A terrarium is a miniature, self-contained ecosystem housed within a transparent container. This glass enclosure simulates a natural environment, allowing you to create a small, aesthetically pleasing garden that requires minimal intervention once established. The appeal of a terrarium lies in its relative ease of maintenance and the fascinating way it demonstrates a simplified natural cycle. Building a successful terrarium depends entirely on selecting the correct plants and understanding the specific layered foundation needed to support them.
Choosing Plants Based on Terrarium Type
The initial decision in terrarium creation is selecting between a closed or an open system, which directly dictates the plant species that can thrive. Closed terrariums are sealed, creating a high-humidity environment that functions like a miniature tropical rainforest. This setup relies on the water cycle, where moisture transpired by the plants and evaporated from the soil condenses on the glass and returns to the substrate, making it largely self-watering.
Plants best suited for this moist environment include small, slow-growing tropical species that tolerate high humidity. Excellent choices are ferns, such as Maidenhair or Boston ferns, various mosses, and Fittonia (Nerve Plant). These plants naturally prefer the low airflow and high moisture retention that a sealed container provides.
In contrast, open terrariums feature a wide opening that allows for constant air exchange, resulting in a much lower humidity level and drier soil conditions. This environment is perfect for plants that prefer arid habitats and are prone to rot in excessively damp conditions. Succulents, cacti, and air plants (Tillandsia) are the ideal inhabitants for open containers, as they require superior drainage and circulating air. Attempting to grow a cactus in a closed, humid terrarium will inevitably lead to its demise through root rot.
Building the Miniature Ecosystem
The foundation of any successful terrarium, particularly a closed one, is a sequential layering of materials designed to manage moisture, as the container lacks a conventional drainage hole. The first layer, typically 1–2 inches thick, is the drainage layer, composed of pebbles, small stones, or lightweight expanded clay aggregate (LECA). This layer serves as a reservoir for excess water, preventing it from pooling directly around the plant roots, which causes root rot.
Above the drainage material, a thin layer of activated or horticultural charcoal is essential, especially in closed systems where air exchange is minimal. This porous charcoal acts as a natural filter, binding impurities, absorbing toxins, and neutralizing odors that accumulate from decaying organic matter. To prevent the substrate from mixing and clogging the reservoir, some builders place a fine mesh or a layer of sphagnum moss between the drainage and soil layers.
The final layer is the substrate, or soil mix, which should be chosen based on the plants selected. Tropical plants require a moisture-retentive mix with high organic matter, while arid plants need a sandy, gritty blend for rapid drainage. When planting, use long tools like tweezers or chopsticks to gently place the roots and cover them with soil, ensuring plants are not overcrowded. Leave space between the foliage and the glass to promote air circulation within the confined space.
Maintaining Humidity, Light, and Temperature
Once constructed, the ongoing care for a closed terrarium primarily involves monitoring the internal environment rather than frequent watering. The key to its health is the daily water cycle, visually confirmed by condensation on the glass. If condensation consistently covers more than one-third of the glass surface, it indicates excessive moisture, and the lid should be removed for a few hours to allow water vapor to escape. Conversely, if no condensation is visible, a small amount of distilled or filtered water should be added to re-establish the cycle.
Terrariums require bright, indirect light, often for four to six hours per day, to power photosynthesis and the internal water cycle. Direct sunlight must be avoided, especially for closed containers, because the glass acts like a greenhouse, trapping heat and quickly causing internal temperatures to rise. This rapid heating can scorch the plants. The ideal temperature range is a stable room temperature, generally around 19–21°C.
Maintenance also includes trimming any plant that begins to outgrow its space, as this prevents crowding and ensures light can reach all parts of the ecosystem. Overgrowth can lead to poor air circulation, which stresses the plants and encourages mold. For open terrariums, watering is more conventional but still infrequent, focusing on providing water directly to the soil only when it is completely dry.
Addressing Common Terrarium Issues
Despite careful construction, the high-humidity, confined environment can sometimes lead to specific problems. Mold and fungus are the most common, often appearing as white, fuzzy patches on the soil or decaying matter. This growth is typically a sign of too much moisture, poor ventilation, or the presence of dead plant material. The immediate solution is to remove the affected material with tweezers, briefly open the terrarium to increase air exchange, and check condensation levels to ensure the system is not overwatered.
Another frequent issue is the presence of small flying insects, most often fungus gnats, which thrive in moist soil and can damage young roots. Treating the soil with a diluted neem oil solution or introducing beneficial organisms like specific nematodes or springtails can help control gnat populations. Springtails are micro-arthropods that feed on mold and decaying organic matter, making them a natural part of a healthy closed ecosystem. If a plant begins to rot, appearing as dark, mushy stems or leaves, it must be removed promptly. This suggests severe overwatering and can spread quickly to other plants.