Berlin's 12,000 Volunteers Water 900,000 Trees via App

Amanda Gläser-Bligh hauls five gallons of water down her apartment stairs every week to water Linda, the eight-year-old linden tree she's adopted outside her building in Berlin. She's one of 12,000 residents keeping nearly a million city trees alive through increasingly brutal summers.

The Gieß den Kiez app, which means "water your neighborhood" in German, maps all 900,000 street trees across Berlin and color-codes them by thirst level. It calculates each tree's water needs based on age, species, and recent rainfall, updating in real time so volunteers can see which trees are desperate and which ones someone else just watered.

CityLAB Berlin launched the project in 2020, perfectly timed with Covid lockdowns when people were searching for reasons to leave their homes. The biggest watering spike came during those early pandemic months, when volunteers poured nearly half a million liters onto thirsty trees.

Project leader Anna Mehrländer expected people would care about trees. She didn't expect them to treat them like family members. "I was really surprised at just how deeply people care about trees. For many people it's almost like caring for a pet," she says.

The app solves a coordination problem city maintenance crews can't handle alone. On June 27, when Berlin hit an all-time high of 103.8 degrees Fahrenheit, volunteers delivered 2,100 gallons of water in a single day. Saplings get flagged as high priority because they lack the deep roots that help mature trees survive droughts.

Gläser-Bligh says the app gives people permission to do something they already wanted to do. "Many people may want to water the trees, but they might feel weird about doing it or they're not sure if it's their responsibility," she explains. Passersby who stop to watch her usually walk away looking at street trees differently.

The Ripple Effect

Urban forestry expert Dr. Cecil Konijnendijk says cities are finally recognizing trees as critical infrastructure, not decoration. Studies show they cool air during heatwaves, reduce flooding, cut air pollution, and improve mental health. The trees already in the ground matter most because new plantings take 30 to 40 years to deliver the same benefits.

The code is completely open-source. Residents in Potsdam, Leipzig, Magdeburg, and Paris have already launched their own versions. "Anyone can copy the code and set up their own Gieß den Kiez," Mehrländer says. "The main requirement is a data set of all the trees in your city."

Most cities already have that tree data sitting in some municipal database. Finding people willing to show up with a watering can turns out to be the easier part.