How-to Guide
How to Make and Care for a Terrarium
The short answer
Build it in layers: pebbles and activated charcoal in the bottom for drainage, a thin barrier of sphagnum moss to keep soil from washing down, then at least 1.5 inches of potting medium. Choose small plants that all want similar light, temperature, and moisture, and do not mix plants with widely different needs.
Keep terrariums out of direct sun, which cooks the plants inside the glass, and place them within a few feet of a bright window instead. Closed terrariums hold humidity and may not need watering for 4-6 months - water lightly only when condensation stops forming or plants wilt, because excess water is nearly impossible to remove and causes rot.
The layers
Start with drainage. MU Extension calls for activated charcoal and pebbles in the bottom of the container - typically a layer of gravel with about a half-inch of charcoal above it. The charcoal helps keep the closed environment fresh.
Add a thin layer of sphagnum moss over the drainage layer to stop the growing medium from filtering down into the pebbles, then add potting medium at least 1.5 inches deep so roots have room. Arrange your plants, firm them in, and add any decorative stones or wood.
Open, closed, or dish
Closed containers retain the most humidity and typically will not need watering for four to six months, which makes them ideal for moisture-loving plants like ferns, mosses, and small tropical foliage. Open terrariums lose humidity faster, so they need more frequent watering but carry less disease risk.
Low-sided dish gardens behave more like ordinary houseplant pots - they need regular watering and suit drought-tolerant plants such as small succulents. Whichever you build, MU Extension's rule is to choose plants with similar cultural requirements and not mix plants needing widely different light, temperature, and moisture.
Light and watering
Keep closed or tall containers out of direct sunlight, which causes heat to build up inside the glass and damage the plants. Position the terrarium within several feet of a bright window instead; low-light plants should be no more than about 10 feet from a bright window, and you can supplement with artificial light run 16-18 hours a day if needed.
Water sparingly. In a closed terrarium, the failure of condensation to form on the inside of the glass, or wilting plants, indicates it needs water. Always water lightly - excess moisture is nearly impossible to remove and promotes root rot.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Missouri Extension - G6520 Terrariums — Specifies drainage of pebbles plus ~0.5in activated charcoal, a sphagnum moss barrier, and potting medium at least 1.5in deep; states closed containers retain humidity and typically need no watering for 4-6 months while open/dish types need more; warns against direct sun for closed/tall containers; advises placement within several feet of a bright window (low-light plants within ~10ft) with optional artificial light 16-18 hours daily; and says to water lightly, using lack of condensation or wilting as the cue.