The type of tree planted is important.  In regions such as the Pacific Northwest where the majority of precipitation occurs in winter, evergreen trees play the largest role in interception.  According to Ramie Pierce, Tacoma’s Urban Forester, “Evergreens have more of a stormwater benefit mostly because they retain their leaves all year (it helps too that they are still growing, although slowly) and the leaves intercept and therefore slow down water flow, reducing the flow that reaches our storm system in the beginning parts of storms.”

Pierce isn’t suggesting that evergreens be the only type of trees planted.  One large deciduous tree can reduce stormwater runoff by over 4,000 gallons per year.  The largest benefit occurs if it is in-leaf when precipitation is at its greatest.  It will take a variety of trees to help Tacoma reach its goal of 30 percent coverage and to maximize stormwater retainment.

When determining tree selection, homeowners should consider tree species with large leaf surface area and rough surfaces.  Obviously, the larger the tree, the greater the impact on stormwater runoff.  When large trees are not an option, small groves of trees make a larger impact than single trees. Trees native to this part of Washington, once established require little supplemental irrigation and more readily tolerate this region’s fluctuating precipitation patterns which can leave them in standing water during the winter and high and dry in the summer months.

By planting appropriate trees, improving the maintenance of existing trees and redesigning parkways, boulevards, parking lots, traffic islands, swales, median strips and residential rain gardens to include more trees, homeowners can substantially reduce the amount of runoff during rain events and thereby reduce the impact to downstream lakes and rivers and eventually Puget Sound.