John Metcalfe is CityLab’s Bay Area bureau chief, based in Oakland. His coverage focuses on climate change and the science of cities.
It’s reportedly easier to maintain than soil and mulch.
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.
The city’s transportation department is testing out the stuff on 13 downtown trees. (It’s being provided for free by manufacturer K.B. Industries in Largo, Florida.) “The highly porous material can displace water quickly and can be installed without migration meaning it doesn’t get washed away or moved by external forces like weather,” the department writes, outlining what it believes are advantages:
- A safe, stable surface for pedestrians
- Allows air and water to pass into the soil to keep street trees healthy
- Zero to low maintenance with no weed or unplanned vegetation growth and removal
- Cheaper than traditional tree grates and doesn’t wash away
It’s also environmentally friendly, as the Flexi-Pave used in Seattle represents about 220 car tires saved from the dump.