A closed terrarium is a miniature, self-contained ecosystem housed within a sealed glass container, operating much like a tiny greenhouse. The unique environment relies on a continuous water cycle to sustain the plant life inside. Moisture from the soil and plant leaves evaporates, condenses on the cooler glass walls, and then trickles back down to the substrate. This natural process creates a high-humidity, self-watering system that mimics the conditions found on a tropical forest floor. The goal is to achieve a stable, low-maintenance environment where the internal conditions remain constant.
Characteristics of Ideal Terrarium Plants
Plants selected for this enclosed environment must be able to tolerate conditions vastly different from a typical houseplant setting. The most important requirement is a high tolerance for humidity, as the sealed container maintains near-constant saturation in the air. Tropical understory plants are naturally suited for this, having evolved in the perpetually damp, low-airflow conditions of the rainforest floor. Another significant requirement is a slow growth rate and compact size to prevent the plants from quickly outgrowing the limited space. The plants must also thrive in low or indirect light, since direct sun can rapidly overheat the glass container and scorch the foliage.
Specific Plant Recommendations for Closed Systems
The best choices for a closed terrarium are small, moisture-loving tropical plants that naturally inhabit shaded, humid areas. Ferns are a classic choice, with miniature varieties like the Lemon Button Fern and the Fluffy Ruffles Fern adding delicate texture. Mosses, such as cushion moss or sheet moss, are also indispensable, functioning as a lush green ground cover and helping to regulate moisture levels. Another element is the Fittonia (Nerve Plant), which thrives in high humidity and offers vibrant colored foliage. The Pilea genus, small-leaved creeping vines like Creeping Fig (Ficus pumila), and small orchids such as Jewel Orchids (Ludisia discolor) also flourish in these conditions.
Plants to Avoid
Avoid plants adapted to arid or semi-arid environments, as they will quickly rot in the sealed, damp conditions. Succulents, cacti, and other desert plants require dry soil and intense light, which are the opposite of a closed terrarium’s environment. Air plants (Tillandsia) should also be avoided because they require consistent airflow and good ventilation, which is not present in a sealed jar. Large or fast-growing varieties, like full-sized Pothos or regular Ivy, will quickly dominate the space.
Building the Terrarium Foundation
A successful closed terrarium requires a specific layered foundation to manage water and maintain a clean environment. The first layer is the drainage layer, typically consisting of pebbles, gravel, or lava rocks, which serves as a reservoir for excess water and prevents the plant roots from sitting in standing water and succumbing to rot. Above this, a separation layer of sphagnum moss or fine mesh keeps the soil from clogging the drainage space. Activated charcoal is then spread over this separator, acting as a natural filter to absorb toxins and impurities, which helps prevent mold and bacterial growth. The final layer is the substrate, which should be a well-draining, yet water-retentive, tropical potting mix, often including peat-based material, perlite, and coco coir.
Long-Term Maintenance and Health
The self-sustaining nature of a closed terrarium requires minimal intervention once it has established a balance. The container must be placed in a location that receives bright, indirect light; direct sunlight must be avoided entirely, as the glass acts as a magnifying lens, rapidly increasing the internal temperature and cooking the plants. Watering is rarely necessary; if the glass consistently shows condensation, the moisture level is sufficient, but if the glass remains clear, only a small amount of water should be added using a mister or dropper. Pruning is the most common maintenance task, done to manage plant growth and prevent overgrown plants from blocking light or touching the container walls, which can encourage mold. Condensation level is the best indicator of internal health: excessive condensation suggests too much moisture and requires venting the lid slightly, while a lack of condensation indicates the system is too dry and needs light misting; any dead leaves or mold should be promptly removed.