Planting succulents in closed glass containers often leads to disappointment. Succulents are a diverse group of plants defined by their specialized ability to store water in thickened leaves and stems. This adaptation allows them to survive in arid environments. Consequently, these drought-tolerant plants generally do not thrive in the conditions created by a traditional, enclosed terrarium.
How Succulents Store Water and Energy
The physiological makeup of succulents reflects their evolution in dry landscapes where water is scarce and sunlight is abundant. These plants exhibit a specialized form of photosynthesis known as Crassulacean Acid Metabolism (CAM), which conserves moisture efficiently. Succulents open their stomata only at night to absorb carbon dioxide, minimizing water loss from evaporation during the daytime heat.
This water-saving strategy results in a low tolerance for constant moisture around their roots or foliage. To prevent decay, they require soil that drains rapidly, mimicking the sandy or rocky substrates of their native habitats. High light levels are necessary to fuel the energy-intensive CAM photosynthesis effectively. Constant air movement around the plant also helps manage the minimal moisture they release.
The High-Humidity Terrarium Environment
The environment inside a standard terrarium is structured to maintain a closed, self-sustaining ecosystem. The glass container acts as a miniature greenhouse, trapping moisture that evaporates from the soil and plant surfaces. This water condenses on the cooler glass walls and drips back down, creating a perpetual water cycle.
This constant recycling of moisture results in consistently high relative humidity within the container, often approaching 100%. Traditional terrariums also lack the necessary drainage holes found in standard plant pots. This combination of saturated soil and a water-filled atmosphere creates a continuously damp environment.
The enclosed nature of the glass vessel severely restricts air movement. Stagnant air contributes to moisture buildup on leaf surfaces and prevents the rapid evaporation succulents depend on. This environment is the opposite of the dry, airy conditions these plants require for survival.
Why Combining Them Leads to Failure
The incompatibility between a succulent’s biology and the terrarium’s conditions quickly manifests as physiological distress and decay. The primary cause of failure is root rot, which occurs when roots sit in perpetually moist, poorly aerated soil. Without adequate oxygen, the root cells die and become susceptible to water molds and bacteria.
High humidity also promotes the growth of pathogenic fungi and mildew on the plant’s aerial parts. Constant moisture on the leaves and stems provides an ideal breeding ground for these organisms, leading to lesions and tissue death. The restricted airflow allows these fungal spores to spread easily throughout the confined space.
Light availability poses a separate challenge for succulents inside a glass enclosure, even near a window. Insufficient light results in a condition called etiolation, where the plant rapidly stretches toward the light source. This stretching causes the stem to become thin, pale, and weak, sacrificing compact, healthy growth for height.
Alternative Enclosed Gardens
Gardeners interested in enclosed planting should select species that naturally thrive in high-humidity, low-airflow environments. Excellent choices for traditional closed terrariums include small tropical plants such as ferns, mosses, and creeping fig. These plants benefit from the recycled moisture and tolerate the lower light levels common in indoor glass containers.
For those who wish to display succulents in a glass vessel, an open container garden, sometimes called a succulent dish garden, is the appropriate alternative. This design utilizes a wide, shallow glass bowl or container that remains completely open to the air. The open design allows for necessary air circulation and prevents humidity buildup on the leaves and stems.
The container must be filled with a fast-draining soil mix and should never hold standing water. While this setup captures the aesthetic appeal of glass, it functions biologically as a standard planter. This ensures the dry, well-ventilated conditions succulents need to flourish and avoids the fatal flaw of the closed terrarium design.