Trees on top
Rewilding rooftops with the Urban Jungle Project
By Dr. Nadina Galle
“Welcome to the jungle,” Daan Grasveld says with a grin, steadying the wobbly ladder as I climb the final rung. What greets me is an unexpected sight: a meadow of wildflowers in full bloom, the hum of busy bees and seven full-sized trees swaying gently in the breeze. At first glance, it might look like an ordinary Amsterdam park — but then I remember: we’re five stories above the ground. The telltale sign? The treetops of the cypresses at street level just barely peek over the roof's edge.
This isn’t just a green roof — it’s a living example of modern-day rewilding. In 2020, Grasveld co-founded The Urban Jungle Project, transforming rooftops into vibrant ecosystems that challenge our perceptions of underutilized space.
“It all started with a heat problem,” Grasveld explains as we walk along the rooftop. “Residents of VvE Blueland were facing unbearable temperatures on the top floors. They wanted to install air conditioning, but some suggested turning this unused gravel roof into something greener.”
VvE Blueland is a homeowner’s association managing a modern apartment complex in Amsterdam’s Oost district. Constructed in 2008, the building houses 50 apartments and boasts a 650-square-metre flat rooftop. Once covered in gravel, its potential as a green space became clear during the pandemic, as residents began exploring ways to address heat stress.
“Was it an easy sell?” I ask, imagining the logistical hurdles.
Grasveld laughs. “Not at first. The roof had a weight limit of just 230 kilograms per square metre, so traditional solutions were out of the question. But we specialize in lightweight, modular green roofs, so we proposed an ambitious idea: what if we could create a real ecosystem here, complete with trees?”
Together with partners like Rooftop Revolution and the Nationale Bomenbank, The Urban Jungle Project replaced the gravel roof with 2,321 plant crates and seven innovative “Urban Trees.” These trees are suspended in cube-like structures designed to evenly distribute their weight while providing essential nutrients and stability that traditional soil would. “Normally, a tree in the ground weighs 16,000 kilograms. With our root ball, we bring that down to just 450 kilograms,” says Grasveld.
Each plant crate contains carefully selected vegetation — 10,000 plants in total — chosen for their ability to thrive in limited soil and attract pollinators like bees and butterflies. The result? A dynamic rooftop ecosystem that buffers 38,000 litres of rainwater, reduces heat stress and provides shade and cooling for the surrounding urban environment.
As we approach the edge of the roof, I notice the view: a sea of barren rooftops. “The neighbors on the other side, who overlook this green roof, must love it,” I remark. “Do you think they should help fund it? After all, they’re benefiting from the greenery without bearing the cost.”
Grasveld nods thoughtfully. “We tried a crowdfunding campaign to involve the neighborhood, but it didn’t gain much traction. The concept was new at the time, but I’m confident that as more projects like this emerge, future campaigns will be more successful.”
The urban trees are a standout feature. Suspended in modular frames, they are sustained by advanced irrigation systems that deliver water and nutrients directly to their roots with precision. Integrated sensors continuously monitor soil moisture, temperature and nutrient levels, relaying data to a central system that dynamically adjusts irrigation and fertilization in real time. “In the future, we aim to implement a closed-loop system,” Grasveld explains. “Water will drip onto the root ball, with any excess captured, recycled and reused — maximizing efficiency and minimizing waste.”
Despite its successes, Grasveld admits a missed opportunity: “We didn’t take baseline temperature measurements before installing the green roof,” he acknowledges. “But the residents' stories speak for themselves. Three summers have passed since installation, and no one has brought up air conditioning again at a homeowners association meeting.”
The project’s funding offers key insights into urban green initiatives. Half of the cost was subsidized by the City of Amsterdam, with the rest covered by the homeowners association. Why would the city invest so heavily in a private rooftop? “It’s simple,” Grasveld explains. “Amsterdam is running out of space at ground level to meet goals for rainwater management, biodiversity and urban cooling. Rooftops are an untapped resource.” By retaining rainwater, supporting pollinators and mitigating the urban heat island effect, projects like this align perfectly with municipal sustainability goals.
This model offers valuable lessons for other cities. Grasveld believes that partnerships between local governments and private entities are crucial for scaling urban green infrastructure. “The key is showing mutual benefit,” he says. “Cities meet their sustainability goals, while residents enjoy cooler homes, better air quality and increased property value.”
The project hasn’t been without challenges. Grasveld recalls the skepticism he faced: “A lot of people said trees belong in the ground, not on rooftops. We don’t disagree, but this isn’t about replicating a forest. It’s about finding ways to bring nature into urban spaces where it wouldn’t otherwise exist.” This mindset has driven The Urban Jungle team to push the boundaries of possibility.
Beyond its technical innovations, the project serves as a blueprint for what cities around the world could achieve. “Eighty per cent of the buildings in 2050 are already here,” Grasveld notes, stressing the importance of retrofitting existing structures to combat heat islands and climate change. Modular green roofs like the one atop VvE Blueland offer a scalable, sustainable solution that can be replicated in dense urban areas.
The Urban Jungle Project’s impact extends past environmental benefits. Residents report cooler apartments during summer heatwaves. Neighbors and passersby enjoy the sight of greenery and the buzzing life it attracts. The project has also become a hub for research, with Wageningen University studying its effects on biodiversity. Researchers regularly analyze soil samples, insect populations and plant health, turning the rooftop into a living lab.
Grasveld's ambitions don’t end here. He envisions connecting rooftops across the city to create green corridors. “We’re already seeing interest from other cities,” he says. “The goal is to turn rooftops into part of an ecological network that supports biodiversity, much like parks do — just elevated.”
This approach to urban rewilding has sparked projects across Amsterdam and beyond. The Urban Jungle Project has completed over 25 landscapes from playgrounds, community gardens and corporate spaces, demonstrating their versatility in enhancing urban life. Each project begins with a thorough analysis of the roof’s load capacity, followed by quick installation. The pre-assembled modular crates allow for fast setup, minimizing disruption. Because the crates can be moved, roof issues can be addressed without removing the greenery — if a leak occurs, the crates are moved aside for repairs and then put back.
“The impact of these projects goes beyond just adding greenery,” Grasveld says. “It’s about showing what’s possible. If we can make this work on a rooftop with limited capacity, imagine what we could achieve with the untapped potential of urban spaces everywhere.”
“And the resistance?” I ask, recalling the initial doubts.
“Of course, there were and are skeptics,” he replies with a wry smile. “But we proved it works, one project at a time. And now, it’s happening.”
As I step down the ladder, leaving this urban jungle behind, I think to myself: If they can build it here, why not everywhere?
Dr. Nadina Galle highlights cutting-edge Internet of Nature (IoN) technologies and the practitioners transforming soil health, urban forest management, the creation of more livable cities — and everything in between.