June 2024. After barely fifteen minutes of heavy rain, the streets of cities across Europe turn into fast-flowing rivers. Cars sit bumper-deep in water, basements flood, trams come to a standstill. Scenes that once shocked us are becoming an increasingly regular part of urban summers. This is no coincidence – it is one of the most tangible consequences of the climate crisis and the relentless sealing of our cities with concrete and asphalt.
The answer does not lie solely in engineering – in concrete storm drains or ever more powerful pumping stations. More and more cities around the world are discovering that the most effective and affordable ally in the fight against floods is nature itself. Trees, rain gardens, meadows and permeable surfaces together form what urban planners and hydrologists call blue-green infrastructure (BGI) – a system that manages rainwater by mimicking natural ecosystem processes. In this article, we explain why trees are a key piece of this puzzle and what we can do to make our cities more resilient to the whims of the climate.
Where Do Flash Floods Come From? Cities That Cannot Absorb
To understand why urban greenery matters so much, it helps to look at what happens to rain when it falls on a typical city. In a natural environment – a forest, a meadow or undisturbed soil – between 50 and 70 per cent of rainfall is absorbed into the ground. Some evaporates, some feeds rivers and streams, and some recharges groundwater. The whole system works like a natural sponge: slowly and effectively.
In a city, this cycle is radically disrupted. Sealed surfaces – asphalt, concrete, paving stones – are impermeable. Rooftops, car parks and roads channel water rapidly into stormwater drains. Sewerage systems, designed decades ago, are often unable to cope with the short, intense downpours that climate scientists call extreme precipitation events. The result? Water backs up and floods the very streets it was meant to drain.
Researchers at meteorological institutes across Europe warn that the frequency of extreme rainfall events has increased by several dozen per cent in recent decades. At the same time, the area of impermeable surfaces in cities keeps growing. These two trends together create a ticking hydrological time bomb. The only way to defuse it is to change the very logic of urban design – from draining water away to retaining and absorbing it.
The Tree as a Plumber – How One Organism Manages Hundreds of Litres of Water
A single mature tree is a remarkable hydrological machine. Its canopy acts as a natural umbrella and retention reservoir at the same time. Leaves and branches intercept raindrops before they even reach the ground – a process known as interception. Research conducted in European cities shows that a mature deciduous tree can intercept between 15 and 40 per cent of rainfall, depending on species and rainfall intensity. Conifers perform even better – capturing up to 50 per cent.
Beneath the canopy, another wonder unfolds: the root system loosens the soil and creates channels through which water can infiltrate many times faster than in unvegetated ground. One large oak can „manage” as many as 100,000 litres of water in a single year – absorbing it through its roots, drawing it up through the trunk and releasing it into the atmosphere through transpiration. That water never reaches the storm drain. Instead, it cools the air, humidifies the surroundings and re-enters the water cycle in an environmentally friendly way.
The Cooling Effect: Trees versus the Urban Heat Island
Water retention is just one aspect of what trees do in cities. Closely linked to it is the cooling effect. Transpiration – the release of water vapour through leaves – acts as natural air conditioning. A single mature tree can on a hot day produce a cooling effect equivalent to several air-conditioning units. This is why the temperature beneath a large horse chestnut in August can be as much as 8-10°C lower than on a nearby concrete car park.
The cooling and retention effects are interconnected: a cooler city means less evaporation, which in turn leads to more efficient infiltration of water into the soil. Less sun-baked soil and air also means less stress on vegetation, which then performs its ecosystem functions more effectively. It is a virtuous cycle – but it only works when there are enough trees in the right conditions.
Blue-Green Infrastructure – What It Is and How It Works in Practice
The term blue-green infrastructure (BGI) emerged in urban planning as a response to the need for systemic thinking about water and greenery in cities. Blue represents water – rivers, lakes, retention ponds, rain gardens. Green stands for trees, grasses, meadows, gardens and green roofs. Together they form a network of solutions that mimic natural ecosystem functions and complement – or replace – conventional technical infrastructure.
BGI is not just about aesthetics or fashion. According to reports by the European Environment Agency, investments in blue-green infrastructure are on average three to five times cheaper over a 30-year horizon than building and maintaining conventional stormwater infrastructure of comparable capacity. On top of that come benefits no concrete collector can provide: better air quality, greater biodiversity, recreational spaces for residents and increased property values.
Key Elements of Blue-Green Infrastructure
The most commonly used BGI solutions include:
- Rain gardens – sunken landscaped areas filled with a special growing medium and plants that collect water from rooftops and streets and slowly release it into the ground.
- Bioretention swales – shallow, elongated planted channels along roads and car parks that slow surface run-off.
- Trees in silva cell systems – underground structural cells that give roots access to soil, water and air even beneath sealed surfaces.
- Green roofs and living walls – vegetation on rooftops and building facades that retains water and provides thermal insulation.
- Permeable surfaces – paving blocks, gravel and other materials that allow water to percolate into the ground rather than run off into drains.
- Retention ponds and infiltration basins – natural or artificial water bodies that collect water during rainfall and release it gradually.
The common denominator of all these solutions is the principle: slow it down, soak it up, put it to use. Rainwater stops being a problem to be discharged and becomes a resource to be managed. One More Tree Foundation, through its tree planting programmes for businesses and urban greening projects, is directly aligned with this philosophy.
Which Tree Species Handle Water Best? Species Selection Matters
Not all trees are equally effective at retention and water management. Species selection appropriate to local conditions – both climatic and soil-related – is crucial. In Central Europe, the question of tree resilience to water stress is increasingly important – both excess moisture (waterlogging) and deficit (drought). Climate change means cities must deal with both extremes alternately.
Species particularly recommended for urban planting in the context of water retention include: elms (Ulmus) – tolerant of temporary waterlogging and drought; alders (Alnus) – masters of wet habitats, ideal near watercourses; English oaks (Quercus robur) – large, long-lived trees with a powerful root system; small-leaved limes (Tilia cordata) – classic street trees with excellent transpiration rates; and maples (Acer) – fast-growing and tolerant of urban conditions.
The principle of species diversity is equally important. Monocultures – such as avenues composed entirely of one species – are highly vulnerable to disease and pests. When one pathogen takes hold, an entire avenue can disappear within a few growing seasons. Species diversification is not merely an aesthetic consideration; it is a strategic resilience measure for the urban ecosystem.
European Cities in Practice – Examples That Inspire
Urban water retention has long moved beyond academic discussion. City after city is incorporating BGI solutions into its climate adaptation strategies. Wroclaw in Poland is implementing a Blue-Green Network programme that envisages open water channels, rain gardens and new plantings. Gdańsk, which experienced devastating flash floods in 2001 and 2016, is investing heavily in retention basins and green tram tracks. Copenhagen has become a global reference point for climate adaptation after investing over one billion euros in BGI following catastrophic flooding in 2011.
Several German cities, including Hamburg and Berlin, have made BGI a standard component of street renovation schemes – every newly resurfaced road must incorporate permeable sections and tree pits with sufficient rooting volume. The lesson learned across Europe is clear: the earlier green infrastructure is integrated into urban planning, the lower the long-term costs. Retrofitting is always more expensive than building right the first time. This is exactly the professional, long-term approach that underpins our employee volunteering programmes.
Rain Gardens – A Small Solution with a Large Impact
One of the most exciting trends in contemporary urban design is the growing popularity of rain gardens. These are relatively straightforward landscaped depressions – often beside houses, on housing estates or near playgrounds — filled with an appropriate mix of soil, sand and compost, and planted with species tolerant of both temporary flooding and drought. A rain garden of just 4–6 m² can handle the water running off the roof of a house covering several dozen square metres.
Rain gardens are also excellent habitats for pollinators, birds and other organisms. The plants chosen for rain gardens – yarrow, meadowsweet, common reed, Siberian iris, yellow flag – are native species requiring no irrigation, fertilisation or specialist maintenance. A rain garden thus becomes not only a retention tool but also a small urban biodiversity reservoir.
What Businesses Can Do – Green Engagement with Real Impact
Responsibility for urban greenery and water retention does not rest solely with local authorities. A growing number of companies – large corporations as well as small and medium-sized businesses – recognise that their activities have a real impact on the built environment in which they operate. Office buildings surrounded by sealed plazas, car parks with impermeable surfaces, warehouses with bare rooftops – all of these contribute to the surface run-off problem.
The good news is that change is possible and often simpler than it seems. Corporate engagement in water retention and urban greening can take many forms: from green roofs and rain gardens on company premises, to replacing sealed surfaces with permeable materials, to funding or co-organising tree planting in public spaces. These actions form part of a Corporate Social Responsibility (CSR) strategy and can be implemented in partnership with organisations like One More Tree Foundation. Find out more about our ecological programmes and field activities on our eco workshops and lectures page.
It is worth emphasising that involving employees directly in physical activities – planting trees, building rain gardens, restoring green spaces – produces an effect far beyond the environmental impact alone. Research in environmental psychology shows that direct contact with nature and a sense of agency in pro-ecological actions significantly increases motivation, wellbeing and a sense of purpose among employees.
The City of the Future Looks Like a Sponge – The „Sponge City” Concept
The concept of the sponge city originated in China, where after catastrophic flooding in several megacities the government adopted it as an official urban planning doctrine. The idea is to design the urban fabric so that cities can absorb, store, cleanse and slowly release rainwater in a controlled and beneficial way. There is no single miracle solution – it is about integrating dozens of small and large interventions that together create a new way of thinking about the city.
The concept is gaining growing traction in Europe. New design standards for roads, squares and housing developments are beginning to incorporate retention requirements from the earliest planning stages. Building regulations are also shifting – towards requiring developers to manage rainwater on their own land rather than discharging it into the sewerage system. This represents a paradigm shift: water stops being waste and becomes a resource.
Realising the sponge city idea requires not only legislation and budgets, but a change of mindset – among designers, municipal decision-makers and residents alike. Every rain garden established by a homeowner, every tree planted on a street, every green roof on a company warehouse is a brick in this construction. Climate change is a collective problem and demands a collective response – and that response begins with individual choices and actions.
How You Can Act Right Now – From Your Balcony to the Company Car Park
Discussions about systemic change in urban planning can feel abstract when your own basement is flooding or you are watching the grass in the park turn to dust. But it is precisely at this scale – individual, local – that real change begins. What can you do without waiting for large-scale government programmes?
At the individual and local level:
- Create a rain garden beside your home or in your garden – modest cost, significant retention and aesthetic benefit.
- Replace sealed surfaces with permeable materials – gravel, grass grids or permeable paving on your driveway or path.
- Install a rainwater butt or underground retention tank – water collected during rain is water available for irrigation during drought.
- Plant native trees and shrubs – even one well-chosen and properly planted tree will work for the climate for decades.
- Support local greening initiatives – volunteering, planting events, ecological community days.
At the corporate level, it is worth carrying out an audit of your property’s water retention potential and greening opportunities, then seeking expert support. Organisations like One More Tree Foundation help companies navigate this process – from needs analysis and species selection through to on-the-ground implementation with employee participation. This is not merely „doing something for the environment” – it is a smart investment in the future of the place where your company operates and your people live.
Conclusion – Trees Are Not Decoration, They Are Infrastructure
For decades we treated urban trees as decorative elements – something nice to have, but expendable when the next road or construction project came along. The climate crisis is brutally correcting that perspective. We now know that a city tree is fully-fledged technical infrastructure – every bit as essential as a sewer pipe or an electricity cable. But cheaper, more beautiful, healthier, and serving not only people but the entire ecosystem.
Blue-green infrastructure is not a utopia or a project for the future – it is a set of solutions available here and now, proven in dozens of cities around the world. What is needed is political will, funding and – perhaps above all – the collective conviction that it is worth it. Every tree planted, every rain garden created, every permeable surface laid is a step towards a city that lives in harmony with its climate rather than battling every storm.
If you would like to find out how you can work with us – plant a tree, join an action, or engage your company – we invite you to contact One More Tree Foundation. Together we are planting not only trees, but the future of the cities we want to live in.
