Tropical plant varieties, such as mosses, orchids, ferns and air plants, are generally kept within closed terrariums due to the conditions being similar to the humid and sheltered environment of the tropics.
Keeping the terrarium sealed allows for the circulation of water, but terrariums must be opened once a week to remove excess moisture from the air and walls of the container. This is done to prevent growth of mould which could damage the plants and discolour the sides of the terrarium.Terrariums must also be watered occasionally, the absence of condensation on the walls of the terrarium or any wilting of the plants is an indicator that the terrarium requires water.
Closed terrariums also require a special soil mix to ensure both good growing conditions and to reduce the risks of microbial damage. A common medium used is 'peat-lite', a mixture of peat moss, vermiculite and perlite.The mixture must be sterile in order to avoid introducing potentially harmful microbes.