Terrariums are usually sealable glass containers containing soil and plants, and can be opened for maintenance to access the plants inside. However, terrariums can also be open to the atmosphere rather than being sealed. Terrariums are often kept as decorative or ornamental items. Closed terrariums create a unique environment for plant growth, as the transparent walls allow for both heat and light to enter the terrarium. The sealed container combined with the heat entering the terrarium allows for the creation of a small scale water cycle. This happens because moisture from both the soil and plants evaporates in the elevated temperatures inside the terrarium. This water vapour then condenses on the walls of the container, and eventually falls back to the plants and soil below. This contributes to creating an ideal environment for growing plants due to the constant supply of water, thereby preventing the plants from becoming over dry. In addition to this, the light that passes through the transparent material of the terrarium allows for the plants within to photosynthesize, an important aspect of plant growth.
The first terrarium was developed by botanist Nathaniel Bagshaw Ward in 1842. Ward had an interest in observing insect behaviour and accidentally left one of the jars unattended. A fern spore in the jar grew, germinated into a plant, and the terrarium was born. The trend quickly spread in the Victorian Era amongst the English. Instead of the terrarium, it was known as the Wardian Case. The story goes that Ward hired carpenters to build his Wardian Cases to export native British plants to Sydney, Australia. After months of travel, the plants arrived well and thriving. Likewise, plants from Australia were sent to London using the same method and Ward received his Australian plants in perfect condition. His experiment indicated that plants can be sealed in glass without ventilation and continue thriving.
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 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.
Open terrariums are better suited to dry plants such as succulents. Not all plants require or are suited to the moist environment of closed terrariums. For plants adapted to dry climates, open, unsealed terrariums are used to keep the air in the terrarium free from excess moisture. Open terrariums also work well for plants that require more direct sunlight, as closed terrariums can trap too much heat potentially killing any plants inside.