the rhacodactylus book does talk a bit about vivariums. several past reptile magazine issues had articles about constructing vivariums as well. the key ingredient to a natural vivarium is having a biologically active soil base, which breaks down fecal matter naturally, leaving no smell in the cage. spot cleaning may be needed to wash off droppings on planets etc. as for what sort of plants to use, any plant that handles humidity well, doesn't require alot of heat, or does well in low light conditions, would be best. as it isnt' necessary to have bright basking spots in a crested gecko cage, having full spectrum lighting for your plants to florish in would be all that's necessary.
doing a google search on vivariums should bring up alot of how to articles on how to build them. keep in mind that crested geckos are not from steaming hot rainforests, but more temperate forests, with temperatures that rarely exceed 85F. the rhac book is great for giving alot of info about the natural habitat of crested geckos and other rhac species.
there is also the chat with anthony caponetto this coming saturday. as he deals alot with crested geckos, he may know some tips on how to design a natural vivarium or at least what plants work best with crested geckos. for more info about the guest chat with anthony, check here;
www.kingsnake.com/articles/HerpChat8.html
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PHLdyPayne