A terrarium is a miniature, enclosed ecosystem housed within a transparent container. Whether a terrarium needs to be sealed depends entirely on the specific environment intended for the plants inside. The decision to use a seal determines if the system will be a self-sustaining, high-humidity environment or an open container that relies on external care.
The Self-Sustaining Closed System
A truly sealed terrarium operates as a closed loop, mimicking the natural water cycle. The seal traps moisture that evaporates from the soil and is released by the plants through transpiration, creating water vapor within the container. This moisture rises until it contacts the cooler glass surfaces, causing it to condense into visible droplets.
The condensed water trickles down the glass walls and returns to the soil, completing the cycle and effectively watering the plants. This continuous recycling means external watering is rarely necessary once the system is established. The sealed environment stabilizes the temperature and maintains a consistently high internal humidity, often between 70% and 90%, characteristic of a tropical microclimate.
Plants within this sealed space participate in a gas exchange cycle. During the day, they absorb carbon dioxide for photosynthesis and release oxygen. At night, plants and microorganisms switch to respiration, consuming oxygen and releasing carbon dioxide. This maintains a stable air balance without the need for external air exchange. This balance allows the sealed terrarium to thrive with minimal human intervention, sometimes for years.
Open Containers and Airflow Needs
An open terrarium remains unsealed, allowing continuous air exchange with the outside environment. This prevents the high humidity buildup characteristic of a sealed system, making it suitable for plants that prefer drier conditions. Constant airflow is important for preventing moisture stagnation, which can lead to mold or fungal diseases.
Since water vapor escapes, the open terrarium does not create a self-sustaining water cycle, requiring manual, regular watering. This design necessitates a proper drainage layer, such as gravel or pebbles beneath the soil, to prevent roots from sitting in stagnant water. Excess water must be allowed to drain or evaporate to prevent root rot.
Maintenance involves monitoring soil moisture and watering when the substrate becomes dry to the touch. The ventilation ensures plants receive a constant supply of fresh air and prevents issues associated with overly damp conditions. This setup requires more attention than a sealed ecosystem.
Matching Plants to Terrarium Type
The need for a seal is dictated by the selection of plants. Species that naturally grow on a rainforest floor, such as ferns, mosses, and tropical foliage like Fittonia, are best suited for the sealed environment. These plants flourish in the consistent high humidity and stable, moist soil that the closed system creates.
Conversely, plants from arid environments require an open container to thrive. Succulents, cacti, and air plants (Tillandsia) are preferred for open terrariums because they need low humidity and excellent airflow to prevent rot. Succulents store water in their leaves and quickly succumb to root rot in the perpetually damp conditions of a sealed jar.
The open design accommodates the requirement for drought-tolerant plants to have the soil dry out completely between waterings. Selecting plants with similar water and humidity needs is important. All species must be suited to either the high-humidity sealed setting or the low-humidity open environment.