Protecting street trees can be arduous. Rain washes mulch and topsoil into the gutter. Pedestrian traffic compacts the ground and harms their health. Installing metal grates to keep people off is expensive.
Seattle believes it might have found a way to make the job easier. It’s a weird substance called Flexi-Pave, and it’s composed of granite aggregate and recycled auto tires. City crews spread it as a crumbly mix around the base of trees, and it hardens into a permeable material not unlike super-airy pumice.