​Successful Tree Pit Design | GreenBlue
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The Process of Successful Tree Pit Design

Urban tree populations in cities around the world are on the decline. For over 23 years, GreenBlue has performed research into why urban trees fail and what can be implemented to change this degeneration. Arming urban landscape professionals of all trades with proven urban tree planting methods and best practice techniques is the best way to ensure the successful establishment of our urban tree canopy cover – and this is exactly what GreenBlue has put together here.

Having already made a decision on planting location and tree species, landscape architects and specifiers should consider the following process when designing the exact profile of the tree pit and ancillaries. Having made a decision on species and location, the following process should be observed when designing the exact profile of the tree pit and ancillaries:

Available root space – Soil volume requirements for trees can be estimated using several methods. As stated earlier, in a natural environment a root system can extend two to three times the radius of the tree canopy. Probably the simplest way of calculating a minimum required soil volume is to take the projected canopy area of the mature tree, multiplied by a depth of 0.6m. The shape of this area can be configured to suit the particular site. Other methods are based on mature trunk girth and are possibly more accurate as they provide for different foliage shapes. The old method of providing an area the size of the pavement opening is clearly insufficient, and commits the tree to an untimely death, or a lifetime of costly repairs.

Engineering requirements – With many trees being planted immediately adjacent to highways and engineered structures, it is vital that root volume beneath or around such is considered. Engineering requirements for hard surfaces are directly opposed to horticultural requirements. Structural soil modules or similar must be considered early enough in a project to be incorporated during the civils or groundwork stage.

  • Root management – paved surrounds or utilities nearby? If so, root management should be specified depending on what needs protecting and where it is in relation to the tree. For continuous paved surround for example, roots will need managing downwards by at least 300mm to design out paving heave. See our section on root management for further details.
  • Irrigation – lack of water and nutrients are the biggest single killers of newly planted trees in the UK. It is very important to incorporate the means to irrigate efficiently, particularly for the first three years.
  • Drainage – water logged tree pits can become anaerobic and this will kill the tree – please ensure that potential drainage issues have been addressed early on in your scheme.
  • Aeration – less widely known but none the less important, soils and roots need air to live. If the root plate of the tree is covered with impervious paving, vital gaseous exchange in the root zone cannot take place. Appropriate tree pit design should include a means of facilitating air supply below ground.
  • Support – how will you ensure the tree is securely located? Underground guying is widely favoured for urban areas as it is unobtrusive. Staking and tying is an alternative but this will require maintenance.
  • Above ground – What sort of environment will you be planting in – in some locations above ground protection from carelessness and/or gratuitous vandalism becomes critical to tree survival. A decision will need to be made on whether there is a need for tree grilles, vertical guards and other protective measures.  Having considered and provided for all the above items, we are well on the way to ensuring that our tree planting programme is going to be efficient and successful. The above factors cover well over 90% of the reasons for urban tree failure. We now need to look at the specific products required to help us design these features into our schemes.