Urban tree pit comparative study gets underway

An urban tree pit comparative study broke ground last month with final installations complete by the end of this month with trees being planted during National Tree Week in December.

Tree pit study
Tree pit study

The University of Greenwich is running the University Urban Tree Pit Comparative Study, with space and assistance provided by Hadlow College and its ground crew, led by head gardener Alex Rennie.

The project is being led by landscape architect and university senior lecturer, Duncan Goodwin, whose research covers the environmental and social benefits of green infrastructure and urban tree establishment.

He said: "For trees to become functionally useful within our urban landscapes, they need to establish and reach a state of healthy, productive maturity. Unfortunately, tree planting is often seen as a piece of ‘window dressing’ to assist a design scheme through the planning process.

"With little thought about their long-term requirements, it is quite common for trees to be planted within insufficient rootable space. As a consequence, tree roots tend to explore moist and less compacted materials close to the surface and along utility excavations, disrupting paved surfaces and other infrastructure.

"This study investigates different tree pit systems currently available in the marketplace, which provide suitable root volumes to enable trees to be successfully planted and establish within our hard landscapes."

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 SilvaCell from Deeproot and Cornell University Structural Soil manufactured, under license, by Landtech.

The first system installed was GreenBlue Urban’s Stratacell. This is a UK manufactured, 100 per cent recycled, high-strength, open composite module which clips together to form a structural matrix of cells, able to support the high vertical and lateral loads created by engineered pavement systems. The system is filled with top soil to provide uncompacted rooting conditions beneath the hardscape above.

The second system was Bourne Amenity’s Tree Sand. This product was used in the Olympic Park as part of the 2012 Olympics and transformation to Legacy works. Tree Sand was originally developed in Holland during the 1980s to provide suitable growing conditions for large trees in dense, urban areas whilst avoiding problems with compaction from engineered paved surfaces.

The first product expected to be installed in mid-August will be the SilvaCell system, provided by Deeproot. This consists of a series of open, high-density plastic crates, topped with structural decks, which are able to support engineered pavements and vehicular loads. As with the Stratacell system above, the crates are filled with soil to provide the tree with an accessible root environment beneath the hard surface.

Hadlow College horticultural faculty head Sarah Morgan said: "We are really excited to assist the University of Greenwich with this project by providing space for the planting to take place. It is a very important topic and will be a great teaching resource for our students, the future specifiers of these systems, to see."

Goodwin said: "This study could not have happened without the generosity of our commercial partners. Each has supplied their systems free of charge. The point of the project is not to set one system against another, but to review them all and see where each can excel. It is intended that this study will be a catalyst for raising awareness of the ecosystem services provided by trees in our urban landscapes."

The Tilia cordata ‘Greenspire’ trees, supplied by Hillier Nurseries, are to be planted in early December during National Tree Week in December. Monitoring and data gathering will begin in 2015. This is a long-term project will investigate various methods of ensuring adequate rooting volumes for trees in constricted urban landscapes.

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