I use Cypress mulch for all my adult snakes and newspaper or finely shredded coconut on my babies, unless I take them out and feed them in another container. The only issue I have had with Aspen (and why I am careful feeding my snakes on the Cypress bedding) is that the wood sometimes gets stuck to the food, especially if it is damp, and the snake can swallow it and get the wood caught in its intestines and cause problems. Cypress is by far the cheapest for 3$ a bag at any warehouse hardware store and is fine with snakes. The finely shredded coconut bedding also sold as Eco Earth, is fine with small snakes and lizards in that if it gets stuck on the food, it will not hurt the animal to a great extent at all. I actually have some of my baby snakes on Cypress, but I do put them in separate containers for feeding. A double benefit of this practice is that the snakes get some human exposure and this is always a good thing.

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