David, if the sphagnum moss (I assume that's what you are referring to) will be green if living and brown if dead. Either way, it will absorb water. You simply have to spray it well with a hose for several minutes. To rehydrate completely dry moss, you might have to do this 2-3 times each day for several days until the water is well absorbed. But as Cheryl pointed out, it does dry out fairly fast in hot environments, and water has to be re-applied daily. Lots of my epiphytic plants are mounted on sphagnum, and during the summer months, they all get watered daily unless we have a good rain.
