Because of the different conditions within, terrariums can be classified into two types: closed and open.
Tropical plant varieties, such as mosses, orchids, ferns, and air plants, are generally kept within closed terraria 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. The terrarium may be opened once a week to remove excess moisture from the air and walls of the container. This is done to prevent growth of mold or algae which could damage the plants and discolour the sides of the terrarium. Terraria 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 terraria 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.
Open terraria are better suited to plants that prefer less humidity and soil moisture, such as temperate plants. Not all plants require or are suited to the moist environment of closed terraria. For plants adapted to dry climates, open, unsealed terrariums are used to keep the air in the terrarium free from excess moisture. Open terraria also work well for plants that require more direct sunlight, as closed terraria can trap too much heat potentially killing any plants inside.
Note that succulents, despite being a popular option, are a poor choice for terraria – open or closed. The intrinsic lack of drainage in a terrarium will inflict root rot or require underwatering.