Urban trees are under pressure: compacted soil, impervious surfaces, heat, and drought stress make it difficult for them to survive. Tree swales are one of the most effective solutions available to open-space and urban planning. They combine stormwater management with a sustainable water supply for trees, create root space beneath paved surfaces, and fit into the concept of blue-green infrastructure. Understanding tree trenches means understanding a central building block of climate-resilient urban planning.

• What tree trenches are, how they work, and how they differ from other tree planting systems
• What hydrological and ecological principles underlie the system
• How tree swales are planned, sized, and integrated into the urban environment
• What substrates, filter layers, and storage volumes are required for a functional system
• How tree swales are linked with other elements of decentralized stormwater management
• Which standards, regulations, and planning guidelines are relevant
• What common mistakes occur in planning and construction and how to avoid them
• What tree swales look like in practice and what insights have been gained from completed projects

What Are Tree Swales? Definition, Principle, and Distinction

A tree trench is an underground infiltration and storage structure installed directly within the root zone of an urban tree or a row of trees. The term “trench” comes from French and originally referred to a ditch or channel. In stormwater management, the term has come to refer to underground structures filled with filter material or storage media that collect stormwater, temporarily store it, and allow it to infiltrate at a controlled rate. A tree swale combines this hydrological principle with the function of providing for the tree: it serves simultaneously as an infiltration system, a water storage reservoir, and a root-permeable soil space.

The tree trench differs fundamentally from the traditional tree pit. A tree pit is an open, vegetated area around the base of the trunk that, while it can absorb precipitation, does not provide a specifically designed subsurface space for water storage and root expansion. The tree trench, on the other hand, is a technically engineered system located below ground level and typically equipped with a defined substrate structure, inlet pipes for surface water, and a controlled overflow. The tree trench differs from a tree box or tree module—which is installed beneath paved surfaces—in its linear or area-wide extent and its focus on infiltration rather than merely protecting root space.

Tree swales are not a new concept, but they have gained significant importance due to the requirements of sponge city planning and decentralized stormwater management. Today, they are considered a standard element in the planning of climate-resilient urban streets and squares because they combine multiple functions in a single component: retention, infiltration, groundwater recharge, tree irrigation, and cooling through evapotranspiration. This multifunctionality makes them a preferred tool when limited spaces must meet multiple requirements simultaneously.

Hydrological Principle: How Tree Trenches Absorb, Store, and Release Water

The basic hydrological principle of the tree trench is based on the controlled flow of stormwater from paved surrounding areas into an underground storage and infiltration chamber. Rainwater that falls on streets, sidewalks, parking lots, or rooftops is directed into the tree trench via inlet structures, slits, gutters, or street drains. There, it encounters a substrate that absorbs, filters, and gradually releases the water into the subsoil. The tree can absorb this stored water through its roots, which significantly reduces drought stress during heat waves.

The storage capacity of the infiltration trench is determined by the pore volume of the substrate used or by embedded plastic storage elements (known as infiltration trench bodies or infiltration trench cassettes). Drainage trench bodies are highly porous, pressure-resistant structures made of recycled polypropylene that have a void volume of up to 95 percent and thus store significantly more water than an equal volume of gravel. They are typically wrapped in geotextile to prevent the entry of fine particles and colmation—that is, the clogging of pores by suspended solids. Colmation is one of the most common causes of damage in infiltration systems and must be permanently controlled through appropriate pre-filtration and maintenance.

Water drains from the infiltration trench in two ways: through infiltration into the natural subsoil and through evaporation via tree roots and the soil surface. If the subsoil has low infiltration capacity—such as in cohesive soils with a high clay content—the infiltration trench must be larger or equipped with a restricted outlet to the sewer system that functions as an emergency overflow. This emergency overflow prevents the system from overflowing during heavy rainfall and flooding the paved surface. The sizing of the storage volume and the outlet depends on the catchment area to be connected, the design rainfall, and the infiltration capacity of the subsoil, which must be determined through on-site infiltration measurements.

Planning and Sizing: Substrate, Construction, and Catchment Area

Planning a tree swale begins with a site analysis. Subsoil conditions, groundwater level, utility lines, traffic volume, and the hydraulically connected catchment area are the key input parameters. The groundwater level is particularly relevant because a minimum distance must be maintained between the bottom of the trench and the average high groundwater level to prevent backflow and waterlogging of the root zone. The relevant regulations, particularly DWA Worksheet A 138 on the planning, construction, and operation of systems for the infiltration of stormwater, provide guideline values that must be taken into account during the planning process.

The substrate of the tree trench must meet two seemingly contradictory requirements: It should store water well while simultaneously providing sufficient air for root respiration. Conventional topsoil is unsuitable for this purpose because it becomes compacted under traffic loads and closes off the pores. Special tree substrates, as recommended by the Research Association for Landscape Development and Landscape Construction (FLL)—particularly the FLL recommendations for tree plantings—define requirements for particle size, organic content, air capacity, and usable field capacity. Substrates rich in skeletal material—that is, mixtures of coarse mineral components (gravel, crushed stone, lava, or brick chips) and a fine fraction containing organic matter—are widely used; they remain stable under load while still providing sufficient water and nutrients for the tree.

The sizing of the storage volume is based on the design rainfall, which is typically specified as a rainfall event with a defined return period (such as five or ten years) and a specific duration. The catchment area to be connected—that is, the paved surface whose runoff is directed into the tree trench—determines the inflow volume. For a tree trench on an urban street with sidewalks and a roadway on both sides, this can quickly amount to several hundred square meters of catchment area. It is important that the inlet pipes be designed to trap coarse debris and leaves before the water enters the storage structure. Pre-treatment basins, sedimentation chambers, or filter media in the inlet section are common measures used for this purpose.

In addition to hydraulic sizing, the structural load-bearing capacity of the system must be taken into account. Tree swales located beneath sidewalks or parking lanes must be able to withstand the resulting loads without settling or damaging the storage structure. Plastic swale bodies are generally designed for specific surface loads and vehicle loads; the manufacturer’s specifications and the relevant standards regarding the load-bearing capacity of infiltration systems are binding in this regard. Under roadways with heavy traffic, tree swales in their classic form are rarely used; instead, tree boxes with base layer substrates are typically employed, as they allow for better load distribution.

Integration into Blue-Green Infrastructure and Linkage with Other Systems

Tree swales achieve their full effect when they are planned not as a standalone measure but as part of a networked system of decentralized stormwater management. In sponge city planning—which aims to retain stormwater as much as possible where it falls and gradually return it to the water cycle—tree swales are one component among many. They can be combined with trough-swale systems, in which an above-ground trough serves as a prefilter and temporary storage basin and directs excess water into the underground swale. This combination increases both storage capacity and filtration efficiency.

Tree swales can also be connected in series so that the overflow from one swale flows into the next. Such cascade systems are particularly useful in street spaces with rows of trees because they distribute the water along the street and supply multiple trees at the same time. The overflow at the end of the chain is directed either into the sewer system or into another infiltration feature. This linear connection follows the principle of a retention basin and has been proven effective in practice.

Green roofs, green facades, and infiltration basins can be hydraulically connected to tree swales if topographical and piping conditions permit. Such integrated systems are part of current planning practices in cities pursuing ambitious goals for stormwater management. In recent years, Berlin, Hamburg, Munich, and Zurich have developed pilot projects and regulations that establish tree swales as a standard element in street planning. The Berlin Senate Administration, for example, has developed planning guides—such as the “Urban Streets Manual” and the “Rainwater Guidelines”—that explicitly identify tree swales as a tool for rainwater management.

With regard to water quality, it is important to note that street runoff contains pollutants: heavy metals from tire wear and brake dust, hydrocarbons from fuel residues and petroleum products, as well as nutrients from organic inputs. Tree swales are not wastewater treatment plants, but a properly sized substrate structure with suitable filter material can retain a significant portion of these substances before the water reaches the groundwater body. The requirements for groundwater protection are regulated by the Water Resources Act (WHG) and state water laws; the infiltration of road runoff requires a permit and generally necessitates an assessment of pollutant levels.

Common Errors in Planning and Construction

One of the most common mistakes with tree swales is insufficient pre-filtration of the inflowing water. If leaves, fine soil, and organic particles enter the swale unfiltered, the substrate or storage media becomes clogged within a few years. The infiltration capacity decreases, water accumulates, and the tree is occasionally standing in standing water, which promotes root rot. Carefully planned pretreatment—such as a sedimentation chamber with a submerged wall or a filter layer of coarse gravel in the inflow area—is therefore not an optional addition but an integral part of any functional system.

Another common mistake is underestimating the catchment area. If additional areas are subsequently connected to a swale that was originally designed for a smaller catchment area, the inflow volume during heavy rain will exceed the storage capacity. The water overflows at the surface, scours away pavement, and damages tree roots due to oxygen deprivation. A careful hydraulic calculation that takes all connected areas into account is therefore essential, as is a functioning emergency overflow that reliably discharges water even during design rainfall events.

Errors in substrate selection also have serious consequences. Substrates that are too fine-grained compact under load and lose their air-holding capacity; substrates that are too coarse-grained do not store enough water for the tree. Substrates with too high an organic content settle and can form anaerobic zones in which roots die off. The use of untested or non-compliant substrates is a common cost-saving measure that proves expensive in the long run. The FLL recommendations and relevant product standards provide clear guidance in this regard, which should be explicitly mandated in the request for proposals.

Finally, maintenance is systematically underestimated. Tree swales are not maintenance-free systems. Sedimentation pits must be emptied regularly, filter substrates inspected and replaced as needed, and supply lines checked for blockages. In municipal practice, there is often a lack of clear responsibilities and budgets for these maintenance tasks. Systems that stop working after a few years due to a lack of maintenance discredit the system as a whole, even though the fault lies in operational organization, not in the design.

Completed Projects and Practical Insights

In practice, tree swales have proven effective in various types of urban spaces. In street spaces with rows of trees, where the space beneath the roadway and sidewalk is used intensively, they enable a combination of transportation infrastructure and green space that would not be possible without underground systems. Projects in Berlin, such as those carried out as part of the implementation of Berlin’s stormwater management plan, show that tree trenches can be retrofitted into existing streets if the pipeline layout permits it and the subsoil is permeable.

In Zurich, tree trenches were systematically integrated into street renovations as part of the “Sponge City” project. Experience shows that trees in locations with tree trenches are significantly healthier than comparable trees without an underground water supply, especially during dry summers. The survival rate of newly planted trees increases, the establishment phase is shortened, and the trees reach a canopy size that contributes to shading and cooling the urban environment more quickly. These observations are consistent with findings from other European cities that use tree trenches as a tool for climate adaptation.

In Hamburg, as part of the redesign of city streets, tree trenches were combined with above-ground basins that serve both as design elements and as buffer storage. This combination makes it possible to connect larger catchment areas and to visibly display the water in the urban space, which has educational value: Citizens witness how rainwater remains in the street space and infiltrates the ground instead of being diverted into the sewer system. This visibility is also politically relevant because it increases acceptance of investments in blue-green infrastructure.

Practical experience shows that coordination between civil engineering, parks and recreation departments, and stormwater management planning is one of the greatest organizational challenges in implementing tree swales. The system involves multiple departments simultaneously, and without clear project leadership, coordination issues arise during planning, construction, and operation. Cities that have successfully integrated tree swales into standard practice unanimously report that the key lies in early coordination among all stakeholders and in the development of standardized planning modules that reduce complexity for those carrying out the work.

Tree-lined swales as a building block of climate-resilient urban planning

Tree swales are not a panacea, but they are a technically mature and well-proven tool that deserves a permanent place in climate-adapted urban planning. They solve the fundamental problem of urban trees: too little water during dry periods, too little root space beneath paved surfaces, and too little connection between the water cycle and vegetation. By making rainwater from impervious surfaces usable instead of diverting it into the sewer system, they close a cycle that is otherwise interrupted in the densely built-up city.

The requirements for planning and implementation are real and must not be underestimated. Substrate selection, hydraulic sizing, pre-filtration, and maintenance planning are not mere details but core tasks on whose quality the system’s functionality depends for decades. Those who view tree swales as a cheap solution will be disappointed; those who plan and operate them as an investment in long-lasting green infrastructure will see urban trees that remain vigorous even under the conditions of urban climate change and continue to provide their ecosystem services over the long term.

The integration of tree swales into overarching concepts such as the “sponge city,” blue-green infrastructure, and decentralized stormwater management provides the framework within which their full potential becomes apparent. Individual swales help individual trees; networked systems transform the hydrological balance of entire streets and neighborhoods. This systems-based approach is the decisive step from individual measures to urban planning, and it is what elevates tree swales beyond a mere technical detail: making them a building block of a city that knows how to manage water and vegetation.