I have read many posts on building terrariums. The majority of these posts recommend a drainage layer at the bottom of the terrarium. In my experience, not only is a drainage layer unnecessary, it’s not healthy for the terrarium plants; overall health of the terrarium; and ends up looking rather unattractive and dirty.
There should never be so much water in your terrarium that it collects at the bottom. As is the rule for potted plants, it is better to err on this side of under watering. If water makes its way down to the drainage layer, the soil just above will be saturated and the water will eventually stagnate. The excess water will also promote root rot and shallow roots in the terrarium plants.
The solution… use a well draining soil mix for your terrarium. After planting your terrarium, water sparingly. Watch your terrarium and add water as needed.
If you would like to include a layer or gravel for decorative purposes, you can cover it with a weed barrier or other horticultural fabric to prevent water and soil from draining into the decorative layer.
In the image above, a mound of moss is planted in a dish and placed on striped tiger stone.