A terrarium creates a miniature, self-contained world for plants, categorized as either open or closed systems. Open terrariums require regular watering, but closed terrariums are sealed environments designed to mimic a natural, self-sustaining ecosystem. Beginners often wonder if these sealed environments require external air. The answer lies in the natural, cyclical processes occurring within the closed system, which continuously refresh the air without needing to open the container.
The Closed Loop of Gas Exchange
A closed terrarium is a balanced micro-ecosystem where the internal atmosphere is continuously renewed by the living organisms inside. Plants and microorganisms engage in a constant exchange of oxygen (O2) and carbon dioxide (CO2), forming a stable atmospheric cycle.
During daylight hours, photosynthesis occurs: plants absorb carbon dioxide and use light energy to convert it into glucose, releasing oxygen as a byproduct. Cellular respiration is the reciprocal process, occurring continuously in both plants and microorganisms inhabiting the soil and substrate. During respiration, these organisms consume oxygen to break down stored sugars for energy, releasing carbon dioxide back into the atmosphere. This dual process ensures that the oxygen produced is utilized, and the carbon dioxide required for photosynthesis is consistently replenished.
Microorganisms, including bacteria and fungi, play a specialized role in balancing the atmosphere. They decompose fallen leaves and organic matter, which releases additional carbon dioxide through their respiration. This carbon dioxide is immediately available to the plants, creating a self-sufficient carbon cycle within the sealed glass container. This symbiotic relationship maintains the necessary gaseous equilibrium, allowing the system to thrive for extended periods.
Atmospheric Role of Humidity
Water vapor plays a distinct role in the atmospheric composition, separate from the oxygen and carbon dioxide exchange. The high humidity inside a closed terrarium is maintained by a continuous, miniature water cycle that is essential for plant health. Water is absorbed by the roots and released as vapor through tiny pores in the leaves in a process called transpiration.
This water vapor, combined with evaporation from the moist soil, saturates the air and rises toward the cooler glass surface. When the warm, moist air meets the glass, the vapor condenses into visible droplets. These droplets eventually fall back down to the substrate, mimicking rainfall and completing the cycle.
This constant recycling ensures plants remain hydrated without external watering. The resulting high-humidity environment creates the tropical conditions that allow many terrarium-suited plants to flourish. This self-regulation of moisture prevents the plants from drying out.
When to Introduce New Air
While the gas and water cycles are designed to be self-sustaining, a closed terrarium may occasionally require temporary intervention to rebalance the atmosphere. The most common sign of an unbalanced system is excessive condensation, appearing as heavy fog or pooling water. This indicates an over-saturation of moisture, which can lead to problems like fungal or mold growth.
The primary purpose of opening the terrarium is to allow excess water vapor to escape, thereby reducing the internal humidity. Fungi and mold thrive in stagnant, overly wet conditions and can rapidly consume oxygen, disrupting the delicate atmospheric balance. Briefly opening the lid for a few hours, or even overnight, helps to dry the air and deter the proliferation of these organisms.
Managing Heat and VOCs
New air may also be introduced to manage heat or release accumulated volatile organic compounds (VOCs). If a terrarium is placed in direct sunlight, the glass container can trap heat, causing the internal temperature to spike and potentially harming the plants. Temporarily venting the system allows for a rapid exchange of heated air with cooler, fresh air. Decomposition of organic matter can also release small amounts of various gases, and a short period of venting ensures these compounds are refreshed, maintaining a clean internal atmosphere.