When architects sketch out a new building, they usually think about steel, glass, and concrete—not bees. But researchers in Plymouth just spent weeks counting insects on city walls covered in plants, and what they found might change how we design our cities.

Why Living Walls Matter More Than We Thought

Dr. Paul Lunt and his team at the University of Plymouth weren't expecting to find much. They surveyed three locations across the city, from the busy centre to the quieter outskirts. What they discovered was hundreds of creatures making homes on these vertical gardens, including twelve different types of bees and hoverflies. These walls weren't just pretty decorations—they were functioning ecosystems.

"We're in the middle of a global biodiversity crisis where wild species are increasingly being threatened by the changing climate and habitat loss," Lunt explained. His team tracked everything from pollinating insects and spiders to soil invertebrates, birds, and bats. The findings revealed something important: not all green walls are created equal.

Plants growing in actual soil attracted far more wildlife than those planted in artificial substrates. That might seem obvious, but it matters for city planners who often choose lighter, synthetic materials to reduce structural load on buildings. The Plymouth study suggests we need to rethink that trade-off.

What Green Walls Actually Do

Halfway around the world in Singapore, ecologist Katharina Hecht from Utrecht University was asking similar questions. Her team analyzed twenty walls scattered across Singapore's dense urban landscape. They wanted hard numbers on what these installations deliver.

The temperature reduction was modest but real—about 0.6 to 0.7 degrees Celsius between the front and back of the walls during daytime. That doesn't sound dramatic, but it adds up. Traditional green roofs can drop rooftop temperatures by up to 40°C compared to standard materials. Plants achieve this through evapotranspiration, releasing water vapor that cools the surrounding air.

The biodiversity results were more striking. These twenty walls hosted over one hundred different animal species—insects, spiders, birds. Compare that to bare concrete or brick walls, which typically support just a handful of species. The walls were working as habitat.

Hecht's team found something else interesting: green walls performed almost as well as natural cliffs in urban settings. Not quite as good, but close enough to matter. The key factor was connectivity. Nearby trees and vegetation patches acted as "stepping stones," allowing species to discover and colonize the walls. A green wall isolated in a concrete desert won't attract much life. Context matters.

The Policy Problem

Here's where things get frustrating. The Plymouth researchers noted that current UK legislation significantly undervalues what living walls contribute ecologically. Planning assessments don't give them proper credit.

Some American cities are doing better. Fife, Washington offers extra points in its Green Area Factor system for green roofs with deeper soil and native plants. Devens, Massachusetts requires native species appropriate to the local climate zone. Portland provides suggested plant lists that emphasize native species and encourages designs with varied soil depths, gravel areas for ground-nesting insects, and water containers during dry summers.

San Francisco took a different angle with its Bird-Safe Buildings standards. The city requires treated glazing near urban bird refuges to prevent collisions—acknowledging that adding green elements without considering birds can create deadly traps.

The National League of Cities is now partnering with the Biophilic Cities Network and researchers at Notre Dame to study the UK's Biodiversity Net Gain policy. They're looking for lessons American cities can apply. A 2025 report called "Catalysing biodiversity on buildings" by Second Nature Ecology + Design, IUCN, and Holcim laid groundwork for these conversations.

Beyond the Buzzwords

The benefits stack up quickly. Green infrastructure filters air pollutants and dust while producing oxygen. It manages stormwater by filtering rainwater runoff before it hits overwhelmed sewer systems. The vegetation dampens sound, reducing noise pollution in dense neighborhoods.

For pollinators struggling with habitat loss, these walls and roofs provide crucial resources. Design them with native, nectar-rich plants and they become feeding stations for bees and butterflies. They offer shelter and breeding grounds in areas where traditional gardens simply don't fit—think downtown cores where every square meter of ground is already spoken for.

The energy savings are real too. Cooler buildings need less air conditioning. That lowers utility bills and reduces the urban heat island effect that makes cities swelter during summer.

What Actually Works

The research points to some clear design principles. Real soil beats artificial substrates for supporting biodiversity. Native plants outperform exotic ornamentals because local insects evolved alongside them. Deeper soil allows more diverse plant communities and better supports invertebrates.

Connectivity cannot be ignored. A single green wall on an isolated skyscraper won't accomplish much. But integrate it into a network of parks, street trees, and other green infrastructure, and species can move through the urban landscape. Think of it as building highways for wildlife.

Variety helps too. Mixed plantings with different heights and structures support more species than monocultures. Leave some bare gravel or sand patches for ground-nesting bees. Include water features during dry seasons. These small touches multiply the habitat value.

The Bigger Picture

Cities are still growing. More people live in urban areas now than ever in human history. That trend isn't reversing. We've typically thought about cities and nature as opposites—places where we've replaced ecosystems with concrete.

But research from Plymouth to Singapore shows we don't have to accept that binary. Buildings can host life. Walls can feed pollinators. Roofs can cool neighborhoods and shelter birds.

The challenge isn't technical. We know how to grow plants on buildings. We understand what designs work better for wildlife. The challenge is whether city planners, developers, and policymakers will prioritize these benefits in their decisions.

When researchers count twelve bee species on a city wall, they're not just documenting insects. They're showing us that urban spaces can actively support the biodiversity we're losing elsewhere. Every building is potential habitat. Every roof could be a meadow.

The question is whether we'll build cities that way.