Quick answer: To build a closed jar terrarium, add 2–3 cm of clay balls, a thin layer of dry sphagnum as a barrier, and 4–7 cm of tropical bioactive soil. Plant slow, humidity-loving species like mini fittonia, Peperomia prostrata, small ferns, Selaginella and cushion moss. Add a few small stones or a piece of cork and a pinch of springtails, mist lightly, close the lid, and keep it in bright, indirect light. Use far less water than you think.
How a closed terrarium works
A sealed jar creates its own water cycle. Water evaporates from soil and leaves, condenses on the glass and runs back down. Plants produce oxygen by day and use it at night. Springtails and microbes recycle dead material. Once balanced, the jar can go months between waterings.
Step 1: Choose the container
- Clear, uncoloured glass lets in the most light.
- A wide opening (8 cm+) makes building and maintenance far easier. Narrow-necked bottles are a fun challenge but need long tools.
- A lid that seals loosely or tightly. A cork stopper, glass lid or clip-top jar all work.
- Size: 3–10 L is the sweet spot. Very small jars dry out or overheat quickly and leave little room for plants to grow.
Clean the jar with hot water and a little vinegar, then rinse well.
Step 2: Build the layers
| Layer | Material | Depth | Purpose |
|---|---|---|---|
| Drainage | Clay balls | 2–3 cm | Holds excess water away from roots |
| Barrier | Dry sphagnum moss or mesh | ~1 cm | Stops soil washing down |
| Soil | Jurassic ABG Terrarium Mix | 4–7 cm | Roots and microbial life |
| Top dressing | Moss, small leaf litter, stones | Thin | Looks natural; holds moisture |
Canadian-made by Jurassic Reptile Products, ABG mix is airy enough to resist going sour in a sealed jar. Some builders add a thin sprinkle of horticultural charcoal between layers. Note that ABG-style mixes typically already include charcoal. Slope the soil higher at the back for depth.
Step 3: Add hardscape
Small hardscape gives the jar scale and structure:
- A small piece of cork bark or a twig of Amur spiderwood
- A few stones of moon rock or Seiryu stone
Keep it simple. One wood piece and two or three stones are plenty. Clean all wood first: how to prepare wood.
Step 4: Choose the right plants
| Plant | Why it works |
|---|---|
| Mini fittonia | Loves humidity, stays small. Fittonia guide |
| Peperomia prostrata / P. emarginella | Tiny leaves, slow growth. Peperomia guide |
| Selaginella | Lush, mossy texture; needs constant humidity |
| Mini ferns (Maidenhair, Lemmaphyllum) | Fine texture. Fern guide |
| Small begonias (B. prismatocarpa, B. thelmae) | Colour and flowers. Begonia guide |
| Cushion and sheet moss | Ground cover. Frog moss, sheet moss |
| Pilea glauca, Ficus pumila 'Minima' | Tiny creepers |
Avoid succulents and cacti (they rot), air plants (rot), fast vines like golden pothos (they outgrow the jar), and big-leaved plants.
Step 5: Plant it
- Wash all soil off the plant roots.
- Use long tweezers or chopsticks to plant, starting with the largest plants at the back.
- Leave space between plants for growth.
- Press moss firmly onto the soil and stones.
- Clean soil off the inside of the glass with a paper towel on a stick.
Step 6: Add springtails
A small pinch of springtails is one of the best things you can add to a closed jar. They eat mould, decaying leaves and fungi, preventing the white fuzz that ruins many jars. See clean-up crew guide. Isopods are usually too big for small jars.
Step 7: Water carefully, then seal
This is where most jars fail. Add water a little at a time with a spray bottle until the soil is just moist and you see a tiny bit of water in the bottom of the clay ball layer. Then stop. It's easy to add more later and hard to remove excess. Close the lid.
Step 8: Placement and light
- Bright, indirect light. Never direct sun, which can cook a sealed jar quickly.
- A small LED above the jar on a 10–12 hour timer gives the most consistent results. See lighting guide.
- Avoid cold windowsills in Canadian winters. Leaves touching freezing glass can be damaged.
Reading condensation
| What you see | Meaning | Action |
|---|---|---|
| Light fog in the morning that clears by afternoon | Ideal balance | None |
| Heavy condensation all day, water pooling | Too wet | Open the lid for a few hours to a day |
| No condensation ever, plants wilting | Too dry | Mist lightly and reseal |
| White fuzz on wood or soil | Mould from excess moisture or decay | Remove with tweezers, air out, add springtails |
Long-term care
- Prune plants that touch the glass or overcrowd others.
- Remove dead leaves.
- Air out occasionally if condensation never clears.
- Add a few drops of water every few months if the jar slowly dries.
FAQ
Do closed terrariums need air?
A sealed jar has enough air for plants, which recycle CO₂ and oxygen. Opening occasionally helps manage moisture.
How long can a closed terrarium last?
Well-built jars can last for years with minimal care.
Why are my jar plants rotting?
Almost always too much water. Air it out and use less water next time.