Soil Cells at Blackheath Hill | 14 Year Review - GreenBlue Urban
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Soil Cells at Blackheath Hill | 14 Year Review

This project was planted in 2001 as one of the first installations in the world utilizing the commercial use of soil support cells for tree root growth. The international engineering group involved in the design improvements to this major roadway had identified a planting location, but realized that without proper provision below ground, the tree would become an expensive failure. Recognizing GreenBlue’s work in the research of urban tree planting and soil support technology, the consulting specifier designed a tree pit incorporating GreenBlue’s world-renowned ArborSystem tree planting system – including soil support cells to offer uncompacted soil volume for root growth, tree pit irrigation/aeration to provide the vital nutrients every tree needs, and root management to direct roots away from paved surfaces & utilities and into intended rooting zone.

Although this tree pit was of a limited size (16 feet long by 3.5 feet wide by 3.5 feet deep) allowing only 5 cubic meters of rooting medium, it has been an excellent demonstration of successful tree planting strategies, showing growth from 16-18 cm girth to approximately 60 cm girth by May of 2010.

The planting location was 12 feet from the nearest building, and the surface around the tree was continuous pavement with RootRain Precinct™ irrigation/aeration inlets set into the granite paving stone surround. The new tree pit was created as part of a pavement realignment and refurbishment project.

The concept here was to provide a high quality tree pit, limited in volume due to the close proximity of buildings, but capable of taking vehicular traffic as the tree pit is adjacent to one of the busiest routes into the city. It was imperative therefore that the soil cell system could cope with the potential of regular delivery vehicle and emergency services overrun.

Having been directed downwards to an appropriate level below the paved surface, the roots grow into the load bearing soil cells. The cells are intended in this instance to provide for the complete life of the tree, principally to limit the trees ultimate size, because of space constraints, and also to protect utilities and services surrounding the tree pit area.

This type of tree pit is designed for constricted space applications where it is not possible to provide large volume ‘complete life’ soil cell structures due to lack of space.

Tree Pit Details & Installation Methodology

The outer perimeter of the tree pit was lined with RootStop high density root barrier. The purpose of this is to contain roots within intended zone, preventing roots from further spreading laterally but allowing them to spread in depth.

ReRoot linear ribbed root barrier lines the root ball to direct roots downward and outward, preventing colonization immediately under pavement
RootStop is used around the perimeter of the pit to prevent lateral root spread outside of the designated zone

Within that perimeter, the load bearing soil cells are placed on two sides of the tree and filled with a sandy loam topsoil mix.

ReRoot root barrier is then laid around the root ball area (shown above). This high density root diverting product has integral ribs to prevent root spiraling and train roots to greater depths, thus preventing pavement heave from surface roots. The perimeter root barrier, also visible in the above photo, was trimmed down to finish just above the soil level in the cells.

Irrigation and aeration vents are placed above the soil cells to provide ventilation. The inlet pipes can be seen in the above photo awaiting trimming to suit paving levels.

Compaction at the base of the tree pit is critical, as is drainage. In this instance, the tree pit was free drainage so no additional drainage was required apart from a 4″ (100 mm) layer of free drainage aggregate.

Once the soil cells were installed, they were backfilled with topsoil in layers, and a compacting plate was used to consolidate the cell structure, vibrating the soil into the cells to eliminate voids. The soil cells were then overlaid with a geotextile to provide separation from the subsequent layers of road stone required to build up to finished surface paving level.

Inspection Methodology

Tree was assessed visually, new shoot extension, color, girth, measured and height calculated. The team then selected five sample leaves from different aspects of the canopy and these were removed for leaf chlorophyll fluorescence. The five leaves were tested to arrive at a mean average to offer a good overall measure of the tree’s health and vitality.

Barcham Trees, a UK-based tree research company, have been pioneering the use of leaf chlorophyll fluorescence testing in their nursery. Over time they have been building up a large database of results taken from nursery grown samples, with the goal of establishing benchmarks for different species. This means that readings from trees in urban locations can be compared with trees in a nursery situation to offer an insight into the impact of stresses, often unknown, on the trees health and vigor.

2002: Newly planted tree with paving works complete

2005: Tree establishing well, despite weeds indicating a possible lack of appropriate maintenance

2013: Tree has reduced 50% by pruning the previous winter


Initial visual results were apparent and very positive, as substantiated in above photos. The tree appears healthy and vigorous with excellent new shoot extension. In the 2013 review, 12 years after planting, the tree had attained a height of 10.8 meters (even after severe crown reduction for maintenance) and a trunk diameter at 1.5 m above ground of 23 cm DBH (72 cm girth). The leaf chlorophyll fluorescence mean average reading result was 0.823 – indicating that the tree is in good health and not under any significant latent stress.

Close assessment of the surface surrounding the planting location confirmed that there was no pavement deflection evident at the site attributable to surface root heave.


The trees studied in this location appear to be growing very well and by assessment show excellent vigor and health, despite the challenging urban environment. While 14 years in not a large percentage of a trees potential life span, it is encouraging to see that this tree is already bringing significant environmental benefits to the area.

It is also noteworthy that the shallow root deflection technique designed into this tree pit has been effective, as the tree flourished without any damage to the paved surface – which is so commonly seen in many urban tree plantings.