I have been thinking about a Cactus Bonsai for years. Colorado does have one cactus that does grow into a small tree, they are commonly called Cholla. This is one tree you would not want to climb. I did find an Opuntia whipplei ‘Snow Leopard’ (Silver Cholla) that I liked so I bought it and have made a page for it under Deciduous trees, list under Tree Information. I really like this tree allot.

2018 Opuntia whipplei ‘Snow Leopard’ (Silver Cholla) Bonasi