Answer: As luck would have it, I did a similar project for a family member a few seasons back. I used succulents and they turned out great. More specifically, I bought a "succulents mix" from a local nursery that contained plants such as Hens & Chicks (Sempervivum tectorum), a variety of sedums, etc. I packed a little soil mixed with a bit of sand (if memory serves me, I used 60% potting soil, 40% sterile sand) into the crevices, and stuck the plants into the crevices gently (I used chopsticks to tuckthem in). Some of the bloomed some of them didn't, but, if I do say so myself it looked great! You should be able to obtain these type of succulents from any garden center, nursery, or greenhouse. I have even seen many varieties at the grocery store.
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