ON A CLOUDY APRIL AFTERNOON in Seattle’s Licton Springs neighborhood, middle schoolers in gardening gloves fan out across what used to be a lawn on the east side of Robert Eagle Staff Middle School, carefully spreading mulch around what is now a 3,000-square-foot strip of waist-high saplings. The air smells faintly of compost. Someone points out a bud that wasn’t there yesterday.

This is Seattle’s newest mini forest, one of several planted in the city in the past few years using the Miyawaki method, a technique developed in the 1970s by Japanese botanist Akira Miyawaki to restore small, degraded, often unconventional spaces quickly.…