Sustainable Drainage Systems – more commonly referred to by the acronym SUDS – have become a central component of London’s response to surface water flooding, a risk that affects an estimated 480,000 properties across the capital and is projected to worsen as climate change intensifies rainfall intensity and frequency. Trees are increasingly recognised as active participants in SUDS schemes rather than incidental landscaping elements, contributing to flood mitigation through canopy interception, evapotranspiration, and the hydrological function of their root systems in improving soil permeability and water retention. The London Plan’s Policy SI 13 now requires all major developments in Greater London to incorporate SUDS, and the Greater London Authority’s Urban Greening Factor framework actively incentivises the integration of trees into those drainage schemes. The consequence is that tree planting within or adjacent to SUDS infrastructure is becoming a standard feature of new development and public realm improvement projects across the capital. What is not always understood – and what represents one of the more consequential errors in urban greening practice – is that the hydrological conditions of a SUDS zone place specific and demanding physiological requirements on any tree planted within them. Species selection that fails to account for those requirements produces trees that establish poorly, decline early, and ultimately contribute nothing to either the drainage function or the canopy cover the scheme was designed to deliver.
Understanding SUDS and the Role of Trees in Urban Flood Mitigation
How SUDS Systems Function and Where Trees Fit In
SUDS manage surface water by slowing runoff, promoting infiltration, and storing water temporarily at or near the point where rainfall lands, rather than routing it immediately into combined sewer systems already operating at or beyond capacity during storm events. The principal components encountered in London’s urban context include bioretention cells and rain gardens – shallow, vegetated depressions designed to receive and infiltrate surface runoff – swales, which are linear vegetated channels that convey water slowly whilst promoting infiltration along their length, and detention basins or attenuation ponds, which store peak flow volumes temporarily before releasing them at a controlled rate. Trees are incorporated into all of these typologies, positioned to contribute their hydrological functions as integrated elements of the drainage system rather than as aesthetic additions. A mature tree in a bioretention cell or alongside a swale intercepts rainfall directly on its canopy – studies indicate that urban trees can intercept between 15 and 40 per cent of annual rainfall depending on species and canopy architecture – and its root system creates macropore channels in the soil that substantially increase infiltration rates relative to unplanted ground.
The Hydrological Demands Placed on Trees in SUDS Contexts
The functional value of a tree in a SUDS scheme is directly proportional to its physiological health, and that health depends on how well the species is matched to the conditions it will experience. SUDS planting zones are not stable, uniformly moist environments. They are by design subject to periodic inundation following rainfall events, followed by intervals of normal or dry conditions between events. The soil alternates between saturated and unsaturated states, sometimes repeatedly within a short period. Root systems must function effectively across this full range of moisture conditions – tolerating temporary oxygen deprivation during inundation whilst maintaining sufficient vigour during dry intervals to sustain canopy growth, water uptake, and the biological activity that maintains root channel structure in the soil. A species that cannot tolerate periodic waterlogging will suffer root death during inundation events; a species with a very shallow root system will fail to contribute meaningfully to deep infiltration. Neither failure is apparent immediately – both manifest as progressive decline over the first few growing seasons, by which time the capital and maintenance costs of establishment have already been committed.
The Challenging Conditions of SUDS Planting Zones
Periodic Inundation and Soil Anaerobiosis
When a soil becomes saturated, the air-filled pore spaces through which roots obtain oxygen are displaced by water. The resulting anaerobic conditions inhibit aerobic respiration in root cells, disrupt nutrient uptake, and – in species with limited tolerance – cause root cell death within a period measured in days rather than weeks. The severity of the impact depends on inundation depth, duration, temperature, and the physiological adaptations of the species concerned. Flood-tolerant species have developed specific responses to anaerobic root conditions: some produce adventitious roots at the stem base above the waterline during inundation; others possess specialised root anatomy – aerenchyma tissue – that facilitates internal oxygen transport from aerial parts to submerged roots. In London’s SUDS context, inundation events following intense rainfall may last from several hours to several days, and the cumulative effect of multiple events across a growing season must be considered alongside any individual episode.
Soil Compaction, Substrate Variability, and Engineered Ground Conditions
SUDS infrastructure in London is almost invariably installed within a wider construction or public realm improvement context, which means that the ground conditions surrounding the drainage feature are frequently far from natural. Sub-base compaction from site operations, the presence of engineered fill material, the proximity of service runs and drainage pipework, and the sharp lateral transitions between SUDS substrate and surrounding urban ground all create a root environment of considerable variability. Bioretention cells are typically constructed with a specified engineered soil mix – often a sandy loam or purpose-blended growing medium – designed to optimise both drainage performance and plant establishment, but the transition between this material and adjacent urban ground can create interfaces that restrict root spread. Species with robust, adaptable root architecture that can establish within a defined volume of improved substrate are better suited to these conditions than those requiring extensive lateral root spread through undisturbed soil.
Species Selection Criteria for SUDS Planting
Flood Tolerance and Root System Characteristics
The primary selection criterion for any tree in a SUDS zone is documented tolerance of periodic waterlogging and temporary inundation. Beyond that threshold requirement, the most valuable species combine a deep or structurally complex root system – supporting infiltration function and soil stability – with a substantial canopy capable of meaningful interception and evapotranspiration.…







