Urban trees are often planted as individual features above ground. Below ground, however, they do not always need to be treated as isolated trees.

A continuous rooting system allows several individual trees to share a connected volume of soil beneath the surface. This gives roots greater opportunity to spread beyond the immediate tree pit and make use of the available rooting space.

One landscape above ground, one connected system below

A street may contain several separate tree openings, each positioned between paving, roads or other hard landscape features.

Below the surface, those trees can be connected through a continuous rooting environment.

Instead of restricting each tree to a small, isolated volume of soil, the underground system can allow roots to move freely between planting locations.

This helps maximise the value of the available soil volume and makes better use of limited urban space.

Supporting a continuous mycorrhizal network

Connected soil volumes can also support the development of a more continuous mycorrhizal fungal network across the rooting zone.

Mycorrhizal fungi form beneficial associations with tree roots. Their fine fungal threads can extend through the soil beyond the roots themselves, helping trees access water and nutrients from a larger area.

In interconnected tree pits, this fungal network has greater opportunity to develop across the wider soil profile rather than being confined to a small, isolated planting area.

This can help support:

• Improved access to water and nutrients
• Greater biological activity within the soil
• Better interaction between roots and the surrounding soil environment
• Increased resilience during periods of environmental stress

The result is a more connected underground ecosystem, helping individual trees function as part of a wider shared rooting environment.

Why continuity matters

The benefit of a continuous rooting system depends on maintaining that connection.

When reviewing a tree pit design, consider:

• Can roots move freely through the proposed soil volume?
• Are there gaps or barriers between individual rooting areas?
• Does the system remain continuous between trees?
• Could changes to the design reduce root connectivity?

Interruptions within the rooting environment can limit how effectively trees access the available soil.

Designing for greater rooting opportunity

In constrained urban environments, it is not always possible to provide a large open planting area around every tree.

Creating continuity below ground provides another approach.

Multiple trees can remain visually separate at surface level while benefiting from a much larger, shared rooting environment beneath pavements and public realm areas.

The principle is simple:

individual trees above ground, connected soil below ground.

By designing for continuity from the outset, landscape architects and engineers can make better use of underground space and provide urban trees with greater opportunity for long-term root development.