Spread gravel, preferably a natural kind like pea gravel, an inch or two thick in a glass container. Rose recommends using a ten-gallon fish tank, which is inexpensive and has a large opening that makes it easier to work in, but smaller containers like fishbowls will work as long as they are transparent. Whatever size you use, it helps if your hand can fit through the opening.
A layer of sphagnum moss or burlap over the gravel is an optional step, to keep the dirt that will go on top from seeping into the gravel. Next, spread about a quarter-inch layer of charcoal over the gravel to absorb odours. Then add at least two inches of potting soil, or more depending on the types and sizes of your plants.
Finally, place your plants inside the terrarium. Smaller containers will hold only two or three, and some might hold only one. Rose suggests using very small, relatively hardy plants that do well in medium-moisture environments, like pathos, ferns, moss, ivy and bromeliads.
Water or mist the terrarium sparingly, but keep it moist. If you’re using a container with a cover, monitor the terrarium for a month or two to make sure it does not get too moist (condensation will form on the glass, and mold and fungus might appear on the plants and in the soil). Adjust the lid, or remove it, to temper the amount of moisture; eventually it should stabilise, and the terrarium won’t need as much care. Terrariums without lids require more water and care, as moisture is lost to evaporation.
Either way, “it’s an ongoing experiment,” Rose said. “You’re trying to create a microclimate, that’s what separates a terrarium from a flower pot.”