I found a way to get very nice moss growth. What you basically want to do is "damage the moss". When I get a clump of moss that I want to tie to a hard structure I put it onto a cookie sheet with a little bit of water on it to keep it moist. I then use scissors to cut it into small nickel sized flat patches. I then take that patch lay it flat onto the cookie sheet and use a bottle cap and press firmly onto the moss. What you want to do is break some cells open to encourage new growth from the damaged areas. You want to damage several areas (hence the bottle cap) but not mush the moss. Then put the patch on the wood and wrap it well with string. Rinse and repeat until the desired area is covered. I found this way the moss fills in a lot faster than if just tied to the wood. And if you don't overdo it, the moss will be fine.