A closed terrarium represents a miniature, self-sustaining ecosystem sealed within a glass container. This environment operates on a continuous water cycle, creating a near-constant state of high humidity. Selecting the right plant species is paramount for the long-term success of this enclosed world, as only certain flora can thrive in such a specialized, high-moisture, and low-airflow setting. Success requires choosing compact, slow-growing tropical plants perfectly adapted to humid conditions, ensuring the miniature landscape remains balanced and healthy over time.
Understanding the Closed Terrarium Environment
The environment inside a sealed glass container mimics a tropical forest floor, characterized by high humidity, consistent temperature, low light, and limited physical space. Humidity levels typically hover near 100% due to trapped moisture recycling itself. Plants must tolerate perpetually damp conditions without succumbing to rot.
Temperatures remain relatively stable, but direct sunlight can rapidly increase heat and scorch the plants. Light exposure must be bright but indirect, as most suitable plants are adapted to filtered light found beneath a dense canopy. The limited soil volume and container size necessitate selecting plants that are naturally small or have a slow growth rate to prevent them from quickly outgrowing the space.
Best Plants for High-Humidity Terrariums
The most successful closed terrarium plants are those that naturally inhabit humid, shaded tropical regions. These species are categorized by their growth habit and their ability to handle the near-saturated air.
Foliage Plants
Foliage plants are selected for their color, texture, and compact size, providing the main visual interest. The Nerve Plant (Fittonia albivenis) is a classic choice, known for its striking, veined leaves in shades of pink, red, or white. Miniature Peperomia varieties, such as Peperomia prostrata (String of Turtles) or Peperomia caperata ‘Rosso’, are also excellent due to their small, textured leaves and tolerance for low light. The Pilea genus, with species like Pilea glauca (Silver Sparkle Pilea), provides delicate, trailing foliage that acts as a fine-textured groundcover.
Ferns
Ferns are well-suited to the terrarium environment because they rely on high humidity to prevent desiccation. Small, non-aggressive varieties are preferred. The Lemon Button Fern (Nephrolepis cordifolia ‘Duffii’) is valued for its petite, rounded leaflets and compact, bushy form. The Button Fern (Pellaea rotundifolia) is another excellent choice, featuring small, coin-shaped leaves that add a unique texture.
Mosses and Groundcover
Mosses are functionally and aesthetically important, serving as a living mulch that helps regulate moisture and provides a lush, carpet-like base. Cushion Moss (Leucobryum glaucum) and Sheet Moss (Hypnum imponens) are highly recommended because they thrive in low light and are effective at absorbing and slowly releasing water, stabilizing the terrarium’s humidity. For a creeping groundcover, Baby’s Tears (Soleirolia soleirolii) forms a dense, bright green mat that tolerates high moisture and low light conditions well.
Plants to Exclude
Just as important as selecting the right plants is knowing which ones to avoid, as introducing unsuitable species will quickly destabilize the closed ecosystem. Plants that require dry soil or high light will fail in the terrarium’s humid, shaded conditions.
Succulents and cacti are fundamentally incompatible with a closed terrarium because they are adapted to arid environments. Their specialized tissues require dry soil and low humidity to prevent rot, which is the opposite of the terrarium’s perpetually moist environment. Similarly, plants that demand direct, intense sunlight, such as most flowering annuals, will quickly decline in the indirect light of a sealed container.
Fast-growing or large-leafed houseplants, even if they tolerate humidity, should also be excluded. Common species like Monstera, Pothos (Epipremnum aureum), and large Ivy varieties will rapidly outgrow the limited space, requiring constant, aggressive pruning that disrupts the ecosystem’s balance. Choosing miniature or dwarf cultivars is the safer approach to maintain the scale of the miniature landscape.
Preparing and Placing Your Plants
The final steps before sealing the terrarium involve careful preparation and strategic placement of the selected plants. Before planting, thoroughly clean the plants to prevent the introduction of pests or fungal spores. This involves gently washing the foliage and rinsing the roots to remove old potting soil, which can harbor unwanted organisms.
Pruning is a necessary step to ensure the plants fit the container and do not immediately touch the glass walls. Roots should be lightly trimmed to encourage new growth and fit into the limited soil volume. Any foliage that is too large or risks touching the sides of the container should also be pruned, as leaves resting against the moist glass are highly susceptible to rot and mold.
When placing the plants, arrange the taller specimens toward the center or back of the container to create depth and scale. Ensure there is adequate space between each plant to allow for future growth and air circulation, even in the low-airflow environment.