The Process of Successful Tree Pit Design

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.

 

Available Rooting Area

Soil volume requirements for trees in urban areas can be estimated using several methods. The occasionally practiced 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. We suggest the simplest way of estimating a minimum required soil volume is taking the projected canopy area of the mature tree and multiply it by a depth of 2ft (0.6m). The shape of this area can be laterally configured to suit the particular planting site.

Engineered Requirements

With many urban tree plantings being positioned immediately adjacent to highways and other engineered structures, it is vital that root volume underneath and around such infrastructure is considered. Engineering requirements for paved surfaces are directly opposed to the horticultural needs of trees. Structural soil cells or similar support modules must be considered early enough in the project process to be incorporated during the engineering specification or groundwork stage.

Root Management

Any tree planting near utilities or paved areas should have appropriate root management specified. Depending on what needs to be protected and where it is in relation to the tree, different root management solutions are available. Continuous paved surfaces for example, require roots to be managed downwards at least 12″ (300mm) to reduce the risk of pavement heave and other surface root damage. See specifications on root management for more details.

Irrigation

Lack of water and nutrients are the single largest cause of death in newly planted trees. It is essential to incorporate the means to efficiently irrigate a tree pit – especially in the critical first three years.

Drainage

Water logged tree pits can become anaerobic, which will often kill trees. It is essential to ensure that potential drainage issues are addressed early on in your planting scheme.

Aeration

Less commonly recognized than irrigation, but equally as important, is aeration. Soil and roots need oxygen to live. If the root plate of the tree is covered by impervious paving, like most street trees are, vital gaseous exchange in the root zone cannot take place. Appropriate tree pit design should include a means of facilitating air supply below the surface.

Root Ball Support

How will you ensure that the tree is securely located and supported? Underground guying is widely favored for urban environments because it is unobtrusive. Staking and tying are alternatives, but these require ongoing maintenance and when not properly cared for becomes a detriment to the tree.

Above Ground

Depending on the sort of environment the tree will be planted in, above ground protection from carelessness and/or gratuitous vandalism can become critical to tree survival. A decision will need to be made whether there is a need for tree grates, vertical guards, and other protective measures.

The above factors account for over 90% of the reasons that urban trees fail. Considering and providing for the above measures ensures that tree planting programs are well on their way to successful and efficient establishment.

Download our free Urban Tree Design Guidelines to learn more!