A terrarium is a miniature, self-contained ecosystem housed within a glass container. This enclosed environment requires a specific, multi-layered structure to mimic natural conditions. The layering process establishes the necessary conditions for managing moisture and promoting plant health within a closed system. Each layer performs a distinct function, working together to create a stable, often self-sustaining, microclimate.
The Foundation: Water Management Layers
The lowest stratum of the terrarium is the drainage layer, often called a false bottom. This layer collects surplus water that percolates through the growing medium, preventing plant roots from becoming waterlogged. Materials such as Lightweight Expanded Clay Aggregate (LECA), lava rock, or small pebbles are used to create void spaces for water to pool away from the soil. This reservoir also contributes to the terrarium’s humidity as the moisture slowly evaporates and condenses within the environment.
Immediately above the drainage material, a separation layer must be installed to maintain the system’s integrity. This barrier, typically a fine mesh screen or landscape fabric, prevents finer soil particles from settling into the drainage layer. Without this separation, the soil would quickly clog the void spaces, compromising drainage and causing the lower section to turn into a muddy anaerobic zone.
A thin layer of activated charcoal is usually placed next, either directly on the separation barrier or mixed into the substrate. This specialized, highly porous material functions as a filter due to its large surface area capable of adsorption. The charcoal traps impurities, absorbs organic toxins, and neutralizes odors that accumulate from decaying matter. This filtration is beneficial in a sealed terrarium where gases and compounds cannot escape.
The Growing Medium
Positioned above the foundational layers is the growing medium, which anchors the plants and supplies them with moisture and nutrients. This substrate must be carefully formulated to balance water retention with adequate aeration. Standard garden soil is unsuitable because it compacts easily and retains too much water, which leads to root suffocation.
A proper terrarium mix often begins with a base of sterile potting mix combined with organic materials like peat moss or coco coir for moisture retention. Amendments such as perlite, vermiculite, or orchid bark are incorporated to create air pockets and promote drainage. Perlite and vermiculite both improve the soil structure by reducing compaction. The total depth of this layer must be sufficient to support the root structure of the chosen plants.
This specialized blend ensures that the roots have access to oxygen, which is necessary for cellular respiration and nutrient uptake. The organic components provide a slow release of nutrients, supporting the long-term health of the plants in the restricted space. The substrate facilitates the growth that drives the internal water and nutrient cycles of the ecosystem.
Finishing Elements and Ongoing Care
The final structural layer consists of topdressing materials, applied directly onto the surface of the growing medium. Sheet moss or decorative pebbles are commonly used to cover the exposed soil. This offers aesthetic appeal and a functional benefit: the top layer helps regulate surface moisture by reducing the rate of direct evaporation from the substrate.
After the plants are situated, hardscape elements like small stones, driftwood, or decorative figurines can be added for visual interest. These elements are non-functional in terms of the ecosystem’s mechanics but complete the miniature landscape design. The process concludes with initial watering, typically a gentle misting, to settle the soil and introduce moisture. The terrarium must be placed in a location that receives bright, indirect light, such as an east or north-facing window. Direct sunlight must be avoided, as the glass container can magnify heat, rapidly elevating the internal temperature. For closed systems, the lid is monitored to establish the correct humidity balance, ensuring condensation is present but not excessively dripping down the sides.