Adequate amounts of lightly compacted, good-quality soil are essential to healthy tree growth. Nowhere are these conditions more challenging than in urban areas dominated by streets, sidewalks, buildings and parking lots. These surfaces are essential for urban living but require heavy soil compaction, which limits the development of large, healthy root systems.
The USDA Forest Service recently determined that U.S. cities are losing around 4 million trees annually — this at a time when average urban canopy cover in North America is still lower in most places than what is recommended by American Forests.
Still, it’s not all doom and gloom. Cities across North America recognize that in order to have healthy, vibrant urban forests, they need to plant trees in more soil. Guidelines already exist that correlate tree size directly with available soil volume, so this isn’t a matter of guesswork. What these cities are doing is simple, but remarkable: They are literally changing their landscapes by mandating that all street trees (used here as a general term to mean any tree planted in a paved area) receive a certain minimum amount of soil.
Here, we’ll look at some of the most interesting and ambitious policies.
Toronto’s Green Development Standard
The most aggressive and ambitious policy that I’m aware of, Toronto’s Green Development Standard mandates that all street trees receive a minimum of 15 cubic meters (529 cubic feet) of high-quality soil per tree if in a shared planter, and a minimum volume of 30 cubic meters (1,059 cubic feet) of soil per tree if in a single planter. To put this in perspective, a tree in a typical four-by-four-foot opening on the street might get as little as 48 cubic feet of soil — just 5 percent of what this standard mandates.
The soil volume minimum set out in the Green Development Standard also dovetails nicely with the stormwater requirements in the same document, which call for:
- Retaining stormwater on-site to the same level of annual volume as before the site was developed (for sites greater than 0.1 hectares). This works out to either retaining the first two inches (five millimeters) from each rainfall on-site, or limiting the maximum annual runoff volume to not more than 50 percent of the total average annual rainfall.
- Removal of 80 percent of total suspended solids — a conventional pollutant whose removal is a way of assessing water quality — from all runoff leaving the site.
- Protecting native soils from becoming compacted (and therefore unusable for tree roots) during construction.
- Retaining soil on site, or adjusting/replacing it with soil of equal or better quality of necessary.
Denver Parks and Recreation Forestry Department
Denver’s "Street Tree Plan Review Checklist" sets a required soil volume minimum for trees of 750 cubic feet. In addition to the minimum cubic feet of soil, it states that “5’ x 5’ pit areas shall no longer be accepted, [and] must use trenches, root paths, break out zones, structural cells, or other un-compacted soil technology.”
This is a great start. The suggested technologies — trenches, root paths, break out zones and structural soils — are all ways of providing lightly compacted, usable soil beneath paved areas. But the language could be more specific. For example, the checklist doesn’t specify whether the 750 cubic feet of soil is per tree, or per trench, or what. I was not able to find any clarification in other online documents. I did call the phone number listed on the checklist for further information, but haven’t gotten a call back.
The Denver checklist does state one thing very clearly: “The below information shall be required on all development plans, omission of any of the below information may result in denial of your plans and/or permit process.”
Emeryville’s “Usable Soil Volume”
Emeryville, Calif. is the only Bay Area city I know of to have a soil volume mandate. It is fairly specific, requiring minimum rootable soil volumes for new trees planted in the public right of way by private developers.
The soil volume minimums are based on the projected size of the tree at maturity:
- Small tree: 600 cubic feet
- Medium tree: 900 cubic feet
- Large tree: 1200 cubic feet
These minimums are a really good start. Even the amount for small trees is more than 10 times the soil a tree in a basic four-by-four sidewalk cutout receives.
One shortcoming of the Emeryville policy is that is doesn’t fully address usable soil volume, which refers to the amount of soil actually accessible to the tree’s roots. As an example, one solution for planting trees in paved areas is called structural soil, which is made up of approximately 80 percent rock (used to support the weight of paving) and 20 percent soil (for roots). This means that if 1,000 cubic feet of structural soil is installed for a street tree, it would meet Emeryville’s requirements — even though the tree could only use 200 cubic feet (20 percent).
West Virginia Department of Environmental Protection
This community created a municipal handbook that made the following soil recommendations based on those in Prince William County, Va.:
- Small tree: 500 cubic feet
- Medium tree: 750 cubic feet
- Large tree: 970 cubic feet
Like Toronto, West Virginia is looking at the whole picture by encompassing both tree growth and the management of wet weather when considering their recommended soil volumes. The soil volume minimums outlined here are not as big as Toronto’s or Emeryville’s, but they’re a huge improvement over how street trees typically get planted. But there is a pesky word in there: recommended. There can be no enforcement with recommendations. I will be curious to see whether designers adopt these soil volume minimums if they remain suggestions rather than mandates.
None of these policies is perfect. There are tweaks, of varying sizes, that I would make to all of them. Still, they represent a critically important step to creating lasting, healthy, thriving urban canopy cover in cities.
And the policies listed here aren’t the only ones. Versions of these minimums exist in Chicago; British Columbia; Minnesota; Athens-Clarke County, Ga.; Aspen and Pitkin Counties, Colo.; Oakville and Markham, Ontario; Alexandria, Va.; Baltimore, Md.; and Charlotte, N.C. For a complete list, you can check out this post outlining all the ones I know about and an overview of their stated policy.
It takes more than a soil volume minimum to cultivate a thriving, mature urban canopy. Good tree stock, soil quality, adequate water and regular maintenance cannot be undervalued. But we’ll continue to lose canopy cover at an alarming rate if we don’t change the way we plant trees in cities. We know what this means, and we simply need to do it. It starts with soil.
Leda Marritz is the creative director of DeepRoot Green Infrastructure and an ISA-certified arborist.