“Ackroyd and Harvey stumbled onto this technique after producing an installation that involved covering an indoor wall with living grass,” he continues. “A ladder was leaning against the wall, and the artists noticed that even after it was removed, a faint outline of the ladder remained on the grass. They set about experimenting with ways of enhancing this effect, and soon they were using a slide projector as an artificial light source for growing their unique photographs. A typical exposure time is just over a week, with the image projected for 12 hours a day.”
Here's a photo from flickr user bowdie of some other grass images they created. Brilliant!
via Creative Review