The first terrariums were closed systems enclosed in glass designed to maintain high humidity for tropical plants. Plants grew in fertile, peat-heavy organic soil and, when the “glass house” worked efficiently, seldom needed water because the plants recycled moisture. Open the top on a traditional terrarium and you'll expose plants to conditions more akin to a desert than a rainforest. With a few other adaptations, a terrarium can showcase cacti and succulents.
Build an Open Environment
The Victorians' Wardian cases were little houses of glass with hinged roofs or doors that created a sealed environment, notes the University of Missouri Extension Service. Some evolved to be greenhouse conservatories and some came indoors as terrariums. For desert plants, you'll want to take the top off a terrarium. Use the most open container you can find, such as a shallow aquarium or globe with a wide opening at the top. If you're skilled in working with glass, cut off the top of a commercial water bottle on an angle and grind the edges. Choose clear glass as colored glass does not efficiently transmit light. Whatever your choice, find a container for desert plants that maximizes light and minimizes enclosure.
Provide an Organic Base
Cacti and other succulents are high-light, moisture-retentive plants, but they need nutrition drawn from the soil just like other plants. Line the bottom of the desert terrarium with marbles and a layer of horticultural charcoal. Fill it with an equal mixture of sharp sand, peat and sterile potting soil. You might also mix equal parts of prepared cactus mixture for houseplants and sand to provide better drainage than needed in a pot with a drainage hole. Plants only need two to three inches of soil – cactus roots are short.
Choose Similar Plants
Populate your little world with water-thrifty cacti only. In a large terrarium, you might want to add a low-growing succulent such as hens and chicks (Sempervivium tectorum), hardy in U.S. Department of Agriculture zones 3 through 8, for a ground cover. Cacti are slow growers, so select plants for the terrarium that are close to their desired size at planting time. Cactus' thick, waxy skin traps moisture and tolerates bright light. Choose from the variety of looks this group can offer -- smooth, bumpy, bristled or spiked skins on upright or squat plants. In the right exposure near a south window, cacti might bloom in white, yellow or red; group blooming plants for a potential display. Cacti suitable for growing inside a terrarium are bunny ears (Opuntia microdasys, hardy outdoors in USDA zones 8 through 11) and red crown cactus (Rebutia minuscula, hardy outdoors in USDA zones 9 through 10). Lightly water the terrarium after planting to moisten the roots, but do not saturate the growing medium where there is standing water or root rot can develop.
Water and Feed Cactus
Cactus, although drought tolerant, needs water and nutrients like other plants -- it just uses both more efficiently. Withhold water for one week after planting, then water lightly only when soil is completely dry. Do not overwater or the cacti will die. Water during winter only when cactus skin begins to shrivel. Terrarium cactus might not need any fertilizer; you don't want it to outgrow its space too soon. If plants turn pale, give them a monthly feeding of the teaspoon-per-gallon of water type of houseplant fertilizer only during active growth from April through August.