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 [email protected]. Here’s Jim.

So how are trees compatible with the concept of the open soil area as a filtering fore-bay?  Will we just simply have to remove the trees after 10-20 years?

First, trees and storm water treatment are entirely compatible. Forested wetlands are the most valuable of our upland storm water treatment systems. Trees can easily accept the level of short term flooding and sediment build up in these systems, which are overly dry, not overly wet, most of the time.

BUT…

Won’t we have to dig up the soil every so often to clean out the system, killing the tree?

While fore-bays are critical to these systems, in intense urban areas the amount of sediment is often not as great as in suburban areas. This sediment can be removed without harming the tree by using air spade and Vactor truck technology. Vactor trucks are already used to clean out inlets that get silted in. We are already using air spades and Vactor technology in tree preservation work to remove soil from the base of trees. Some designers are working on concrete fore-bays that will segregate the majority of the coarse material, making it easier to clean. Assuming that the tree is provided with adequate soil under the pavement, I do not see why we cannot remove sediments above the base soil level, over time, as needed.

James Urban, FASLA
Urban Trees + Soils