On a drizzly late-autumn day in the northern English city of Sheffield, Nigel Dunnett is gazing lovingly at a plant-clad patch of roadside. We’re in an unloved corner of Sheffield – all roads, roundabouts and neglected vacant lots – but through it runs this 1.5-kilometre ribbon of walkways, cycle paths, planted verges and beds. This is Britain’s largest street garden, known as Grey to Green.

Dunnett – the horticulturalist, landscape architect and ecologist who designed it – informs me there’s a riot of wildflower colour dormant within, awaiting its season. A normal clump of urban greenery might be just a couple of neatly clipped bushes. Here there are 70 or 80 different species. “The diversity and health of this area is as good as an English garden,” Dunnett enthuses. “People come from all over the world to see this. To see the scale, the character. And to understand how a council can afford it.”