Planting trees in drainage features without conflict

Trees and surface water features can share the same ground, but only if four things are settled before the drawings are issued: which species tolerate periodic inundation, how much rooting volume each tree gets, what clearance the buried services and sewers need, and who maintains the result for the first five years. Most of the conflict arrives because the drainage layout and the planting plan are produced by different consultants from different base drawings.

The usual symptom is a planning condition requiring a revised landscape scheme because the proposed tree pits sit on top of an attenuation tank.

Why landscape and drainage designs collide

Drainage design works from levels, invert depths and volumes. Landscape design works from soil, canopy and visual effect. Both want the same places: the open, unbuilt margins of a site.

Three specific collisions recur. A swale drawn along a boundary occupies the strip reserved for the planting that was meant to soften the development edge. An attenuation basin needs a mown engineered profile that will not support the trees shown on the illustrative masterplan. And a geocellular tank under a parking court takes the only rooting volume available for the street trees the design code expects.

None of these is unresolvable. All of them are expensive to resolve after permission, because the drainage design is usually fixed by a condition requiring details to be approved and the landscape scheme is fixed by another.

Species tolerance of waterlogging

A rain garden or swale floods for hours at a time, several times a year, then dries. That cycle kills most ornamental species and suits a narrow group.

Two cautions. Willow and poplar are the species most often found inside drainage pipes, so keeping them away from the pipework is a design rule rather than a preference. And a species list that works on a free-draining sandy site will fail on heavy clay where the same feature holds water for days.

Specify the planting by position in the feature rather than as a single schedule for the whole site. An arboriculturist and a drainage engineer looking at the same section drawing will resolve this in an afternoon.

Tree pits, structural soils and rooting volume

A tree needs an uncompacted soil volume proportionate to the canopy it will carry at maturity. Where that volume is under paving, the usual answers are a structural soil or a cellular root system that carries the load above and leaves the soil uncompacted below.

Rooting volume is the number to agree with the arboriculturist, and it should appear on the drawing as a volume and a depth rather than as a pit size. Pair it with a specification for the growing medium, because a tree pit filled with site-won subsoil will not deliver the canopy the landscape drawing shows.

Where the pit also receives run-off, as in a tree pit designed to take road drainage, the inlet detail decides whether it works. Silt traps, a settlement zone and an overflow to the piped system keep the pit from silting up in its first winter, and the maintenance schedule has to include cleaning them.

The survey work underneath all of this follows BS 5837:2012, and the root protection areas it generates for retained trees constrain the drainage layout as much as the planting plan. The Tree Survey should be read alongside the drainage layout, not filed separately.

Clearance distances from services and sewers

Buried services and trees fight twice: at installation, when trenching cuts roots, and later, when roots find joints and ducts. The usual rules are straightforward.

1. Keep trenches outside the root protection area of retained trees, and where that is impossible, use thrust boring or hand digging under arboricultural supervision.
2. Keep new tree positions clear of public sewers and water mains, since a water or sewerage undertaker will normally require an agreement for works close to a public sewer and will resist planting over one.
3. Group services into a shared corridor on one side of the street so that the other side is available for planting.
4. Record the agreed clearances on a combined services and planting drawing, which is the document the authority will condition.

Trenching near retained trees is the most common cause of post-permission trouble, and the practical mitigations are set out in utility trenching close to trees on a development site.

Attenuation structures and root barriers

Root barriers work when they are continuous, deep enough to redirect roots downward rather than merely sideways, and detailed at the joints. They fail when they stop short, when a service duct passes through them, or when they are installed after the tree.

Around a geocellular tank or a permeable pavement, the barrier protects the geomembrane and the sub-base from root intrusion. Around a lined basin, it protects the liner. In both cases the barrier is part of the drainage detail, so it belongs in the drainage package with a cross-section, not in a note on the planting plan.

Scaffolding, site compounds and material storage do at least as much damage as roots do to structures, and the ground protection measures described in scaffolding, site access and the root protection area should be agreed at the same time as the drainage sequencing.

Highway trees and adoption standards

If the street is to be adopted, the highway authority decides whether trees can sit within it, and several will not adopt carriageway or verge trees without a commuted sum for future maintenance. Others require the trees to sit in private land immediately behind the highway boundary, which changes the layout and the maintenance route.

Ask the highway authority three questions before the layout is fixed. Will it adopt streets containing trees? What commuted sum applies? What specification does it require for pits, guards and irrigation? The answers often move the trees two metres, and that is much easier at layout stage.

Maintenance responsibility for planted features

A swale planted with trees needs a different maintenance regime from a mown basin: no close mowing at the base, selective cutting on the slopes, inlet and outlet checks after storms, and tree establishment care for at least the first three summers. BS 8545 covers the establishment process from nursery to independence and is a useful reference to name in the specification.

Decide who holds the obligation. A management company funded through an estate charge is the usual answer on a private scheme, and the obligation needs to name the maintenance plan rather than refer to it generically. Where a tree bond or financial guarantee is being discussed, the mechanics in tree bonds, financial guarantees and the planning obligation apply to planting in drainage features as much as to retained trees.

An illustrative example. A 48-unit scheme shows a boundary swale with 14 standard limes along it. The drainage design puts the swale base 600 millimetres below surrounding ground with a clay liner. Moved to the upper slope, with alders at the wet end and the liner detailed around a root barrier, the same 14 trees survive the review. The change took one coordination meeting and no additional land.

Coordinating the two designs before submission

Issue one drawing that carries the drainage features, the services corridors, the root protection areas of retained trees and the proposed tree positions. Have the arboriculturist and the drainage engineer mark it up together. Then produce the Landscape Strategy and the drainage layout from that single coordinated base.

It sounds obvious. It is also the step most frequently skipped, and it is the reason so many schemes spend three months after permission revising a landscape scheme that could have been right at submission.

This is general guidance, not legal advice. Tree surveys and arboricultural assessments should be prepared by a suitably qualified arboriculturist and drainage design by a suitably qualified engineer, and clearances from public sewers and mains should be confirmed with the relevant undertaker. Check requirements with your local planning authority before relying on anything here.

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