From the demise of the old glasshouse comes the rise of new garden developments. One of these is a new path across the Trials Field linking Battleston Hill to the Arboretum, where the hardcore needed to provide the base is coming from the old glasshouse site.
Where it crosses from the Trials Field into the Arboretum it passes a line of magnificent Sequoiadendron giganteum AGM (wellingtonia) trees, and for this section a different approach is needed in the construction of the path. We need a 'no-dig' solution, and this is why.
A tree stays upright in the same way as a wine glass, with the roots forming a plate that is similar in width to the crown. Most tree roots are within the top 600mm of the soil depth and within 60-70% of the tree's height. Our new path goes close to these conifers so they need protection from heavy traffic as mature trees are particularly susceptible to root damage.
Compaction around the roots can directly crush them, impede the flow of water and nutrients, and reduce the air spaces which allow the tree to 'breathe'. So this week we have been installing root plate protection.
Our solution has been to install a permeable structural sub-base directly onto the existing ground level. This is a cellular panel material 8000mm x 2500mm x 150mm called Geoweb, laid upon a fleece membrane and edged with 150mm x 25mm wooden boards. The panels are stapled to the edging boards and also reinforced with 1000mm x 12mm steel rods. Finally the cellular panels are filled with 20mm shingle. The cellular structure dissipates downward loads by a horizontal transfer, and imparts structural integrity to free draining aggregates. Water and air can freely permeate down to the roots, and the health of the tree is maintained.