This summer I was invited to observe and assist in an urban tree pit comparative study at Hadlow College at the University of Greenwich. The project is led by Duncan Goodwin, a landscape architect and senior lecturer at the University and Alex Rennie, head gardener at Hadlow.
The study consists of twelve tree pits measuring 4 x 4 metres by approximately 1.2 metres deep, lined with a geotextile membrane to restrict root growth beyond the extents of the pit and filled with four different types of tree pit system. Twelve Tilla cordata will be planted in the pits in December and their progress will be monitored at intervals over the next five years.

Two systems to be trialled are GreenBlue Urban’s Stratacell system and Deeproot’s SilvaCell system, both load bearing modular cell systems that are backfilled with soil to create an uncompacted growing environment for tree roots. The other two systems are Bourne Amenity’s Tree Sand, the tree medium used extensively at the Olympics and Cornell University Structural Soil, a gap-graded gravel made up from crushed stone, clay loam, and a hydrogel stabilizing agent. Both of these systems are compacted at various stages during the installation process to become load bearing whilst still permitting natural root growth.

The benefits of trees in the urban landscape are well known and include improved air quality, cooling effects, increased biodiversity, storm water attenuation, added character and a potential increase in property value. However, we only feel the advantages of these benefits when trees are allowed to reach their full growth potential. Trees planted in small tree pits surrounded by heavily compacted soil in hard landscaped areas lack vigour are often short lived, and so become token plantings that add little value to the urban fabric.
I believe the importance of quality tree planting is becoming increasingly recognised. However, from professional experience and my studies at Greenwich I know there are hurdles that need to be overcome. Firstly, more local councils and developers need to adopt a holistic approach to tree planting and acknowledge that investment in trees and the landscape as a whole can provide great returns for the future, both environmentally and economically. In fact, this is inevitable as councils realise the significant role that tree pits systems can play in storm water attenuation.
Secondly, planning for tree planting should take part early in the design process so that a) adequate soil volume and supportive systems are provided underground for tree root growth alongside the needs for existing and new utilities and b) trees are not squeezed into small spaces to meet a recommended quota but planned for to allow for larger canopy trees in appropriate places where they can thrive (or simply plan the trees then the rest can follow!).
Lastly, budgets must be protected, as tree planting takes part in the final phase of development it can be greatly affected by the amount of money left in the pot.
I believe the key to strong tree growth is adequate protected soil volume and I watch with interest to see what conclusions are drawn from this valuable research. From a professional point of view I am interested to see how the different systems strengths will reveal how they can be applied in different situations, how systems can be combined to maximise potential and also the financial implications of adopting such systems, both in the short and long term.