Guys:

Problem with lichen is that it eventually dries out and gets brittle, and you have to cook it back with a glycerine and water solution. Advantage to that is you can re-dye it to add another color.

There is a new extruded foam-plastic stuff that looks a LOT like lichen, but is 100% synthetic, and won't dry out and shatter. Even comes in different colors. Look in the model railroad area of the model shop, and ask a staffer about the synthetic lichen if you don't see it.

I had bags full of lichen ( bought and home-cooked) when I played Classic BattleTech w/ miniatures (200+)