How Much Soil Do You Need to Grow a Big Tree?

It seems pretty universal that everyone likes trees. We find them pretty, and calming, and they make us feel healthy.

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 10 years for most city trees). We all want big, gorgeous trees in our cities.

Different species of trees  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, 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 soil volume alone should also be able to treat around 200 ft3 (5 m3) of water on-site through interception and absorption.

The stormwater quantity and rate control and quality benefits of soil are significant. All that water that is stored by the soil saves on irrigation, reduces non point-source pollution and flooding, and helps prevent combined sewer overflow. From a policy and regulatory perspective, a solution like this will also reduce stormwater utility fees by helping to maintain the site’s pre-development hydrology.

Not all areas can provide this kind of soil volume for each tree. Budgets, site constraints, and more are all factors in the final decision. However, between tools like soil sharing, creating root corridors from tree pits to nearby soil volumes like lawns and parks, and engineered solutions like suspended pavement, we can get a whole lot closer to getting our city trees what they need.

Image: Friends of the Urban Forest

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