I would do like a 3 inch layer of coir with a 1 inch layer of cypress mulch on top of that. Ive done this before and many different plants grow in the coir pretty well and the cypress keeps the moisture around the roots. Alot of the "grassland" plants grow too big for a terranium. I had devils ivy(pothos), bamboo, and some of the air orchids growing in my tank. The pothos THRIVES lol. I got the air orchids at one of the expos. I just used some fishing twine to tie them to branches I had in the tank. They didnt really grow but they do really look cool. I put a few drops of miracle grow around the roots of the plants like once a month and I think that helped them. My snake has never burrowed so i wasnt worried about him being hurt by it and it was only a few drops once a month. I hope this helped, if you have any other questions let me know