The Science of the Glass
Understand the water cycle and microclimate that make a closed terrarium work.
The Miniature Water Cycle
In a closed terrarium, water is never lost—it only changes form. When sunlight warms the glass, water evaporates from the soil and transpires from the plant leaves. It condenses on the cool glass walls, runs down, and waters the soil again.
This infinite loop means a perfectly balanced terrarium may not need watering for months, or even years.
The Balancing Act
However, the glass acts as a magnifier for mistakes:
- Too much water: The roots suffocate in mud, leading to rot and mold.
- Too little water: The cycle stalls, and the plants dehydrate.
- Too much light: The jar becomes a greenhouse oven, cooking the inhabitants.
Light and Temperature
Plants in closed terrariums need bright, indirect light. Direct sunlight hitting the glass will exponentially raise the internal temperature. North or East-facing windows are ideal in the Northern Hemisphere. Alternatively, LED grow lights provide steady, cool illumination.
In the next lesson, we will gather the materials required to build our ecosystem.