Citygreen® tree pit systems play essential role in Bankstown council streetscape upgrade

Citygreen treepit systems’ rootcells played an essential role in a Bankstown City Council streetscape and transport corridor upgrade project that won first prize in the local government/public works category of the 2009 Engineering Excellence Awards of the Institute of Public Works Engineering Australia NSW (IPWEA), and also first prize in the Cement and Concrete Association of Australia (CCAA) 2009 Public Domain for Streetscape,” said Emily Forrest, Sustainable Development Unit, Bankstown City Council.

“We had been planting our new advanced trees in structural soil for two years prior to that, so we took a leap of faith. It’s definitely worked out for us. The trees that used the new system are all looking fantastic,” she said.

The 40 trees planted using the Citygreen root protection infrastructure have healthy and vibrant canopies.

The award-winning $10 million bus corridor and interchange was constructed over three years and opened in December 2008, providing an improved transport system for both local and regional bus routes. It also provided an opportunity for further improvements to the Bankstown City Plaza streetscape and other council infrastructure in the area.

Citygreen treepit system – how it works

“Soil compaction, insufficient drainage, lack of aeration and restrictive pavement openings are the cause of consistent tree planting failure in urban landscapes,” said a spokesperson from Arborgreen, the Australian distributor of Citygreen’s urban tree product range.

Rootcells provide the soil space that is essential for healthy tree roots to grow into and the strength to prevent them being compressed by traffic above or adjacent to where they are planted. This ensures that the roots receive vital life-giving nutrients, including water and oxygen, so that they can flourish in the challenging city environment.

A typical tree planting pit in the street upgrade was 1200 mm by 1200 mm by 800 mm deep, with structural rootcells buried around the root ball and a tree grate surrounding the tree base for protection.

“We placed rootcells beside the root balls of the trees, giving them a substantial volume of uncompressed soil to grow into,” Ms Forrest said.

“Where there was a row of trees, we also formed a linking trench backfilled with structural soil, with non-permeable pavers over the top. The trench is about a metre wide and typically sits behind and parallel to the kerb. It helps the movement of both water and oxygen between the trees.”

RootDirectors buried around the perimeter of the pit direct the growing roots into the rootcells and prevent them disrupting the pavement. A RootRain irrigation inlet is connected to buried perforated pipes that loop around the tree above the rootcells.

Tree guards and grates

Vertical tree guards from Citygreen shield the growing trees from damage until they are large enough to stand on their own without protection from vandalism or accidents.

“The tree guards were purpose-designed with Citygreen to a distinctive cone-shape to allow branching of the tree species selected. They’re finished in a Bronze Dulux powder-coat colour that is not frequently used,” Ms Forrest said.

“The guards have a visual effect on the streetscape and are a point-of-difference of the upgrading. They’re part of the unique and restrained palette of material used for the plaza.”

Tree grates at pavement level were purpose-designed with small apertures that allow water penetration but are too small for debris such as cigarette butts that might otherwise collect in the tree hole.

Species selected

Trees planted in the Bankstown streetscape include Pyrus calleryana ‘Bradford’ (Ornamental Pear), which was chosen for its broad canopy and tree shape, single trunk form, hardiness, proven performance and reddish autumn leaf colour.

It grows tightly upward when young, with the canopy spreading as the tree matures. A 200 litre rootball was the size nominated at planting of these trees.

“The Pears are showing a very vigorous growth result. The trunks have increased their diameter about four-fold in two years,” Ms Forrest said.

Also used in the streetscape was Fraxinus ‘Urbanite’ (Ash), selected for its tree form and size, multi-fingered leaf shape and effect. These trees were planted in 200 litre size pots, with structural soil in between the trees. “They’ve also established very well,” she said.

Elsewhere in the Bankstown city plaza, Ash trees that were planted directly into the ground about 13 years ago without the benefit of structural soils or

rootcells and associated protective equipment have not thrived and are stunted for their age.

These will be replaced soon with new Sapium sebiferum (Chinese Tallowood) planted with the Citygreen root protection infrastructure. Sapium were selected for their small tree shape and form, autumn leaf colour and leaf shape.

“This species is an appropriate choice in a business precinct dominated by Vietnamese and other Asian businesses. The scale of the tree at full height is also in keeping with the predominantly two-storey character of the nearby buildings,” Ms Forrest said.

A couple of Ulmus ‘Lutescens’ (Golden Elm) were chosen as accent trees in the latest streetscape upgrade because of the bright green foliage that contrasts pleasingly with the dark grey paving.

Lagerstroemia indica ‘Tuscarora’ (Crepe Myrtle) was selected for flower colour, mottled foliage and bark patterns. Andreasens Green Nursery had some magnificent specimens available in 1000 litre root size perfectly suitable for this purpose.

Corymbia maculata was chosen to provide a majestic and fast-growing native screen to the railway.

Why selected?

“The chosen trees maximise public amenity and aesthetics, creating an individual ‘sense of place’,” she said.

While many plane trees were planted by the Bankstown council in the 1980s, these were no longer the tree of choice due to concerns about asthma, the effects of the tree’s size on engineering infrastructure, and the low-rise scale of the Bankstown CBD and Plaza buildings.

“The council now chooses deciduous trees for the commercial streetscape to create a point of difference to urban street plantings, which are predominantly native species. Deciduous trees contribute to the removal of air pollutants, which are captured in the leaves and disposed of when leaves are removed from the site.

“The trees also reduce pollution in urban runoff by filtering waterborne particles and nutrients, while entrapping dust and other air-borne particles.

“Programming for leaf litter removal can be more easily planned with deciduous trees than with natives that lose their leaves year-round. Deciduous trees that are bare in winter also let through more sunlight when it’s needed,” Ms Forrest said.

Increasing attention

Rootcell tree root protection products are attracting increasing attention in all states of Australia and throughout New Zealand, according to supplier Arborgreen. Many arborists and landscape architects are now recognising that the modular, structural soil cells represent current best practice for trees in cities.

The new, patented Stratacell™ Generation V rootcell is the strongest module of its kind available globally.

“This unique module incorporates all of the benefits of the previous RC120 rootcells utilized in this project, with the added advantages of larger spaces for root growth, faster installation times, and reduced shipping costs.”

Contact your Citygreen distributor for more information about this product.