Soil volumes and pit design for new street trees

Soil volume for street trees is the single variable that decides whether a tree reaches the size shown on the landscape drawing or stays stunted and dies at fifteen years. A tree planted in a one metre cube of topsoil surrounded by compacted subbase has nowhere to go. Everything else in the specification, species, stock size, irrigation, staking, matters less than the volume of rooting medium the tree can actually occupy.

That is also the number most often missing from a planning submission. Landscape drawings show species, spacing and nursery size. They rarely show the rooting volume, and a landscape officer who asks for it is usually asking because an earlier scheme on the same street failed.

Why volume, not depth, governs survival

Tree roots in urban conditions spread laterally rather than descending. Most fine root activity sits in the upper 600 mm or so of soil, which means a deep narrow pit provides far less usable volume than its dimensions suggest.

Compaction is the other half of the problem. Soil compacted to the density a footway subbase requires has too little pore space for root penetration or for the air and water roots need. So the usable volume is not the volume of the excavation. It is the volume of uncompacted or structurally supported medium within it.

The consequence is counter-intuitive for engineers. A wide shallow rooting zone under a footway performs better than a deep pit, provided the surface above it is supported by something other than the soil itself.

Sizing the volume against mature canopy

BS 8545:2014, which covers trees from nursery to independence in the landscape, expects rooting volume to be calculated from the species and its expected mature size rather than taken from a standard detail. In practice designers size the volume from the projected canopy area at maturity, using an allowance of roughly 0.6 m³ of soil for every square metre of canopy projection.

Worked through, that gives the following. These figures are illustrative arithmetic from that allowance rather than a published table, and the species and site conditions should drive the final number.

Two things follow immediately. Large trees in a footway are only deliverable with shared or continuous rooting zones, because nobody is excavating a 68 m³ pit around a single stem. And if the volume cannot be found, the honest response is to specify a smaller species rather than to plant a large one in a small pit and label the drawing optimistically.

Pits, trenches and shared rooting zones

Three arrangements cover most situations.

An isolated tree pit suits a single specimen where the surrounding ground is unavailable, and it is the weakest option. A continuous trench running beneath the footway between trees shares the volume and performs far better for the same excavation cost. A shared rooting zone under an adjacent planting bed, verge or rain garden gives the best result where the layout allows.

Linking pits costs very little and almost always pays off. Where services prevent a continuous trench, root paths or bridging sections through the obstruction can restore some continuity, and they should be drawn rather than left for the contractor to improvise.

The level change question is a related one. Where a rooting zone sits against a retaining structure or a change in level, the detail needs the same care as the equivalent problem around existing trees, discussed in our note on retaining walls and level changes in a root protection area.

Structural soils and load-bearing systems

Where the surface above the rooting zone has to carry traffic or pedestrians, there are two broad approaches.

Load-bearing cell systems, often called soil cells or root vaults, use a modular frame to take the surface load while the voids hold uncompacted soil. They deliver the highest usable volume per cubic metre excavated and are the normal choice under footways and trafficked areas, at a higher material cost.

Structural soils are engineered stone and soil mixes in which a skeleton of angular stone carries the load while the soil occupies the voids between. They are less expensive and more forgiving to install, and they provide less usable soil per unit volume, so the excavation has to be larger for the same effective rooting volume.

Sand-based rooting media sit somewhere between. Whichever is used, the specification needs the particle grading, the organic content, the compaction regime and the testing requirements stated, because the material is invisible once the paving is down and substitution on site is common.

Water, air and drainage

A tree pit that cannot drain becomes a sump, and waterlogged roots fail as surely as dry ones. Specify a drainage route from the base of the rooting zone, and test the surrounding ground before assuming infiltration will work.

The better schemes make the tree pit part of the surface water design rather than isolating it from it. Directing footway or carriageway runoff into the rooting zone through a kerb inlet gives the tree the water it needs and takes volume out of the drainage system, which is the approach behind bioretention tree pits. It requires thought about pollutants, silt and winter salt, and it needs the drainage engineer and landscape designer to be working from the same drawing.

Vertical irrigation tubes have a poor record. They channel water past the fine roots to the base of the pit and they fill with debris. Permeable surfacing, a mulched or planted surface to the pit, and a proper establishment watering regime achieve more.

Establishment, maintenance and the first five years

BS 8545 frames the objective as independence: the point at which a tree no longer needs intervention to survive. Reaching it takes years, not one planting season.

The establishment period should be specified, with watering frequency, mulch renewal, weed control, stake and tie inspection and removal, and formative pruning. Watering is the part that gets dropped, and summer water stress in the second and third years kills more street trees than any planting defect. Planting season matters too: bare root and rootballed stock go in during the dormant season, container stock is more flexible but needs more water if planted in summer.

Specify replacement during the maintenance period, with the replacement tree subject to the same establishment obligations, otherwise a tree that dies in year four is replaced with a whip and abandoned.

Getting this into the application and the conditions

Councils secure planting through landscape conditions, and the strength of the condition depends on the detail submitted with the application.

Put the rooting volume on the drawings. A Landscape Strategy should include a tree pit section showing the volume, the load-bearing system, the drainage route and the services relationship, alongside the planting schedule. Where the scheme is in London or in an authority with a canopy cover or greening policy, a Green Infrastructure Plan is the document that carries the urban greening calculation and the canopy argument.

Keep the new planting specification separate from the existing tree work. BS 5837:2012 governs the survey and protection of existing trees, which belongs in the arboricultural impact assessment, while BS 8545 governs new planting. Conflating the two is a common reason a tree officer asks for the whole package again. Early engagement on allocated sites avoids most of this, as set out in our note on getting trees right at the plan stage.

A condition worth accepting names the soil volume per tree, the rooting system, the planting specification by reference to a submitted detail, and a maintenance and replacement period. One that simply requires a landscape scheme to be approved later gives you no protection when the civils package is value-engineered.

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