Scroll down to plastic plants made by ZOO MED. One store canceled my order because this line of product is no longer being made so buy them now if you need them. I bought 5 bags of plants to make clumps of trees for a Darkest Africa wargame so I need affordable / easy to make bases.
In effect its a mat of sand and gravel glued to a fabric base for terrariums.
I use it cut into irregular shapes as rough terrain areas, though I do give it a coat of paint to consolidate the sand and match other terrain features. I guess you wouldn't need to paint it if you didn't want to.