A closed terrarium is a miniature, self-contained ecosystem housed within a transparent, sealable container. This setup creates a small-scale, high-humidity environment that mimics a tropical climate inside a glass vessel. The seal traps moisture and air, allowing the internal environment to largely regulate itself. Unlike an open terrarium, which is suited for drier plants like succulents, the closed environment sustains plant life with minimal outside intervention.
The Science of Self-Sustenance
The self-sufficiency of a closed terrarium is driven by three interconnected natural cycles: the water cycle, gas exchange, and nutrient recycling. Light warms the sealed container, causing water to evaporate from the soil and from the plants through transpiration. This warm, moisture-laden air rises, meets the cooler glass surface, and condenses into droplets.
These droplets trickle back down into the soil, creating a miniature form of precipitation that hydrates the plants. This continuous evaporation, condensation, and precipitation cycle means the terrarium rarely needs watering once properly established.
The plants manage their own atmospheric balance through gas exchange. During the day, plants perform photosynthesis, consuming carbon dioxide and releasing oxygen. At night, the process reverses as plants respire, consuming oxygen and releasing carbon dioxide and water vapor. This equilibrium maintains stable oxygen and carbon dioxide levels within the system.
The ecosystem also includes a nutrient recycling process powered by microorganisms, such as bacteria and fungi, in the soil. When organic matter dies and falls, these microbes break down the material into fundamental nutrients. This decomposition converts waste back into usable minerals that the plants absorb through their roots, ensuring a continuous supply of food without external fertilizer.
Essential Components
A successful closed terrarium requires specific materials layered systematically to support the ecosystem. The clear glass container provides the sealed environment, allowing light transmission while trapping heat and moisture. The initial layer is the drainage material, often small pebbles, gravel, or lava rock. This layer collects excess water, preventing the soil from becoming waterlogged and causing root rot.
Above the drainage layer, a thin layer of activated charcoal is added. The charcoal acts as a filter, absorbing organic impurities and odors that can build up in the sealed environment. Sphagnum moss is sometimes placed between the charcoal and the soil to prevent finer substrate particles from clogging the drainage layer.
The main growing medium is the substrate, which should be a specialized tropical potting mix. This mix retains moisture while remaining loose enough for proper aeration, providing structural and nutritional support. Plant selection is limited to species that thrive in high humidity and low light, such as ferns, mosses, and tropical plants like Fittonia.
Assembly and Initial Setup
Assembly begins by ensuring the glass container is clean to remove any harmful residues. Materials are added sequentially, starting with the drainage layer spread across the bottom to a depth of a half-inch to an inch. Next, a thin layer of activated charcoal is sprinkled over the drainage material to maintain water and air cleanliness.
The substrate layer is then introduced, requiring one to two inches of depth depending on the container size, to provide adequate space for root growth. Before planting, tropical plants should be gently removed from their pots and positioned inside using long tools. Avoid overcrowding to allow space for future growth and ensure air circulation.
Once the plants are set, the soil is firmed around the roots, and decorative elements are placed on the surface. The final step is the initial watering; only a small amount is needed, usually a light misting, until the soil is evenly moist but not saturated. The lid is then sealed, though it is recommended to leave it slightly ajar for the first week to allow the system to acclimate before closing it completely.
Ongoing Care and Troubleshooting
After the terrarium is sealed, its primary external requirement is light, which must be bright but indirect to power the internal cycles. Direct sunlight can quickly cause the temperature inside the glass to rise excessively due to the greenhouse effect, harming the plants. Consistent, moderate temperatures are best, as extreme heat encourages too much evaporation and condensation.
Excessive condensation, where the glass remains completely fogged over, indicates too much trapped moisture. To correct this, remove the lid for a few hours or overnight to allow excess water to evaporate, then reseal and monitor. Conversely, if there is no condensation, the terrarium is too dry and needs a small amount of water added.
Mold or fungal growth, appearing as white fuzz on the soil or decaying matter, signals stagnant air and high humidity. Removing the mold with a cotton swab or tweezers, and then ventilating the terrarium for a few days, can resolve the problem. Regularly removing dead or yellowing foliage also prevents future mold outbreaks and contributes to the long-term health of the environment.