Instead of tossing trees into the shredder, the city is launching a program this month to collect them and turn them into an environmental workhorse known as biochar. This charcoal product can be mixed into soil to greatly improve its drainage and nutrient levels, spurring vibrant growth for more plants. Meanwhile, the heat created by the charcoal-making process will be siphoned off and fed into the city’s district heating system.
So far, so great. But while the idea of Christmas trees re-entering the soil and helping new trees to flourish is delightful, it’s only the tip of the iceberg here.
That’s because Christmas trees are just among the first sources of green waste to be used in what could be one of the most ambitious, potentially influential projects coined by a European city thus far. By bringing together the parks department, the city’s waste disposal service, energy providers, and urban gardeners, Stockholm’s biochar project will create a virtuous cycle so ingenious—and ultimately so simple—that it could provide a template for cities across the world. [...]
Embrén and his colleagues started helping city trees with a new type of soil covering that proved effective in stimulating growth: crushed bedrock on top of sand, clay, and peat. By making the ground more porous, this substance also helped the ground absorb more stormwater, creating an urban soil management process that has already gained some renown as the so-called Stockholm Solution.
