Be careful not to over-water your terrarium, as this could cause the roots to rot and eventually kill the plants. Water infrequently, especially in closed containers. It is also a good idea to use distilled or filtered water for insectivorous plants, mosses and lichens, as they are highly sensitive to pollution.
Carnivorous plants like Venus's fly trap (Dionaea muscipula) and pitcher-plant (Sarracenia purpurea) are well-suited for terrarium planting but they require specific conditions and a dormancy period. It is also important to avoid collecting plants in the wild.
In a completely closed terrarium, water droplets may sometimes form on the walls after you have watered. If this happens, open the lid and allow the terrarium to dry out a bit. Fertilize once a month for healthy plant growth during the growing period, from April to September. Use a diluted organic fertilizer (7-7-7) like fish emulsion; synthetic fertilizers can cause salts to accumulate.
Remember to maintain your plants regularly as well, by removing dead leaves and branches before they start to rot. You may need to trim ground-cover plants if they become too invasive.
Finally, do not place your terrarium in full sunlight, for the sun’s direct rays can burn the leaves.