7.6 Tree pits and root volumes

Tree roots need adequate volumes of soil in which to grow, along with access to nutrients, water and oxygen. Tree roots can also impact adjacent utilities and foundations of buildings. It is important to get the below ground design and specification right alongside choosing the right tree in the right place.

Tree pits in hard surfaces should have a surface opening that is as large as the space allows but must be a minimum of 1m square. The pit should be left open and incorporate layers of ground cover planting. It should not be surfaced with paving materials. Initially the tree pit should be filled with organic mulch to a depth of 50mm to 75mm, which will help the tree establish itself by regulating temperature, moisture and providing nutrients.

Permanent ground cover planting should be added to the pit once the tree is established, species dependent. Dense, native planting, including shrubs, should be used on busy streets that will prevent litter accumulating and deter parking on the verge. Highly maintained, formal planting should be avoided in most situations to reduce the maintenance burden. Local residents could be encouraged to take an active role in aftercare and participate in SCC organised community planting events.

Tree grilles should be avoided as these can accumulate rubbish. Where hard surfacing is required, permeable resin bound gravel, self-binding gravel or rubber crumb should be used.

Below ground, adequate space should be given for root growth. The volume of soil required depends on the tree size and species, so the available space below ground may dictate the tree selection. For example, a medium sized tree will typically require 12m³ of soil to establish fully, which may be difficult to achieve in many locations. In paved areas, it may be necessary to create more room for tree roots using structural soils and other load bearing systems. While these will help create excellent growing conditions, they are not always required so independent advice should be sought on the appropriate solution. The cost of these systems must not become a barrier to planting street trees. Such systems include:

• Structural soil systems. Structural growing mediums such as Amsterdam tree sand;
• Stockholm tree system. Structural soil using large stone aggregate;
• Crate and root cell systems. Proprietary systems, either plastic or concrete, that provide a structure to support pavements above; and
• Raft systems. Proprietary systems that provide a structural base that floats above the tree roots.

Further guidance on these systems can be found in the publication Trees in Hard Landscapes [Reference 20]. Reference should also be made to the Surrey County Council tree strategy and SuDS guidance.

Trees, pits and verges could be linked together in a tree trench to maximise the available root volume. If combined with a structural solution, these trenches could be extended under paved areas, for example linking tree pits between parking spaces.

Tree roots also need to be contained and deflected away from buildings and utilities through the use of suitable root barriers. Root deflectors should be used to guide roots down a minimum of 300mm to avoid damage to paving layers.

References

• Reference 20: TDAG (2014) Trees in Hard Landscapes (Return to content for reference 20).