A terrarium is a miniature, self-contained garden or ecosystem housed within a transparent container. These glass enclosures offer a unique way to cultivate plants indoors, appealing to many because they are aesthetically pleasing and require minimal care once established. The success of a terrarium depends primarily on selecting plants that are compatible with the specific environment created by the container’s design.
Defining the Terrarium Environment
The fundamental distinction in terrarium design lies between open and closed systems, which determines the internal growing conditions. A closed terrarium uses a sealed lid, creating a self-watering microclimate with extremely high relative humidity, typically ranging from 80 to 100%. This sealed environment traps moisture released by the soil and plants through transpiration, which then condenses on the glass and cycles back down to the substrate. Airflow is minimal in a closed system, which is ideal for tropical species that naturally grow in dense, humid conditions.
An open terrarium, conversely, has a wide, unsealed opening that allows for continuous air circulation and the evaporation of excess water. This results in a much lower humidity level, generally between 40 and 60%, similar to a standard indoor potted plant. Open containers are better suited for plants that dislike stagnant, moist air and require a drier environment to prevent rot.
Essential Plants for Closed Terrariums
Closed terrariums mimic the conditions of a tropical rainforest floor, making them the perfect home for plants that thrive in constant moisture and indirect light. The most reliable choices are typically slow-growing to prevent them from quickly overwhelming the limited space.
Ferns are a classic choice for closed systems, as they naturally require the consistent moisture and high humidity that a sealed container provides. Delicate varieties like Maidenhair Fern (Adiantum raddianum) are particularly well-suited because their fragile fronds easily dry out in low-humidity conditions outside of a terrarium. Mosses and Selaginella, often called spike moss or frosty fern, also create a lush, moisture-retaining ground cover and thrive in the low light and damp soil.
Small, colorful foliage plants add visual interest and flourish in the humid air. The Nerve Plant (Fittonia albivenis) is a common and excellent choice due to its compact growth and preference for high humidity. Similarly, various miniature Peperomia species, such as Peperomia caperata, are suitable because they tolerate low light and are not prone to stretching in the enclosed space. Miniature orchids, such as Ludisia discolor (Jewel Orchid), also excel in these conditions, providing unique texture and flowers while staying small enough for the enclosure.
Other successful options include the Friendship Plant (Pilea involucrata) and the Artillery Fern (Pilea microphylla), which are both small-leaved and tolerate the consistently moist soil. Regular, gentle pruning is the main maintenance required to keep these species from outgrowing their glass walls.
Choosing Plants for Open Terrariums
Open terrariums are designed for plants that prefer dry conditions and plenty of air circulation, directly contrasting the needs of closed terrarium species. The increased airflow prevents the fungal growth and rot that plants from arid environments would experience in a humid, sealed container. The most suitable plants for this environment are those with adaptations for water storage and drought tolerance.
Succulents are the most popular choice, including rosette-forming types like Echeveria and architectural species such as Haworthia. These plants store water in their thick, fleshy leaves and require soil that dries out quickly between waterings. Cacti are also ideal, provided they are miniature, slow-growing varieties, as they demand bright light and are extremely susceptible to rot in moist environments.
Air plants, or Tillandsia, are a unique addition to open terrariums because they do not require soil at all. They absorb all necessary moisture and nutrients directly through specialized scales on their leaves, making good air circulation absolutely necessary for their health.
Substrate, Light, and Care
The foundation of any terrarium relies on a specific layering system to manage moisture and provide aeration to the roots. At the base, a drainage layer of material like small gravel or leca (lightweight expanded clay aggregate) is necessary to collect excess water, preventing root saturation. Directly above this, a thin layer of horticultural activated charcoal is highly recommended to filter impurities and maintain a fresh-smelling environment.
The main growing medium, or substrate, sits on top and must be tailored to the plants’ needs. For closed terrariums, a well-draining yet water-retentive tropical mix is used, often with materials like coco coir and sphagnum moss. Open terrariums, by contrast, require a fast-draining mix, such as a blend of potting soil with coarse sand or perlite, to mimic the arid conditions that succulents prefer.
Light requirements vary between the two systems, but direct, intense sunlight should be avoided for most terrariums because it can rapidly overheat the glass enclosure and scorch the plants. Both open and closed setups thrive in bright, indirect light, such as from a north-facing window or a few feet away from a sunny window. Watering is minimal: closed terrariums may only need a few drops of water every few months, while open terrariums are watered only when the soil is completely dry, similar to standard succulent care.