Bitumen –an oil based ingredient in asphalt- is organic and
gives nutrients to the moss' spores. The growth has begun.
The coarse surface of the asphalt gives good hold for the moss and it bounds damp and dirt –which also contains nutrients. Therefore the development increases and the moss' is a fact.
Moss will constantly recur as long as spores are left in the surface, and the only way to counteract the affections is to sterilize the growth and to disinfect the surface.