Terrariums can take many forms; all you really need is a clear glass container without drainage holes that's large enough to fit the plant or plants without them hugging the sides, which can lead to disease. You don't need an aquarium-sized home; a small round vase or candle holder can accommodate one special plant. You can find some unique containers by searching antique shops and second-hand stores. Glass bell jars are especially beautiful.
1. Choose compatible plants. Plants that appreciate the atmosphere in a terrarium are those that thrive on humidity and indirect light. Choose plants that will not grow large or quickly. Some good options are ferns, bromeliads, Swedish ivy, baby tears, creeping fig, oxalis, iresine, fittonia, pilea, miniature begonias and anthuriums, and carnivorous plants such as pitcher plants, venus fly traps, and sundews, which can be very temperamental. Combine plants of different heights, colors, and leaf shapes just like you would in a garden bed.
2. Creating the planting area. Since the container doesn't have drainage holes, spread a 1- to 3-inch layer of pea gravel over the bottom so excess water can drain from soil.
Add a 1/2-inch layer of activated charcoal (the type recommended for aquariums) to act as a filter and purify the water and air as the organic materials gradually decompose.
Spread a thin layer of sphagnum moss on top of the charcoal to keep the soil layer from sifting down into the gravel and clogging it.Finally, spread a thick layer (depending on container size) of a soil mix consisting of one part coarse builder's sand to two parts sterilized potting mix. Moisten the mix beforehand. If space allows, sculpt some hills and valleys.
3. Plant your garden. Use miniature hand tools or a fork to rake the soil surface smooth, then make small planting holes. Remove plants from their pots and plant, firming soil gently.
Add rocks, or shells, or even miniature pink flamingoes if that tickles your fancy. Mist the plants and leave the lid off for a couple of days until the foliage dries. Then close the lid.
In a completely closed system, the terrarium should need very little water. Moisture will collect on the insides of the glass and drip back down onto the soil and plants. If condensation becomes excessive, just open the lid to air it out for a day or two. When the plants need water, do so gently and sparingly.
Even in a terrarium without a lid, err on the side of too little water rather than too much because there's no place for the water to drain.
Keep the terrarium in bright light but out of direct sunlight, which can cause the temperature to build up inside the glass.
Don't fertilize the plants regularly because you don't want to encourage rapid growth and fertilizer build-up, but if plants seem to need a boost, use a very dilute (1/4 strength) solution.
Snip any leaves that touch the glass, and pinch the tips and trim plants as needed to keep them in bounds. You may need to replace a plant now and then.