Here is this week’s “Ask Jim Urban” column. Jim Urban, FASLA is an expert on urban trees and soils and his recent book, Up By Roots, is the industry bible on these topics. Jim was also involved in the development and design of the Silva Cell system. In this column, which we’ll be running once a week through the end of February and on a case-by-case basis after that, Jim will answer some of the most common questions he hears about Silva Cells. If you have a question you’d like Jim to answer, please email us at firstname.lastname@example.org. Here’s Jim.
Won’t we eventually have to remove all the soil (and the tree) because the soil has become polluted?
Urban storm water is typically fairly clean of chemical pollutants. In a field study of the effectiveness of bioretention in California, the pollutant levels of the runoff from the parking lot site was so low that the researchers had to fabricate polluted water to make the study effective.
The levels of pollutants removed by these systems are measured in hundreds of parts per million. The tree can actually photo-remediate much of these pollutants and maintain soil organic matter that supports soil biology that also does a great job of immobilizing pollutants. All plants do some level of phyto-remediation, some better than others. Trees are usually very efficient at this function. The levels of pollutant concentrations where the contaminant starts to harm the tree are measured in thousands to tens of thousands of parts per million. In all but the most polluted environments, the tree should be able to successfully grow in the slowly accumulating pollutant levels. I do not believe that we ever have to dig up these soils during the effective life of the tree.
James Urban, FASLA
Urban Trees + Soils