With increasing urbanization, and more highly concentrated populations within cities, strengthening the green infrastructure is becoming increasingly important. One of the largest opportunities for impact is maintaining and enhancing the urban canopy. This is addressed most readily by advanced tree pit design, which refers to the subterranean structures put in place during planting.

In Minneapolis, the local government conducted research that revealed well-planted trees provide a strong financial incentive in addition to the ecosystem benefits. The research found a $2 million savings between a storm water conveyance system, or subterranean cell systems.

Peter MacDonagh, a landscape architect, said in an ASLA interview, “larger, older trees are far more valuable than younger ones, so work needs to be done to preserve these and use new techniques to enable younger trees to stay in place longer.”

As trees were planted in the past, the soil they were placed in was compacted, causing a lack of nutrients, storm water management, and root establishment. As a result, the trees struggle to thrive and provide their benefits to the local environment and infrastructure. Often, these struggling trees will either die, stop growing, or begin to push through and ruin sidewalks and roads.

“The Center for Urban Forest Research calculates that large-canopy trees …outperform small trees…and they do not start adding significant environmental performance until they reach 30 feet,” states Matthew Gordy, a landscape and urban design professional.

By utilizing cell systems, the strain put on the trees’ growth is almost completely eliminated, resulting in lower costs, and increased shade, stormwater management, and overall well being of the populaces and local infrastructure.

Get more information on advanced soil cell systems here.

Supporting Urban Trees:

It is common knowledge that in order for a tree to thrive, the root network must be able to access enough water. If there is insufficient water then the tree will be unable to absorb nutrients from the soil and will deteriorate as a result of the water loss that occurs during transpiration.

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What You Need For a Successful Urban Treepit Design:

The benefits that healthy urban trees provide to the environment are significant, but the environmental conditions in urban spaces can often be challenging for their establishment and long-term survival.

In order to provide urban trees with the best chance of survival, it is important to consider the conditions of soil, climate, and water availability that trees face when they are planted in towns and cities. The correct design and installation of tree pits will mitigate the negative effects of the urban environment.

There are several key factors to consider, including root volume availability. The following soil volumes at a minimum are recommended for healthy tree growth:

Small tree 5-15 m³

Medium tree 20-40 m³

Large tree 50+ m³

If in case this amount of space is not available, tree establishment can still happen provided that great care is taken with regard to species selection and root management. In all cases, the deployment of structural root cells can help to prevent soil compaction and ensure that the available rooting volume is fully utilized.

This is very important if the tree is to be located next to a road or another engineered structure, as the soil structure requirements for hard surfaces capable of sustaining large weights are diametrically opposed to those of a healthy tree root network.

Aside from providing sufficient volume for growth for tree roots, it needs to be appropriately directed to ensure that they do not damage surrounding surfaces or underground structures. Paved surfaces and utilities are particularly vulnerable to tree root damage and various types of root management products can be specified depending upon the item that requires protection.

For example, if a tree pit is to be located amidst a continually paved surface then the tree roots will need to be managed downwards by at least 300mm or the depth of the pavement structure to remove the possibility of paving heave.

Next, we’ll discuss more about proper irrigation, drainage and aeration to raise a strong tree network.

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

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.