The Stockholm Tree Pit Method is a pioneering urban tree planting technique designed to foster healthy root development while preserving the integrity of surrounding infrastructure in urban areas. By using structural soils, this method addresses the challenges of growing trees in confined spaces, such as beneath pavements and roads, without compromising the stability of hard surfaces. Developed in Stockholm, Sweden, the method has become a standard in urban forestry for ensuring both tree health and sustainable urban development.
A key contributor to the early trials of this method was Carbon Gold partnered with Björn Embren, a Swedish arboricultural expert, who integrated biochar into the tree pits during the method’s development.
History and Initial Trials for Stockholm Tree Pits
The Stockholm Tree Pit Method was developed through trials conducted by Björn Embren in Stockholm, Sweden. These trials aimed to address the challenges faced by trees in urban settings, such as compacted soil and limited root space. The trials successfully demonstrated that using a mix of angular stone, enriched biochar, and compost created optimal conditions for tree survival and growth. Carbon Gold partnered with Embren, supplying biochar-enhanced products that helped improve soil structure, water retention, and nutrient availability. The early trials showed that trees planted with biochar-enhanced structural soil had significantly better establishment rates and long-term growth, leading to the method’s widespread adoption.
Suitable Applications for Structural Soils
Urban Tree Planting: The method is ideal for planting trees in high-traffic urban environments, such as roadsides, pavements, and Plazas. Municipal and Infrastructure Projects: Suitable for urban renewal projects where trees are integrated into public spaces. Hardscape Integration: Perfect for areas where trees are planted alongside or beneath hard surfaces, preventing damage to roads and pavements.
Stockholm tree pits should be specified properly under NBS Q31. For the clause wording, dose rates and substitution traps to avoid, see our guide on how to specify biochar under NBS Q31
Performance and Benefits for Structural Tree Pits
Improved Root Growth: Supports deep root expansion beneath hard surfaces, helping trees thrive in urban environments while preventing surface damage. Reduced Infrastructure Maintenance: Encouraging roots to grow deeper reduces the risk of root-related pavement damage, thereby minimising maintenance costs. Sustainable Drainage Systems (SuDS): The method promotes effective water infiltration, helping manage urban stormwater and reducing flood risks.
Key Components of the Stockholm Tree Pit Method
- Clean Angular Stone
Structural soil is a blend of clean angular stone and a specially designed growing medium, creating a stable base for hard surfaces while supporting tree root growth. Composition: Crushed angular stone (granite, limestone, basalt, or recycled concrete) combined with Carbon Gold Biochar Tree Soil Improver and PAS100 compost. Function: Supports deep root expansion beneath hard surfaces, reducing the risk of surface upheaval caused by roots. Promotes Sustainable Drainage Systems (SuDS), allowing for effective water and air movement.
- Carbon Gold Biochar Tree Soil Improver
Carbon Gold’s biochar mix is a crucial component, improving the structure of the soil, enhancing water retention, and promoting nutrient uptake. During the original trials, enriched biochar played a significant role in increasing tree establishment and long-term growth. What are the Benefits?: Increases soil porosity and supports microbial activity. Enhances water retention and nutrient availability. Plus improves soil structure and water retention.
- PAS100 Compost
The addition of certified PAS100 compost provides essential organic matter, boosting nutrient levels and supporting tree health in high-stress urban environments. Specification for structural soil – Stone Size: 32-63mm clean angular stone. Growing Medium Particle Size: 1-20mm, pH Range: 7.5-8.5 (Note: Crushed limestone will impact soil pH, making it suitable for alkaline-tolerant species only).
Mixing Instructions
- Preparation: Calculate the required soil volume based on the mature size of the tree. Ensure the tree pit is excavated according to local guidelines.
- On-site Mixing:
- Blend clean angular stone with Carbon Gold Biochar Tree Soil Improver and PAS100 compost using an excavator.
- Add water as necessary to ensure the biochar and compost properly coat the stones.
- Installation: Fill the tree pit with the prepared structural soil mix, compacting it in layers to ensure stability.
- Post-Installation: Water the tree pit thoroughly to settle the mix and support initial root establishment.
Pre-mixed structural soils, are available
Including Carbon Gold enriched biochar in a Stockholm tree pit or structural soil specification? See our Specification Defence Brief for what to say when the line is questioned at cost review.
