Case Study

Water Quality

Boise, ID: Soil matters


Suspended pavement systems allow adequate soil volumes, even for large trees.

Boise’s forest management plan calls for larger trees, more shade.  Larger trees need more soil volume.

Boise, Idaho

Semi-arid continental


Metro: 88% White, .5% Black; 9% Latino; 1.4% Asian; .8% Native American

City: 88% White; 1.5% Black; 7% Latino; 3% Asian; .7% Native American

Below Poverty Line: 16% Individuals

Since determining that green infrastructure stormwater strategies produced a high return on investment while reducing stormwater run-off, Boise has invested in close to 30 projects since 2009, including expanding its urban canopy.

How much is enough? More!

The city’s newest Forestry Management Plan released in 2015 estimates that the city’s urban forest annually contributes $485,000 in stormwater runoff reduction value, as well as $381,000 in reduced energy cost, $3.3 million in health benefits and services that make neighborhoods more vital and livable.

“We need more soil volume. We need a place for tree roots, so that they can get what they need to grow to a more significant size and offer ecosystem services, including air quality, stormwater management and many others.” — Brian Jorgensen, City Forester

The plan calls for a growth of the urban canopy from ~10 percent to 25 percent with an emphasis on growing larger shadier trees. Larger trees require significantly more soil volume than is customarily provided for city street trees — up to 1,000 cubic feet. Many communities specify soil volume and composition in their street tree planting guidelines.

Boise engineered additional soil volume through a suspended pavement system that doubles as a GSI strategy. As of 2016, more than 100 trees have been planted in Silva Cells, totaling about 100,000 cubic feet of soil, which support better tree growth while providing runoff reduction and improved water quality for the city.


While suspended pavement systems are expensive, the healthy trees they support provide more ecosystem services than other approaches to urban stormwater management.

  • City recognized past practices weren’t working. Soil compaction, limited soil volumes combined with hot, dry summers caused early mortality
  • Local partners came together to conduct urban tree canopy assessment, in-depth study of ecosystem services
  • Data provided foundation for strategic urban forestry planning
  • Idaho Department of Lands
  • Treasure Valley Canopy Network
  • Ada County Highway Department
  • City of Boise
  • Capital City Development Corporation


Lessons Learned
  • Don’t skimp on soil volumes
  • Upfront costs for suspended pavement systems may be lower than repeatedly replacing root-bound street trees
  • Value of co-benefits must be considered when comparing costs
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