A terrarium is a miniature, self-contained garden housed within a transparent container, acting as a small-scale ecosystem. The concept traces its roots back to the Victorian era when it was known as the Wardian case, named after its inventor, botanist Nathaniel Bagshaw Ward.
Ward discovered in the 1820s that plants could thrive in sealed jars. These early enclosures were used to transport exotic plant species across long ocean voyages, protecting them from saltwater spray and extreme temperature changes. The modern terrarium still captures the imagination as a low-maintenance way to bring a piece of nature indoors.
Physical Components and Layering
A terrarium requires a specific layering of materials inside the chosen container, which is typically clear glass or plastic to allow light penetration. The foundation starts with a drainage layer, usually composed of small gravel, pebbles, or lava rock. This layer collects any excess water that filters through the soil, preventing the plant roots from sitting in soggy conditions, which commonly causes root rot.
Directly above the drainage material, a thin layer of activated charcoal is often included to act as a natural filter. This charcoal helps absorb impurities, odors, and toxins that can accumulate in the soil and water over time. A barrier layer, such as mesh or sheet moss, is then placed on top of the charcoal to prevent the main soil layer from mixing with the drainage material.
The largest layer is the substrate, which is the potting mix or soil that provides nutrients and a growth medium for the plants. This soil needs to be well-draining yet capable of retaining enough moisture to support the plants in the humid environment. Plant selection focuses on small, slow-growing species that tolerate high humidity, such as ferns, mosses, and tropical plants.
How the Ecosystem Functions
A closed terrarium successfully replicates several natural cycles. Light passing through the clear walls powers the entire system, primarily through photosynthesis. During the day, plants use this light energy, along with water and carbon dioxide, to produce glucose for growth and release oxygen.
The water cycle ensures the plants receive continuous hydration. Water evaporates from the soil and plant leaves through transpiration, turning into vapor in the container’s air. As this humid air touches the cooler glass walls, the vapor condenses into droplets, which then “rain” back down onto the soil in a process similar to precipitation.
This sealed environment maintains a constant gas exchange, known as the air cycle. While plants photosynthesize and release oxygen during the day, they, along with soil microorganisms, perform respiration both day and night, absorbing oxygen and releasing carbon dioxide. Decomposition plays a role as fallen leaves and dead plant matter are broken down by bacteria and fungi, releasing nutrients back into the soil.
Distinguishing Open and Closed Terrariums
The difference between open and closed terrariums is the presence or absence of a lid, which dictates the internal environment and plant choices. Closed terrariums, with sealed lids, trap moisture and humidity, creating a perpetually damp and warm microclimate. This high-humidity setting is ideal for tropical plants like mosses and ferns, and their self-watering nature means they require minimal intervention, often needing water only once every few months or years.
An open terrarium has a wide, unsealed opening, allowing for constant air circulation and the dissipation of moisture. Since humidity is not trapped, open systems are better suited for plants that thrive in drier conditions, such as succulents, cacti, and air plants. Because the water cycle cannot fully establish itself, open terrariums require regular watering, much like a traditional potted plant. The choice between the two depends on the desired plant species and the level of maintenance the caretaker is willing to provide.