While graffiti has been a vital form of artistic expression since the age of cave drawings, those toxic spray paint cans are an environmental nightmare. However, we've discovered two forms of green graffiti that breathe new life an old idea.

"Imagine an art form that uses living, respiring material; an art form that creates mini eco-systems in often complex and beautiful patterns," writes Anna Garforth at EnvironmentalGraffiti.com. "Welcome to the world of green graffiti—masterpieces crafted in moss and grass. Here you'll find some of the most incredible works of this rare and unusual art form and how to make it yourself."

Interested? TreeHugger.com has the recipe:

- 1 can of beer

-

-Several clumps garden moss

"You will also need a plastic container (with lid), a blender and a paintbrush.To begin the recipe, first of all gather together several clumps of moss (moss can usually be found in moist, shady places) and crumble them into a blender. Then add the beer and sugar and blend just long enough to create a smooth, creamy consistency. Now pour the mixture into a plastic container. Find a suitable damp and shady wall on to which you can apply your moss milkshake. Paint your chosen design onto the wall (either free-hand or using a stencil). If possible try to return to the area over the following weeks to ensure that the mixture is kept moist. Soon the bits of blended moss should begin to recuperate into a whole rooted plant—maintaining your chosen design before eventually colonizing the whole area."

An alternative, completely carbon neutral concept comes courtesy of GreenGraffiti.nl (although they do couch it in more "advertising" terms): "With the help of a template and a high pressure water sprayer an advertising message is cleaned out of the dirt. The result is a high impact message and a cleaner street."