The morning the Kleistgasse re-opens to people, the asphalt is still missing in places. Where there was a parking lane in 2019, there is now a small cluster of Stadtbäume — street trees, planted, irrigated, and stubbornly alive after a summer of repeated heat days. The ground around them is a porous mineral mix that takes in rain like blotting paper. The street is quieter than the still-asphalted thoroughfare a few blocks away, and, in the shade the canopy now throws across the carriageway, it is measurably cooler.
This is what the City of Vienna calls the Schwammstadt-Prinzip. The official translation is “sponge city”. The colloquial translation, in the Magistratsabteilungen that have been quietly building it since 2019, is closer to: stop fighting the rain, stop sealing the ground, and stop pretending the climate that built this city is the climate it has now.
The first hot summer that nobody forgot
Vienna’s climate-adaptation politics began the way most European climate-adaptation politics begin, which is to say: too late, and in the middle of a heatwave. June 2019 was, by the national weather service’s account, one of the hottest Junes on record in Austria — the monthly mean ran several degrees above the long-term average across the country. The inner districts, where the buildings are tighter and the trees fewer, ran hotter still. The body keeping those records was then the ZAMG (the Zentralanstalt für Meteorologie und Geodynamik, since 2023 folded into GeoSphere Austria). It had been modelling exactly this for years and watching nobody act on it.
By August, the city had a pilot scheme on the street. The first Coole Straßen — “cool streets” — opened in early August 2019 as a one-month trial on three sites: the Kleistgasse in the 3rd district, the Hardtmuthgasse in the 10th, and the Hasnerstraße in the 16th. Each was made car-free and fitted with seating, planting and mist showers — light-touch, plausibly cheap, designed to test whether residents would accept what was effectively a parking removal. By the city’s own survey, they did, in large numbers. The 2020 rollout added eighteen more streets. The 2021 programme, rebranded Coole Straßen Plus, added more than a dozen, several with lighter paving, water elements and tree planting meant to last beyond a single summer. By 2022 the city had committed, in writing, to two linked frameworks — the Smart Klima City Strategie and the Wiener Klimafahrplan — setting a course to climate neutrality by 2040 and a retuning of the city’s street space, building stock and green network for a climate it had not been designed for.
What gets unsealed, what gets planted
The Schwammstadt approach has three operational pieces and they are unromantic. The first is unsealing — pulling up asphalt on what used to be parking lanes, side streets and parts of squares, and replacing it with permeable mineral mixes that allow rainwater to percolate. The second is planting — new street trees, but set into oversized substrate trenches that connect underneath the pavement, so that root systems share moisture instead of competing for it. The third is collecting — redirecting roof and pavement runoff into those trenches instead of into the storm drains, so that the trees drink the rain the city used to flush away.
Each of these is technically modest. The engineering, though, was not invented in Vienna. The substrate-trench technique — a deep, load-bearing bed of coarse stone with soil and biochar washed into the voids, so that trees get root space under a paved surface and the bed itself stores stormwater — was developed in Stockholm, where a survey in the early 2000s found most of the city’s street trees declining for want of root space. Vienna adopted that “Stockholm model” and made the sponge principle standard practice on its larger street rebuilds. The reconstruction of the Praterstraße, one of the inner city’s wider boulevards, ran across 2023 and 2024 and reopened in mid-2024: a redesigned stretch on the order of 900 metres, with roughly eighty new trees, widened cycle tracks and reworked squares, integrating the sponge components into a single climate boulevard.
Stop fighting the rain. Stop sealing the ground. Stop pretending the climate that built this city is the climate it has now.
Smart Klima City Strategie, City of Vienna
The political trick
The most-asked question among visiting city halls is no longer how the engineering works. It is how the politics survived. Vienna is not a uniformly progressive city. The governing coalition since 2020 has been an SPÖ-NEOS partnership; the FPÖ sits in opposition and has long argued that every removed parking space is a vote against the working-class outer districts. Adding a green street where there used to be parking bays should, on the standard model, be politically costly.
It has been less so than expected. Part of the explanation is the climate portfolio itself: Jürgen Czernohorszky took the role of Klimastadtrat — the executive city councillor for climate and environment — when the coalition formed in November 2020, and the cool-streets messaging since has leaned less on morality than on summer comfort, child safety and access — the kind of vocabulary that survives a chat in a Beisl. Part of it is incrementalism: no district was forced into the rollout, and districts that wanted in could request the next pilot, so the programme spread by demand rather than decree.
And part of it is data. Vienna publishes its climate progress in monitoring reports tied to the Klimafahrplan that read more like a building society’s annual statement than a climate manifesto: neighbourhood-level, comparable across years, footnoted. When a district councillor wants to argue against a green street in their territory, they have to argue against the same kind of data the next-door district is using to ask for one.
The hardest streets are still the hardest streets
None of this means the work is easy. The streets that need depaving most urgently are often the streets that are hardest to depave. The dense outer districts in the city’s south-east have some of the lowest tree cover and the worst summer heat exposure, and they tend also to have higher car-ownership and the smallest fiscal margin for street rebuilds. Vienna’s climate planning does not pretend otherwise; closing the gap between the leafy inner streets and the hard-baked outer ones remains the slow, unglamorous part of the job.
The same pattern shows up in another corner of the work. New construction can be designed sponge-compliant from the start. The sealed courtyards inside the dense Gründerzeit blocks — the late-19th-century buildings that make up much of inner Vienna — cannot. Unsealing one typically requires a body-corporate vote, an agreement about runoff, and, often, a rethink of where the bins go. The city runs grant schemes to encourage courtyard greening, but the uptake, by all accounts, is gradual.
Who else is reading the manual
Vienna is not alone in this, and it is not the originator of all of it. The sponge-city idea has older roots — in Stockholm’s tree-planting work, and in a wider European and Asian wave of water-sensitive urban design — and Vienna’s contribution has been less the invention than the patient municipal mainstreaming of it. German cities are watching closely. Munich, for one, has been studying sponge-city elements and has moved to apply them in new development and public-space projects, though it has approached the idea as a set of principles to fold into ordinary planning rather than a single headline programme.
What sets Vienna apart is less any one boulevard than the move from pilot to default: the point at which a sponge street stops being a demonstration and becomes simply how a street gets rebuilt. That shift — from the exceptional to the standard — is the line that turns an experiment into infrastructure, and it is the part other cities find hardest to copy.
What it actually feels like
On the Kleistgasse on a weekday afternoon at the end of a hot summer, the street is busy in a way it would not have been a few years ago. Where a parking row used to be there is now shade, seating and room for children to invent the kind of complicated game that involves a chalk circle and a good deal of negotiation. The foot traffic comes in pulses rather than volume, and it stays.
That is the part that doesn’t make it into the monitoring tables. Vienna’s sponge city is, on the official metrics, an infrastructure-adaptation programme. On the unofficial ones, it is a project about what city centres are for in summer. The data is supposed to win the argument; the shaded benches are what make the data feel obvious in retrospect.
The next decade of Viennese street design will be done under conditions the people who built Vienna never anticipated. The temperature numbers are real; the runoff numbers are real; the political memory of the 2019 heat is real enough, several years on, to keep the budget lines largely unchallenged. Whether the rest of the European urban decade follows depends, on the city’s own telling, on whether other cities can copy the patience as well as the engineering.
Sources: City of Vienna (Magistrat der Stadt Wien) — Coole Straßen programme and press releases, 2019–2021; Smart Klima City Strategie (2022) and Wiener Klimafahrplan (2022); Praterstraße redesign (2023–2024); ZAMG / GeoSphere Austria June 2019 temperature reporting; the Stockholm tree-planting (“Stockholm model”) substrate-trench method; and reporting on Munich’s sponge-city planning (Landeshauptstadt München / bdla Bayern).
