My mind is officially blown.
Ever heard of moss paint? Me neither. But today I came across this phenomenon via my current favourite website craftgawker and you can find the instructions here on instructables but essentially all you need to make a masterpiece like this is some buttermilk, sugar and a few lumps of moss. Put them into a blender, mix into a paste and then just paint it on to a wall to whatever design pleases your good self.
The hulking great concrete wall in our back yard is just screaming out for this treatment but I'm not sure what our landlady would make of it. If you own a house or business, please go out and do this immediately and let me see the results!