It seems pretty universal that everyone likes trees. We find them pretty, and calming, and they make us feel healthy.
(Metric)
(Standard)
Space, pollutants and engineering requirements are all major factors that affect the growing conditions for urban trees. But other than picking a climate-appropriate tree, soil availability is one of the biggest elements in their success or failure.One thing that isn’t well-understood, especially on a policy level, is just how much soil it takes to actually grow a big tree in the built environment.
When I talk about growing a big tree, I mean nurturing it to maturity and aiming to have it live 40, 50, 60 years or more (a far cry from the deplorable average lifespan of 13 years for most city trees). We all want big, gorgeous trees in our cities.
Naturally, it’s important to remember that different species of trees — small, medium and large — will require different soil volumes to reach maturity. The graph shown here is a general guideline, and very useful as a reference. (We have it courtesy of James Urban, FASLA — thanks, Jim!).
As you can see on the chart, 1,000 ft3 (28 m3) of soil is a pretty good target to aim for. With that kind of soil volume available you should be able to grow a good size tree that is positioned to live a long time. Even better, with proper site design and soil selection that tree should also be able to treat around 200 ft3 (5 m3) of water on-site through interception and absorption. That’s no small thing. All that water that is stored by the soil saves in irrigation costs, reduces flooding and the spread of non point-source pollution, and helps prevent combined sewer overflow. From a policy perspective, it has the added benefit of reducing stormwater utility fees by helping to maintain the site’s pre-development hydrology.
Of course, we realize that not all sites can provide this kind of soil volume for each tree due to budgets, site constraints, and other factors. But between tools like soil sharing, creating root corridors from tree pits to nearby soil volumes like lawns and parks, and engineered solutions like cantilevered sidewalks or the Silva Cell, we can get a whole lot closer to getting our city trees what they need.