This is so cool: Some artists created artwork made out of grass for the 2008 Wimbledon tennis championship. From CR Blog (click the link to see a picture of the Wimbledon artworks):
The artists essentially use grass as a form of photographic paper, projecting a black-and-white negative image onto a patch of grass as it grows in a dark room, and using the natural photosensitive properties of the grass to reproduce photographs.
I don't see why you couldn't use a similar process to make signage for your conference, particularly if your organization has something to do with horticulture (or has some kind of growth theme). It would get my attention, that's for sure.