Plants can grow and thrive in glass containers, a practice commonly known as creating a terrarium. These glass enclosures function as self-contained ecosystems, allowing plants to flourish in environments distinct from traditional potted settings. By harnessing basic scientific principles, a terrarium transforms a simple jar or bowl into a controlled habitat. This method offers a way to cultivate plant life indoors, whether aiming for a lush, humid jungle or a dry, arid landscape.
How Glass Containers Create Unique Growing Environments
Glass containers fundamentally alter the environment surrounding the plants by creating a localized microclimate. In a sealed system, the glass traps moisture that evaporates from the soil and the plants’ leaves, initiating a continuous water cycle. This process involves water vapor condensing on the cooler inner walls of the glass. The water droplets then “rain” back down onto the soil, consistently hydrating the plants without frequent manual watering.
The glass structure also serves as a mechanism for humidity regulation, which is the primary benefit for many tropical species. The trapped moisture raises the ambient humidity significantly, mimicking the conditions of a rainforest floor. This high humidity reduces water loss through transpiration, allowing moisture-loving species to thrive.
Light transmission through the glass requires careful placement. While the glass allows light for photosynthesis, direct sun exposure can rapidly heat the interior, causing the temperature to spike dramatically. This solar magnification can scorch leaves and upset the moisture balance, potentially harming the ecosystem. Therefore, bright, indirect light is preferred to maintain a stable temperature suitable for growth.
Selecting Plants for Open Versus Closed Systems
The choice of plants depends on whether the glass container is an open or a closed system, as each mimics a different natural habitat. Closed systems, sealed with a lid or having a narrow opening, are designed to retain moisture and create a tropical, high-humidity environment. These are suitable for plants that thrive in damp, shaded conditions and prefer consistent moisture.
Ideal inhabitants for a closed system include small, slow-growing tropical species such as ferns, which require high humidity to prevent their fronds from drying out. Mosses are also well-suited, as they help retain water at the soil level and flourish in the damp, enclosed air. Other humidity-loving plants include Fittonia (Nerve Plant) and various species of Pilea.
Open systems, typically wide-mouthed bowls or containers without lids, allow for greater air circulation and moisture evaporation. This setup results in a drier, lower-humidity environment, mirroring arid or desert climates. Plants selected for open containers must tolerate dry soil and lower atmospheric moisture.
Drought-tolerant plants like succulents and cacti are the standard selections for open terrariums because they store water in their leaves and stems. Air plants (Tillandsia) are also excellent candidates, as they absorb moisture through their specialized foliage and do not require soil. Matching plants with similar low-water needs is necessary to prevent root rot from overwatering.
Building and Maintaining Your Container Garden
The physical construction of a glass container garden requires specific layers to manage water and maintain a healthy environment. The foundation is a drainage layer, typically 1–2 inches of small pebbles, gravel, or lightweight expanded clay aggregate (LECA), which collects excess water and prevents root rot. Directly above this, a thin layer of activated charcoal is added to act as a filter, absorbing impurities, minimizing odors, and suppressing bacterial growth.
The main planting medium is the substrate layer, which is tailored to the specific plants—a moisture-retentive soil mix for closed systems and a fast-draining, sandy mix for open systems. Once planted, the maintenance routine diverges based on the container type. Closed terrariums require the least attention, often needing water only a few times a year, as the internal water cycle takes over.
For closed systems, monitoring condensation is the best guide. A light morning fog on the glass indicates a balanced environment, while heavy dripping suggests too much moisture, requiring the lid to be temporarily opened for ventilation. Maintenance also includes pruning any foliage that grows too large or begins touching the glass to prevent decay. The interior glass should be gently cleaned with a paper towel wrapped around a long tool to ensure maximum light penetration and visibility.