Ask Jim: “Might the trunk flare grow so large that it alters the soil hydrology?”

Ask Jim!

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 March 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.

What about the tree trunk flare growing so large that it changes the hydrology of the surface in the open soil bed?

hope the trunk flares grows large, as this indicates the tree is very healthy. The design must recognize how a tree grows and put the tree in a location that works both for the tree and the hydrology. I have seen designers plant the tree in the same location as the slot that lets water into the soil fore-bay. This will plug the system as the trunk flare rises out of the ground. The best design would have the lower receiving soil and slot to let in curb water be at only one end of the open soil area, and the tree planted at the other. Unfortunately I almost never see this.  We must have some misguided notion that the tree and other features be always symmetrical.

Clearly all the above ideas require that the designer think about all the issues and design the system to respond to the hydrology and tree growth patterns  They must understand that the tree will change all the time.

James Urban, FASLA
Urban Trees + Soils

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