Retrofitting soil cell systems to retain established trees

Citygreen’s soil cell systems are commonly used to support the growth of new trees in urban environments. Many people are unaware though that the systems can also be used very successfully to support the continued growth and longevity of existing trees.

Clearly, to justify this process, the trees in question need to have some retention value. Perhaps they’re well established, historically significant, or an integral part of the surrounding landscape. In places like Singapore, for example, where there is little room to create new green space laterally, existing urban trees are particularly precious.

So, when would we look at retrofitting a soil cell system to an existing tree?

Good question. We tend to see two common scenarios:

1. The existing tree is clearly struggling and needs an intervention to save it; or
2. A new development is planned which is likely to infringe on the root zone and threaten the health of the tree.

What does retrofit involve?

The primary objective is to keep excavation to a minimum and minimise the impact on the tree.

First of all, you need to look at the encroachment over the root zone and assess how much excavation will be required to install an appropriate system. We recommend engaging an arborist or forester if there’s likely to be root pruning required so that they can oversee this process and ensure the stability of the tree. Particularly in high-wind areas, you need to be very cautious about a large excavation near to the tree destabilising it.

Maintaining as much of the existing root system as possible is crucial, so consider hand digging or vacuum excavation to minimise root damage. Once the roots are exposed, they can then be fed back through the cell system.

For areas where it is not possible to excavate directly next to the tree, nearby installations can be effective. For example, if a car park is being built adjacent, soil cells can be placed under the car park, with nearby roots eventually making their way into the system, and avoiding damage to the car park above in the process.

The Tree Protection Zone

One big caveat is to be sensitive to the Tree Protection Zone (TPZ). Most countries will have guidelines that enable you to calculate the TPZ, and then you should aim to protect this zone as much as possible. We recommend marking it out clearly and making this a no-go zone for all kinds of construction traffic. In the event that the TPZ must be encroached, engage an arborist to assess your specific site and manage this as best as possible for the protection of your tree.

Recent Singapore retrofit

The following photos show the retrofit process end to end, recently completed in a housing development in Singapore. Keen to retain a well-established existing tree, Stratavault was retrofitted to provide the required soil volume and reduce compaction around the roots. You can see the care taken to retain the existing root system without damage.

All in all, retrofitting soil cell systems to existing trees can be very effective, provided adequate planning and the required level of care is applied. When the result is retaining a beautiful established tree for the enjoyment of all, we think it is well worth it.

Do you have a potential project in mind? Please contact us to find out how we can help.

Images courtesy of Elmich Singapore, agents for Citygreen