Urban vertical gardens are not as green as they look

Spend it Better: Forget costly landscaped walls and preserve natural habitats instead

There are trends in the new green world ahead. One of them is the idea of green walls. These vertical gardens use the sides of buildings to add to our green spaces. They put the jungle in urban jungle, clean the air, maybe even cool the buildings, their creators promise.

Botanist and nature conservation advocate Dr Melinda Lyons is sceptical of the green wall trend. “It’s a fad that’s not relevant to biodiversity,” she says. She was astonished to see a green wall promoted recently as a habitat for pollinators. It was made up entirely of ferns, which was the bee equivalent of a buffet of beautiful fake plastic food. Because ferns don’t produce pollen.

We have a green wall in our garden. It didn’t need scaffolding, or shipped-in plants that need watering when it’s dry. It grows wild from the ground without any help from us, costing nothing and giving back so much.

Hello ivy. Glossy green leaves provide a home for the holly blue butterfly. During these early spring hungry gap days, contented pigeons sit in its branches and peck berries off it. The foxes use it as a wildlife corridor. Ivy is a brilliant coloniser of walls but also a loathed plant, the gardener’s enemy, swallower of space, opportunistic scrambler.

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“In the context of gardening I think ivy is great,” says Dr Lyons. It flowers later in the year and berries are great for the birds. “I think ivy is vastly superior to anything else.” One drawback is that when ivy takes over the small ferns, mosses and lichens that might colonise old walls are shaded out. And the keepers of places often respond to an invasion of ivy with a scorched earth approach.

On a field trip with students to an old graveyard recently, Dr Lyons noticed “these very old walls were far too clean”, evidence possibly of an ivy-clad wall that had been recently cleared.

Rather than creating horticultural versions of habitat, Dr Lyons says the imperative is to protect and conserve existing natural habitats. This means understanding where they are, what conditions they are in, valuing their complexity and ensuring that they’re not damaged. “If we mind the habitats, we mind all the species, all the things we don’t see, the invertebrates, the fungi, the spiders … Conserving the habitats is the single biggest thing. We need to stop damaging or losing our natural habitats.”

Left alone, many walls will green themselves. The builders of towns and cities, be they local authorities or private developers, could spend the funds that might be allocated to expensively landscaped green walls on preserving the natural habitats we already have.

Catherine Cleary is co-founder of Pocket Forests