I have a bit of an odd inquiry and not sure where the best place to post it would be, so I figured I would try here. I have numerous recently built retaining walls and steps which are made using Allen Block (some pics below). My question is, to give an impression of age and character I have added sedum amongst any cracks I can find but would also like to incorporate some Lichen as well? Is this possible? If so, how do I go about finding some Lichen and then "planting" it on to my walls and steps? Will Lichen do okay on a porous concrete based product? Any suggestions as to which Lichen may do well and look good in this area? Thanks - any advice you can provide would be very much appreciated.