The University of Greenwich is hosting the University Urban Tree Pit Comparative Study in collaboration with Hadlow College. The research is being led by landscape architect and university lecturer, Duncan Goodwin, whose research includes focus on determining the environmental and social benefits of urban trees and the establishment of green infrastructure.

“For trees to become functionally useful within our urban landscapes, they need to establish and reach a state of healthy maturity. Unfortunately, tree planting is often seen as a piece of ‘window dressing’ to assist a design scheme through the planning process, with little or no thought about long-term requirements,” he explains. “It is quite common for trees to be planted with insufficient rooting volume, and as a consequence, tree roots explore moist and less compacted materials close to the surface and along utility excavations, which disrupt paved surfaces and other infrastructure.”

This study investigates different tree pit systems currently available, which provide suitable rooting volume to enable street trees to thrive within urban hardscape settings.

During the last two weeks of July, 12 tree pits were excavated by groundworks contractor A. Eastwood and, to date, GreenBlue Urban’s StrataCell and Bourne Amenity’s Structural Tree Sand have been installed in six of the pits. This will be followed in mid-August by the installation of Silva Cell from Deeproot and Cornell University Structural Soil.

The first system installed was GreenBlue Urban’s StrataCell. These 100% recycled, lead bearing  modules clip together to form a structural matrix of cells able to support vehicular traffic areas. The skeletal system is then filled with soil to provide uncompacted rooting volume beneath the pavement above.

The second system was Tree Sand by Bourne Amenity. Tree Sand was originally developed in Holland during the 1980’s to provide growing conditions for trees in urban areas whilst avoiding problems with compaction from engineered paved surfaces.

The first product expected to be installed in mid-August will be the Deeproot Silva Cell. This consists of a series of plastic crates topped with structural decks, which are able to support engineered pavements. As with the StrataCell system, the crates are filled with soil to provide the tree with an accessible root environment beneath the pavement.

“We are really excited to assist the Greenwich University with this project by providing space for the planting to take place,” says Hadlow College horticultural department head Sarah Morgan. “It is a very important topic and will be a great teaching resource for not only our students, but for future specifiers of these systems.”

The Tilia cordata ‘Greenspire’ trees used were supplied by Hillier Nurseries, and are to be planted in early December during National Tree Week. This long-term project will investigate various methods of ensuring adequate rooting volumes for trees in constricted urban landscapes. It is intended as a catalyst for raising awareness of the services provided for trees in our urban landscapes, with monitoring and data gathering beginning in 2015.

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