Here's one way of creating eco-friendly street art. Graffiti artist Stefaan de Croock used a pressure washer to carve out his designs into the natural dirt and moss. And the results are quite amazing:

This series of images showcases Croock's signature "subtractive graffiti" style, which was cut into a wall of moss growing outside the STUK art center in Leuven, Belgium. Since the moss grows back quickly within a few weeks, the art is only there temporarily. But you can see the rest of Croock's work over at his website.