A terrarium garden is a garden grown in a container that lets in light, and sometimes moisture. Usually, it means that plants are being grown in a jar or aquarium. The top can be open or sealed. Sealed terrariums have reached a point of near perfect moisture and do not need to be watered.
Most terrarium gardens should be kept in bright, filtered light. Direct sun might cause them to heat up too much, especially in the summer. Direct sun might also encourage algae. Too little light can cause plants to grow very "leggy" and unhappy as well.
Make sure to keep the environment moist for most plants, or on the dryer side for cacti and succulents. If the container is sealed or covered, it may virtually never need water. Covered containers should also be monitored for a month or two in order 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 stabilize, and the terrarium won’t need as much care. Terrariums without lids need more water and care, since moisture will be lost to evaporation. Watering is usually best done with a few squirts of a mister, in order not to disturb the environment too much. The water will make its way into the mini ecosystem.
Building a terrarium garden
A terrarium garden can be as simple as a little dirt and a plant in an old pickle jar. If your plants are sensitive to too much water, or you are afraid you might overwater, then adding gravel and carbon at the bottom first are a good idea.
You container can be an old jar, a fishbowl, an aquarium or anything else. It should be glass (or clear) to let in light. The top can have a lid or not, glass or otherwise. A bigger container, like a 10-gallon fish tank give you more room to maneuver, work in and room for error in watering, but even a fishbowl or old jar will work. If your hand fits in the opening, that of course makes setup easier.
Spread gravel, preferably a natural kind like pea gravel, an inch or two thick on the bottom if you might overwater the garden. An optional layer of sphagnum moss or burlap over the gravel will help prevent the dirt from settling into the gravel. Then, if you wish, you can put a quarter-inch layer of charcoal over the burlap or moss in order to absorb odors. Finally, add at least 2 inches of potting soil, or more, depending on the size of your plants and garden.
Finally, you plant the aquarium with the plants you've chosen. If it's very small, it might hold only one or two. You might first try with very small, pretty hardy plants that do well in medium-moisture environments, like pathos, ferns, moss, ivy and bromeliads.