As Willow as said already they will manage to find a way. It just takes much longer and is hard for them to. Wood is usually not a problem in getting plants to attach too. Rocks on the hand can be. If they are too smooth it is difficult for them.When you say you get better rooting from a rougher surface. When attaching to bog wood would it be worth sanding the surface to make it not so smooth, or even putting some cuts into the wood.
As these are not in substrate would I need to add fertiliser?