Sustainable Urban Trees Require Healthy Soils

San Francisco, CA (November 16, 2013) – James Urban, Fellow at the American Society of Landscape Architects and author of Up By Roots, wrote a recent “Deep Root” post on the need for landscape architects to design with healthy soils in mind. “The success of a tree is fundamentally linked to the soil in which it grows,” say Urban. He had this same message at the 2013 ASLA annual meeting. Urban lays out some of the ways landscape designers rethink planning to ensure that trees and plants in urban environments get healthy soils and appropriate growing space.

OLYMPUS DIGITAL CAMERAAccording to Urban, “the designers of today’s built environments and city planners tasked with creating sustainable, livable, resilient communities continually make mistakes that doom their trees to failure,” giving trees limited opportunity to fully mature and offer all the benefits to a community. Trees need proper soil and space.

Whether natural or man-mixed, soils have physical, environmental, and chemical properties. These are all important to the health of a growing medium. To determine what kind of soil is needed for a project, Urban advises that goals and requirements are needed early on in the design process.

For example, “What type of trees and plants are you trying to grow? How big do you want these plants to get?” Depending on the requirements, an oak can grow to 25 feet and last 50 years, or grow to its full extent and live hundreds of years. Landscape architects need to think through these things with soil in mind.

Urban describes eight critical properties of soils, which soil biologists can test to determine if soils meet specifications—structure, texture, density, nutrients, pH, organic matter, and density, which are all “inter-connected.” More often than not, Urban says, trees and plants don’t do well because of the physical properties of soils rather than the chemical. More landscape architects need to measure soil structure, as proper soils is the most important factor in growing healthy trees.

Urban suggest putting trees into the built environment in new ways.  While there is much support for “right tree in the right place,” soil in urban areas has become more degraded and there are fewer trees that will match local site conditions. This results in less diversity of tree species, which creates its own set of problems.

Urban proposes designing landscapes with the larger role and benefits of trees in mind. “If the goal is to create a diverse urban forest with healthy trees, we must design soil environments with that goal in mind,” says Urban. He also suggests that landscape designers design for fewer trees, but with soil conditions that will support longer life and a more diverse stock of tree species to give trees a chance to live longer and with fewer maintenance requirements. Read the complete article, “An Open Letter to the Complete Streets Movement.”

Sources:
An Open Letter to the Complete Streets Movement,” Deep Root
Designing with Soils,” The Dirt