Rooting Volumes
Trees within urban environments are often planted within small restricted spaces, with only a small amount of space available for roots to grow. Smaller available rooting spaces for trees means less access to water and nutrients, which can often be fatal for proposed trees.
The primary direction of tree roots is outwards (not downwards as is often misunderstood), and up to 3 times further than the canopy. The vast majority of the tree’s roots present within the top 600mm of soil.
For trees planted within hard landscape, root cells can be installed beneath adjacent hard surfacing. The root cells support the hard landscape above, minimising soil compaction and resulting in increased available rooting space for trees. This will result in healthier trees that are more likely to survive.
While the cost of implementing root cells can seem an expensive solution in the short term – in the long term it can end up being cheaper than without, as lower mortality rates mean there are less tree replacement costs.
The minimum required rooting volume for trees is calculated by GreenBlue Urban with the following equation: π x r² x 0.6. Or you can use the Soil Volume Calculator on their website:
https://www.greenblue.com/gb/resources/soil-calculator.
Unfortunately due to factors such as restricted budgets, poor planning and/or a general lack of knowledge; countless landscape schemes have proposed trees in tiny volumes which are ultimately doomed to fail. It is important to consider the implications of specifying larger trees early on to ensure their long-term survival.
Protecting Services
One of the biggest concerns from engineers is the potential of trees to damage adoptable services.
See figure 2.3 from the commonly referenced Sewers for Adoption 6th edition, tree planting adjacent to sewers guidance. This states minimum required distances of proposed trees from adoptable services. When trees are within this easement, a commonly proposed solution is to wrap tree pits in a root barrier. This ‘solution’ is often fatal as it drastically reduces the available rooting volume. Root barriers should be strategically placed to allow for the maximum rooting volume while still protecting services.
