Healthier trees,
healthier cities

It’s common to see urban trees in poor health, and trees planted in hard landscaping often die young. There may be a variety of reasons for this but in most cases it’s because they don’t have access to enough rootable soil or they don’t receive enough water. If urban trees fail to reach maturity they only provide a small fraction of the benefits that they could potentially be bringing to towns and cities.

The Stockholm system offers a practical and effective solution for planting trees in hard landscaped areas. We supply the equipment and know-how to landscape architects, engineers, and construction companies throughout the UK.

How it works

A structural soil is a stone-based growing medium that can support pedestrian and vehicular traffic, and so their use allows tree pits to be extended beneath hard surfacing. This enables designers to provide enough soil for a tree to be healthy and reach maturity. When designing tree pits with structural soils rainwater that has been collected from roofs, roads and paved surfaces is diverted into the tree pit. This is good for the trees and tree pits constructed using this approach also contribute to sustainable drainage systems (SuDS).

The tree planting programme carried out by the City of Stockholm has demonstrated that exceptionally healthy trees can be cultivated when using structural soils. This website aims to explain how tree pits can be successfully designed with structural soils.

Standard machinery found on most building sites can be used to install structural soils which means that they are easy to use. Also, the simple design allows structural soils to be dug up after the pavement has been laid if any future ground works are required.

Download the guide that explains the principles of designing of tree pits using structural soils.

Click on the numbers to find out more about each design feature.

Interactive diagram showing a cross-section of a Stockholm Tree Pit
1 2 3 4 5 6 7 8 9
Interactive diagram showing a cross-section of a Stockholm Tree Pit within the context of a street
1 2 3 4 5 6 7 8 9 10 11

Standard paved surface with base course

Impermeable tarmac or any sort of paving can be laid on top of tree pits constructed using the Stockholm system. Once the aeration layer has been installed a geotextile is laid over the entire tree pit followed by a base material for the wearing course and the surface layer of the road, pavement, or cycle path that is desired.

Channel to divert rainwater into tree pit

In most built-up areas rainwater is diverted away from the trees into stormwater attenuation systems but this is a waste of an important resource because the local street trees would benefit from receiving that water. A key feature of the Stockholm system is that it collects rainwater from roofs, roads and paved surfaces and diverts it into the tree pit.

In the modern age stormwater management and urban tree establishment should be part of one integrated design.

Inlet for water ingress and gaseous exchange

The aeration wells are installed within the structural soil mix, they receive rainwater from the hard surfacing above. The aeration wells are key to the success of structural soils because they not only a conduit for surface water to enter the tree pit but they also provide a pathway for air diffusion. The aeration wells are designed to trap silt and debris that is washed down with the water.

Prefabricated steel aeration vents can be purchased from Stockholm Tree Pits.

Silt trap at base of inlet

The aeration wells are designed to trap silt and debris that is washed down with the water. These need to be emptied one a year, which can be done using a standard gully sucker.

Concrete frame

A pre-cast concrete planting frame is used for the planting hole to house the rootball. This also functions as a root deflector by ensuring that roots can only grow deep beneath the surface. The concrete planting frame has rigidity which allows stone to be introduced and compacted around the planting hole during the construction of the tree pit.

Concrete planting frames can be purchased from Stockholm Tree Pits.

The concrete frame is filled with topsoil

Structural soils are highly permeable and so they drain quickly and a have a relatively low water holding capacity. The topsoil in the planting hole can provide an important water storage function in times of drought.

Separation geotextile

The separation geotextile prevents the surface’s base material from migrating down into the aeration layer and it also prevents roots from growing up into the base material. In this way the aeration layer ensures that root growth is well below the final surface and so problems caused by roots lifting pavements are avoided.

The aeration layer

A 200mm layer of clean stone (20‒40mm) needs to be installed above the structural soil and beneath the surface courses, this layer provides pathways for air and water to reach the whole of the tree pit via the aeration wells and kerbside inlets.

The aeration layer is a vital part of the design and so it is essential that this feature is included in the specification of any tree pit that uses structural soils beneath hard surfacing.

Structural soil

A structural soil is a stone-based growing medium that can meet engineering standards for supporting pedestrian and vehicular traffic, this allows tree pits to be extended beneath hard surfacing and so enable designers to provide enough soil for a tree to be healthy and reach maturity.

For most installations 32-63mm crushed rock is used for the structural soil, and because it has to support heavy loads it needs to be a hard rock such as crushed granite, basalt, limestone, or recycled concrete. The similar-sized stones are installed in layers and once the material has been laid it meets the performance criteria of a macadam sub-base. Currently the best outcomes are being achieved with crushed rock combined with 1:1 mix of enriched biochar¹ and compost (15% volume).

Download a guide to designing tree pits with structural soils

¹There is a growing body of research to support the use of biochar to increase soil quality and plant growth. Biochar is produced through pyrolysis (burning at 350 to 800 under partial exclusion of oxygen). The resulting material is durable and its large surface area has a high affinity for nutrients. Adding biochar to soils directly sequesters carbon and produces soil that have a high carbon content. There are a variety of ways that biochar improves soils including reducing the bulk density, enhancing soil aggregation, improving the water retention capacity of the soil and reducing the leaching of trace metals from the upper horizons of the soil profile. It also has excellent filtration properties which remove waterborne contaminates. Another benefit of biochar’s structure is that it provides micro-habitats for soil organisms. For this application it is important that enriched biochar is used.

Standard paved surface with base course

Impermeable tarmac or any sort of paving can be laid on top of tree pits constructed using the Stockholm system. Once the aeration layer has been installed a geotextile is laid over the entire tree pit followed by a base material for the wearing course and the surface layer of the road, pavement, or cycle path that is desired.

Channel to divert rainwater into tree pit

In most built-up areas rainwater is diverted away from the trees into stormwater attenuation systems but this is a waste of an important resource because the local street trees would benefit from receiving that water. A key feature of the Stockholm system is that it collects rainwater from roofs, roads and paved surfaces and diverts it into the tree pit.

In the modern age stormwater management and urban tree establishment should be part of one integrated design.

Inlet for water ingress and gaseous exchange

The aeration wells are installed within the structural soil mix, they receive rainwater from the hard surfacing above. The aeration wells are key to the success of structural soils because they not only a conduit for surface water to enter the tree pit but they also provide a pathway for air diffusion. The aeration wells are designed to trap silt and debris that is washed down with the water.

Prefabricated steel aeration vents can be purchased from us.

Tree guard

In an urban setting trees are vulnerable to vehicle impacts and vandalism. Planting trees with tree guards protects them in the early years of establishment until they develop a thick layer of bark around their trunk.

The planting hole

The tree planting opening in the centre of the tree pit designed to accommodate the rootball of the tree is called the planting hole. The tree pit is the larger volume around the planting hole that is designed to extend the volume of rootable soil available to the tree. When structural soils are used, they are installed around the planting hole beneath the adjacent surfacing.

Kerb inlet

This is a kerbside gully grate that receives road runoff and diverts it into the tree pit. To an urban tree road runoff can be an important source of water. Also passing this water through a tree pit is a pre-treatment step that removes potential pollutants from the water before it reaches downstream watercourses or aquifers. The kerb inlet also allows air to reach the underground tree pit.

Maintenance access for kerb inlet

When there is a kerbside inlet the water is channelled to a more central part of the tree pit. In order to prevent silt and debris from clogging the system there is a need to have a silt trap included in the design. This is typically a 450mm diameter inspection chamber riser topped with a manhole cover. Any silt or debris that is washed down with the water collects in the inspection chamber. These need to be emptied one a year, which can be done using a standard gully sucker.

Download the guide to kerbside inlet silt traps

Separation geotextile

The separation geotextile prevents the surface’s base material from migrating down into the aeration layer and it also prevents roots from growing up into the base material. In this way the aeration layer ensures that root growth is well below the final surface and so problems caused by roots lifting pavements are avoided.

The aeration layer

A 200mm layer of clean stone (20‒40mm) needs to be installed above the structural soil and beneath the surface courses, this layer provides pathways for air and water to reach the whole of the tree pit via the aeration wells and kerbside inlets.

The aeration layer is a vital part of the design and so it is essential that this feature is included in the specification of any tree pit that uses structural soils beneath hard surfacing.

Structural soil

A structural soil is a stone-based growing medium that can meet engineering standards for supporting pedestrian and vehicular traffic, this allows tree pits to be extended beneath hard surfacing and so enable designers to provide enough soil for a tree to be healthy and reach maturity.

For most installations 32-63mm crushed rock is used for the structural soil, and because it has to support heavy loads it needs to be a hard rock such as crushed granite, basalt, limestone, or recycled concrete. The similar-sized stones are installed in layers and once the material has been laid it meets the performance criteria of a macadam sub-base. Currently the best outcomes are being achieved with crushed rock combined with 1:1 mix of enriched biochar¹ and compost (15% volume).

Download a guide to designing tree pits with structural soils

¹There is a growing body of research to support the use of biochar to increase soil quality and plant growth. Biochar is produced through pyrolysis (burning at 350 to 800 under partial exclusion of oxygen). The resulting material is durable and its large surface area has a high affinity for nutrients. Adding biochar to soils directly sequesters carbon and produces soil that have a high carbon content. There are a variety of ways that biochar improves soils including reducing the bulk density, enhancing soil aggregation, improving the water retention capacity of the soil and reducing the leaching of trace metals from the upper horizons of the soil profile. It also has excellent filtration properties which remove waterborne contaminates. Another benefit of biochar’s structure is that it provides micro-habitats for soil organisms. For this application it is important that enriched biochar is used.

Underground services

Ideally the underground services should be routed around the tree pit. In this example the underground services are shown between the tree pit and the buildings.

Fundamentals of tree pit design

To get the best long-term benefits from urban trees we need to be designing tree pits that allow them to reach the mature phase of their life cycle where they will survive for many years with little requirement for maintenance.

Trees can be an important part of a sustainable drainage system. They draw large amounts of water up through the soil, and their leaves catch and slow down water before it reaches the ground. Tree roots can also absorb pollutants from soil and soil-water. Therefore, it’s important that tree pits are integrated with the overall site drainage design.

There are three key aspects of a successful tree pit design:

Soil volume

Designers need to specify enough soil to support the tree at its mature size, download our detailed guidance on soil volumes for different sized species.

Water

Trees need a continual supply of water if they are to thrive. In the urban environment a tree’s water supply can be naturally sustained if rainwater is collected and diverted into the tree pit.

Air

Roots and soil organisms respire in order to generate energy which is a process that requires oxygen. Therefore, pathways need to be provided for gaseous exchange between the soil and the above ground atmosphere.

Modular plastic structures (crate systems) or structural soils can be used to support surfacing above tree pits. Both systems can support healthy urban trees, but tree pits made using crate systems are much more expensive than tree pits constructed with structural soils.

Recommended volumes for tree pits

People have been building tree pits for over a century and deciding how big to build them has always been a trade-off between how much space the tree needs and how much can be afforded in the design. The most limiting factor in the growth of urban trees is the lack of usable soil for root growth, and inadequate underground rooting space is one of the main contributors to the premature mortality of urban trees.

There is a direct relationship between the volume of below ground growing space and how a tree is likely to develop, the greater the soil volume:

  • the faster the tree will grow
  • the bigger it will become
  • the healthier it will be
  • the better it will look
  • the longer it can be expected to live

Therefore, when designing urban spaces, we need to make sure that the species that we wish to plant is provided with enough soil to be healthy and reach a degree of maturity that will deliver benefits to the local community. The minimum volume of soil recommended for trees of different sizes is provided in Table 1.

Table 1: Minimum requirements for tree pit specifications.

Mature size of tree*†
Very small
(<5m)		 Small
(5 - 10m)		 Medium
(10 - 15m)		 Large
(15 - 25m)		 Massive
(>25m)
Recommended minimum volume of uncompacted loam soil 6m³
(5m³ if shared)		 12m³
(9.5m³ if shared)		 20m³
(16m³ if shared)		 28m³
(24m³ if shared)		 36m³
(30m³ if shared)
Recommended minimum volume of stone-based structural soil 8m³
(6m³ if shared)		 15m³
(12m³ if shared)		 26m³
(20m³ if shared)		 36m³
(28m³ if shared)		 45m³
(35m³ if shared)
Recommended number of air/water inlets‡ 1
(0.5 if shared)		 1
(0.5 if shared)		 1 2
(1.5 if shared)		 2

*Mature tree sizes are listed in Tree Species Selection for Green Infrastructure - a guide for specifiers.

†Fastigiate trees will require less rooting space than trees with wide canopy shapes. As a rule of thumb, one should assume that a tree with a narrow and columnar crown form would require half as much soil volume as a tree of the same height that has a wide crown.

‡Ideally the surface of the tree pit should be open, rough in texture, and protected from compaction. If there is hard surfacing above the tree pit designers must provide pathways for water ingress and gaseous exchange. This could be provided by a permeable surface over the whole of the tree pit or by using a non-permeable surface with specially designed inlets. Suitable inlets would be substantially larger than an irrigation tube and service the whole of the tree pit.

Download our guide to soil volumes for tree pits designed with structural soils.

Products

Stockholm Tree Pits supplies four products:

STP01: Concrete planting frame (1.2m x 1.2m)

Concrete planting frame
(1.2m x 1.2m)

STP01

STP02: Concrete planting frame (1m x 1m)

Concrete planting frame
(1m x 1m)

STP02

STP03: Steel aeration well

Steel aeration well

STP03

STP04: Modal filter planter

Modal filter planter

STP04 + Factsheet

Stockholm Tree Pits can now supply Carbon Gold Tree Soil Improver biochar in bulk and at a discounted rate. This is a enriched biochar that improves soil structure and is proven to boost the health and vitality of trees. We are pleased to add this product to our range because it is a key component of structural soils.

Please get in contact if you would like a quote for your project.

Consultancy services

Stockholm Tree Pits specialises in the design of urban tree pits and offer this as a consultancy service. We can assist in all stages of a project, from initial masterplanning through to detailed design and the supervision of tree pit installations.

Get in contact if you have any questions about how to design or construct tree pits using structural soils and we will work with our team to provide you with detailed answers.

Bosky Trees

Stockholm Tree Pits is an offshoot of Bosky Trees arboricultural consultancy. As a professional consultancy business, we regularly collaborate with landscape architects when designing new development schemes. We also use our practical experience to assist contractors when they are working near established trees.

Above all we try to manage trees in the best way possible for the health of people and the natural environment.

Visit boskytrees.co.uk

Arboricultural Association - Registered Consultant Institute of Chartered Foresters - Registered Consultant
Green, healthy tree within square concrete bed

Technical downloads

Diagrams are available to help designers specify structural soils, these can be downloaded in DWG or PDF formats.

Download diagrams individually below, or download all our resources.

  • Tree pits installation guide Illustrated step-by-step guide to tree pit installation
  • Structural soil tree pits section drawing Diagram showing tree pit design using structural soils
  • Kerb inlet section drawing Diagram showing kerb inlet design for a submerged tree pit
  • Concrete planting frame (1.2m x 1.2m) Diagram showing dimensions of planting frame (1.2m x 1.2m)
  • Concrete planting frame (1m x 1m) Diagram showing dimensions of planting frame (1m x 1m)
  • Aeration well specification Diagram showing dimensions of aeration well
  • Materials calculator Tree pit materials calculator
  • Recommended reading List of recommended reading

Subscribe to our newsletter

In the future we'll be producing case studies, expanding our product range, and offering fresh guidance on tree pits and raingardens. Join our mailing list to receive updates on the latest news!

Subscribe to our newsletter

In the future we'll be producing case studies, expanding our product range, and offering fresh guidance on tree pits and raingardens. Join our mailing list to receive updates on the latest news!

Get in touch

We are here to help, please get in contact if you have any questions about tree pit designs or if you would like a quote for any of our products or services.

If you are interested in purchasing our products and would like to speak to someone in our sales team, please call us on 07872 609 633. Alternatively send us an email using the form below.