Terrarium facts and figures
Eleven figures for building and caring for a terrarium, taken from two public horticultural sources. Each figure comes with the original sentence and its link, so you can check it, quote it and reuse it.
These figures are rules of thumb for a terrarium of tropical houseplants. The source's measurements in inches are converted to centimetres (1 inch = 2.54 cm) and rounded. Sources checked on November 1, 2026.
30 cm · The ideal jar size
About 30 cm (12 in) wide and tall. Smaller works too, but fewer plants will fit and the choice narrows.
Original sentence: “About 30cm (12in) wide and tall is the optimum size.”
Source: Royal Horticultural Society (RHS), Bottle gardens and terrariums
2.5 cm · The drainage layer
About 2.5 cm (1 inch) of gravel at the bottom of the container.
Original sentence: “Add 1 inch of gravel to the bottom of the container.”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
6 mm · The charcoal layer
About 6 mm (¼ inch) of horticultural charcoal on top of the gravel.
Original sentence: “Using the paper funnel, slowly add about ¼ inch of charcoal.”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
5 cm · The soil layer
About 5 cm (2 inches) of soil mix for the plants to root in.
Original sentence: “Next, add about 2 inches of soil mix to the container.”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
4:1 · The growing mix
Four parts peat-free multi-purpose compost to one part drainage material, such as horticultural grit.
Original sentence: “Use a mix of peat-free multi-purpose compost and a drainage material, such as horticultural grit, at a ratio of about 4:1”
Source: Royal Horticultural Society (RHS), Bottle gardens and terrariums
24 h · Open after planting
Leave the jar open for about 24 hours, then alternate 24 hours closed and 24 hours open until no more condensation forms.
Original sentence: “After finishing the project, leave the container open for about 24 hours to allow excess water vapor to escape. … Replace the lid for 24 hours. … Remove the lid for another 24 hours, allowing condensation to evaporate. … Repeat this process until no moisture collects on the inside glass of the terrarium.”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
6 h · Daily light
At least six hours of bright, indirect light: near a north- or east-facing window, or a south- or west-facing one in winter.
Original sentence: “Terrarium plants need bright, indirect light for at least six hours a day, so a position near a north- or east-facing window is ideal, and a south- or west-facing window in winter.”
Source: Royal Horticultural Society (RHS), Bottle gardens and terrariums
1 h · When condensation covers the glass
Open the lid for about an hour to let excess moisture evaporate.
Original sentence: “When water droplets (condensation) form on the inside walls or lid of the terrarium, open the lid for about an hour to allow excess moisture to evaporate.”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
A few times a year · Watering a closed terrarium
A closed or narrow-necked terrarium needs watering only a few times a year, and only a little.
Original sentence: “A closed terrarium, or one with a narrow neck, won’t need much watering – usually just a few times a year, and only a small amount.”
Source: Royal Horticultural Society (RHS), Bottle gardens and terrariums
Weeks to years · Without watering, once balanced
Once it reaches equilibrium, a closed terrarium can go a long time without added water: from weeks to years.
Original sentence: “At this point, the terrarium ecosystem has reached equilibrium and can withstand long periods without additional watering—from weeks to years!”
Source: Mississippi State University Extension, How to Design a Closed-System Terrarium (Publication 3253)
19th century · The first terrarium
The Wardian case, in the 19th century, let plant collectors bring exotic plants home alive.
Original sentence: “The first example of a terrarium was a Wardian Case, which helped plant collectors bring back and introduce new, exotic plants in the 19th century.”
Source: Royal Horticultural Society (RHS), Bottle gardens and terrariums