A terrarium, whether open or closed, relies on light as the most important factor for plant survival. Light energy drives the system through photosynthesis, even though the glass container regulates temperature, humidity, and gas exchange. Balancing the intensity, duration, and type of illumination is challenging for new hobbyists, as too much or too little light can cause the miniature world to fail. The goal is to replicate the soft, dappled light of a forest floor, not the harsh intensity of direct sun.
Quantifying Terrarium Light Needs
The specific light requirement depends entirely on the plant species chosen for the enclosure. While professionals use scientific tools like Photosynthetically Active Radiation (PAR) meters, hobbyists rely on qualitative categories: low, medium, and high light. Low-light plants, such as mosses, require minimal, heavily diffused illumination and are easily scorched by moderate intensity.
Medium-light plants, including tropical ferns and creeping fig (Ficus pumila), need bright, indirect light for healthy growth and color. This level is achieved near a window that avoids direct sun or with a moderate-intensity artificial fixture. High-light plants, such as succulents or carnivorous species, are generally poor choices for closed terrariums due to high humidity and require intense illumination. For the average tropical terrarium, bright, indirect light that avoids shadows is the practical standard for success.
Evaluating Natural and Artificial Light Sources
The choice between natural light from a window and an artificial source affects the terrarium’s environment. Natural sunlight is simple but must be managed to prevent overheating. Placing a terrarium in direct sun, especially from a south or west-facing window, causes the glass container to trap heat, creating a severe “greenhouse effect” that can damage the plants. The ideal natural placement is near an east or north-facing window, which provides gentle, indirect light.
When natural light is insufficient, artificial illumination, particularly Light Emitting Diode (LED) fixtures, offers a consistent and controlled alternative. LEDs are preferred because they produce high light output with minimal heat transfer, preventing temperature spikes that damage plants in a sealed environment. Full-spectrum LED bulbs, typically rated between 5000 and 6500 Kelvin, are recommended because they mimic natural daylight, providing the necessary wavelengths for robust photosynthesis. Fluorescent bulbs (T5 or T8 types) are also acceptable and energy efficient, but they often require more distance from the plants to avoid heat buildup compared to LEDs.
Determining the Ideal Photoperiod
The photoperiod refers to the total duration of light exposure within a 24-hour cycle. Plants require a consistent photoperiod to regulate biological functions and maintain a healthy circadian rhythm. For the majority of tropical plants used in terrariums, including ferns and nerve plants, an optimal light duration is 10 to 14 hours per day.
A consistent dark period is equally important, allowing plants to undergo necessary metabolic processes, such as respiration and nutrient transport, that occur at night. Using an automated timer is recommended to ensure the light cycle remains consistent, preventing stress from irregular exposure. Mosses, which prefer dimmer conditions, may thrive with a shorter photoperiod (sometimes as low as 8 hours), but maintaining a predictable schedule is key.
Troubleshooting Light Issues and Plant Health
Plant appearance serves as the most practical diagnostic tool for identifying light imbalances. Insufficient light is commonly indicated by etiolation, a condition where plants stretch and develop thin, elongated stems toward the light source. Leaves may also become pale, or chlorotic, due to inadequate energy to produce chlorophyll, resulting in minimal overall growth.
Conversely, excessive light and resulting heat can cause noticeable damage, particularly in a closed terrarium. Symptoms include scorching (brown or white spots and crispy edges on leaves) or a sudden, rapid dieback of the entire plant. Corrective actions are straightforward: move the terrarium away from the window or increase the distance between the plants and an artificial light source to reduce intensity. Rotating the terrarium every few weeks helps ensure all plants receive equal illumination and promotes even growth.