On Riyadh’s King Abdulaziz Road, construction workers sweep sand off a brand-new sidewalk. A gardener pops a spindly plant out of a plastic pot and gives it a new home in the ground, smoothing the soft red earth with his hands.

Behind them, 15-foot-high banners advertise the future gardens and canals of King Salman Park, a plan to turn an air base in Saudi Arabia’s desert capital into a public green space four times bigger than Central Park in New York.