drdawg said: 
Gravel works, but for me I use cypress mulch (4" deep), overlaid with greenhouse fabric. I think the cypress holds and evaporates water for a much longer period.
I forgot to say that the floor right now is hardware cloth to keep out burrowing critters, covered with pea gravel and I have square pavers over that. Haven't finished the pavers. I need to cut some to fit the dome shape. If the floor is wet, it does stay wet a long time, and should raise the humidity.
Yes, Ursula, the holding pond inside does raise the humidity a bit. It would be much lower if it didn't exist. You can see pics of the holding ponds on the Growing spaces website. I have a shelf across mine for plants that like more humidty. I could hang a few orchids above it.
