure and facing greater challenges to survival than ever. In addition to the problematic removal of existing trees, the UK does not have a robust policy or plans for UK-wide soil volume minimums for urban street trees.
Protection of existing trees is one very important element of preserving our current urban tree canopy. The other is improving development schemes that provide yet-to-be-planted trees with the soil they need to thrive and grow in a much more urbanised and dense environment than their predecessors faced.
Trees need large volumes of soil to grow big and healthy. This is established science. And the small plots they receive on a typical city street generally don’t provide enough for them to reach maturity. Urban tree and soil expert James Urban, FASLA, recommends that trees receive at least 28 m3 of soil (slightly less if the soil is being shared between two or more trees). In order to grow the kinds of trees the world wants —ecological workhorses, the kinds of trees the world needs in an ever-warming world —the trees them a solid head start with adequate amounts of soil.
The UK is very behind on soil volume mandates. There are few soil volume standards in the UK and when there are, they tend to be very fragmented or specific to one locality like the attached from Leeds City Council. They are not requirements but guidance. The Sovereign Square project in Leeds followed the guidelines of medium and large trees by giving 12.6 m3 and 16.2 m3 for 15 trees in the new green space. LCC uses the following guidance measures:
In North America, cities are addressing this need by mandating minimum quantities of soil to be used in planting urban trees. Not all of these policies are perfect, but they are a quantum leap forward from planting a tree in a tiny plot and simply praying that it will thrive. The most ambitious policies are in Ontario, Canada, where several cities—including Toronto—require that all trees planted on city grounds be allotted 30 m3 of soil each. To see our list of the other cities in Canada and the U.S. that have joined Toronto in establishing soil volume minimum requirements, click here.
Trees ought to be considered a form of infrastructure, as much as buildings and roads are. Let’s focus not just on the quantity of trees being planted but also on the quality. And in order to ensure healthy trees, we must provide them with an adequate amount of soil to grow and thrive. U.K. cities and towns ought to consider implementing minimum soil volumes for street trees if U.K. cities are ever to see urban tree canopies that will help battle climate change and improve biodiversity. Cities other than Leeds must adopt minimum soil volumes to contribute to healthier, happier, economically vital communities.