“What are the key aspects I should think about when planning trees into my projects?”

tree pit design in city using stratavault for root support

If you’re an Urban Forester, Landscape Architect, Tree Officer or Consulting Arborist, I’m sure you’d be aware of the essential points that should be considering tree pit design or planning green infrastructure into urban areas. Even so, this is a common question and a good question. We cannot expect to drop an urban tree into a 3-by-3ft hole in the pavement and hope it’ll be a 30ft giant in 20 years time. So, we’ll briefly outline these points for those who are not up to speed in this area.

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:

  1. 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.
  2. 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 civil or groundwork stage.
  3. 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.
  4. Irrigation – lack of water and nutrients are the biggest single killers of newly planted urban trees around the world. It is very important to incorporate the means to irrigate efficiently, particularly for the first three years.
  5. 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. See our guide on ‘How to Promote Deeper Root Growth and Drought Tolerance’
  6. 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.
  7. Support – How will you ensure your tree pit design and 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. One thing you have to remember with staking and tying is trees are lazy. Trees will do as little as possible if they can help it. So if you give it outside support from staking and tying it will use those support systems, which could lead to structural problems with the tree later on in its life.
  8. 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.
  9. Maintenance – When a tree is first established in a new environment a care plan needs to be established for the tree. Most people assume that trees don’t need constant watering and upkeep to survive and natural rainfall will be sufficient to ensure the tree grows but this is not the case. A tree needs constant watering during its establishment period to ensure optimal root growth so it can establish itself in the environment.

Having considered and provided for all the above items, we are well on the way to ensuring that our tree pit design planting programme is going to be efficient and successful. The above factors cover well over 90% of the reasons for urban tree failure. Our team at Citygreen can help design urban trees for all environments. Contact us.