Silva Cells Can Be Installed with (around) Existing Trees – Part I

A common question we receive is can Silva Cells be used with an existing tree? Yes!

While Silva Cells are typically used to provide soil volume and stormwater management with new tree plantings, there are many circumstances where saving an existing tree is the goal. With care and consideration, Silva Cells can be installed to provide the soil volume needed to support an existing tree, but there are a set of stipulations to consider.

Here are some of the questions to ask when determining if Silva Cells should be used around an existing tree in your project:

Health and Condition

Jim Urban, FASLA suggests considering:

  1. How much can the roots expand?
  2. How is the tree’s condition?
  3. What are the long-term results if you do nothing and don’t add Silva Cell?
  4. Review Jim Clark and Nelda Metheny’s book, Trees and Development. Jim Urban notes the authors ask arborists to “consider the trees specie, age, and health.”
  5. Are there historic site preservations that require keeping the tree?
  6. Consider allowing support roots that pass through the Silva Cells to remain. Just build the Silva Cells around the roots the same way you would if a conduit was passing through the Silva Cell are.

Excavation

If the health and condition questions lead to a “yes, let’s keep the tree”, the next question is to determine how and to what extent it will be excavated. Excavation is critical in determining the feasibility of existing tree applications. Here are two things that should be considered early on:

  1. Size of the opening. Existing trees require larger openings than newly planted trees. The size of the opening dictates how close the excavation needs to be to the tree and ultimately how much of a tree’s root structure will be impacted. Tree openings need to be sized proportionally to the size/root development. It is not realistic to think you can excavate close enough to a mature tree to accommodate a standard 4’ x 4’ tree opening.
  2. Excavation costs can vary substantially depending on the means required. In order to budget accurately, it is important to understand how you will be able to excavate. Cost will vary significantly based on how much excavation can be done with a machine (an excavator) compared to hand shovel, vac truck, or air spade. While costs will be higher than a typical Silva Cell installation, when working with significant and valuable trees there is a strong financial incentive to make it work. Here is an example of the value some people place on their trees.

Contractor prepping site for Silva Cell installation surrounding one of two 20-year-old Elm trees at Hanover Inn in New Hampshire.

Consult an Arborist

Al Key has worked on dozens of high-profile projects including Waterline Square and Lincoln Center and speaks with 25+ years of experience. He suggests:

  1. Hire an ASCA Arborist to decide where to sever the roots of the tree and its ability to feed itself.
  2. Consider mapping the root structure with the use of Ground Penetrating Radar to help determine the extent of the root structure.
  3. Root pruning is necessary because you need to compact the subbase around the tree to support the Silva Cells.
  4. Only an Arborist can decide where to sever roots without creating a hazardous tree condition.

Part II of this blog highlights four projects that successfully installed Silva Cells around existing trees. The projects were able to evaluate the health and condition of the trees and determine budget and cost given the size and opening surrounding the tree pit. They are in New Hampshire, Ontario, Florida, and New York.