Urban areas are rapidly evolving to meet climate resilience goals, and trees are at the centre of many green infrastructure strategies. Yet the success of urban tree planting doesn’t begin above ground, it starts beneath our feet, with the soil.
The Underground Challenge
Trees in natural environments thrive in loose, well aerated soil full of organic matter and microbial life. In contrast, urban trees are often planted in compacted subsoils, surrounded by impermeable surfaces, and subjected to limited rooting volume, air flow, and water availability. These conditions not only stunt tree growth but can lead to premature tree failure.
Engineered soils are the solution, specially developed to meet dual demands:
- Structural stability to support roads and paths
- Biological viability to sustain healthy root growth and long-term tree performance
At Enrich Environmental Ltd, we specialise in engineered tree soils that support the establishment, growth, and long-term vitality of urban trees, particularly through our Type 11 Structured Tree Soil and Type 13 Urban Tree Sand Mix.
Enrich Type 11: Structured Tree Soil
Our Type 11 Structured Tree Soil is designed to function as a load bearing medium while still maintaining porosity and water permeability. This soil type blends large 75-150mm stone with compost enriched topsoil to form a stable matrix that allows tree roots to extend without compacting the surrounding soil. It is ideal for tree pit designs where soil must support vehicular or pedestrian loads without compromising on root health.
Enrich Type 13: Urban Tree Sand Mix
Our Type 13 Urban Tree Sand Mix is engineered with high drainage capacity in mind. Used primarily in tree pits where rapid infiltration and stormwater movement are critical, this mix includes a precisely graded sand combined with compost and topsoil. It provides an optimal rooting zone that also functions as a stormwater bioretention medium, playing a dual role in urban water management.
What is Structural Soil?
A well-established concept in green infrastructure, structural soil is a specially designed substrate that allows simultaneous root development and load-bearing capacity, making it ideal for trees planted beneath roads, footpaths, or plazas. Structural soil combines angular stone, to preserve pore spaces for root growth while remaining structurally sound under compaction.
This concept is the foundation of Enrich’s Type 11 Structured Tree Soil, developed to meet Irish urban planning needs. Its composition allows roots to penetrate freely through a load-bearing matrix, enabling trees to establish in areas that would otherwise be too compact or dry. As urban spaces expand, engineered solutions like structural soil are essential for planting resilient urban trees that thrive, not just survive.
Stormwater Management and Green Infrastructure
Urban trees are increasingly recognised as essential tools in stormwater management. Tree pits designed with engineered soils act as natural sponges, slowing down and filtering rainwater before it reaches drainage systems. According to the Greater Dublin Drainage Scheme and CIRIA SuDS Manual, integrating vegetation with permeable subsoils is the recommended method for managing urban runoff and reducing flood risks.
By using engineered soils like Enrich Type 11 and Type 13, landscape architects can transform tree pits into multifunctional bioretention zones, meeting both horticultural and hydrological needs.
Landscape Architecture and Best Practice in Ireland
Urban tree pit design is not just about digging a hole and planting a sapling, it’s a complex interaction of soil science, hydrology, and structural engineering. Irish planning guidance, including the Green Infrastructure policies in the Regional Spatial and Economic Strategies (RSES) and the Design Manual for Urban Roads and Streets (DMURS), strongly encourage the use of integrated green infrastructure, including tree planting supported by appropriate soil systems.
Landscape architects play a crucial role in specifying soils that:
- Align with tree species needs
- Allow for sufficient rooting volume and oxygen exchange
- Meet structural load-bearing requirements
- Support SuDS integration
Our team at Enrich frequently collaborates with landscape designers and local authorities to ensure that each engineered soil mix is matched to site-specific needs, tree species, and expected loading conditions.
Trees as Urban Infrastructure
These ecosystem services are only possible when subsurface conditions support healthy tree growth, which is where Enrich’s engineered tree soils make a critical impact.
Healthy cities need healthy trees, and healthy trees begin with the right soil. As Ireland’s cities move toward climate adaptation, the integration of engineered tree soils in landscape design will be vital for long term resilience, urban liveability, and ecological performance.
