houseplant care
How to build a terrarium — open vs closed, plants, drainage
Step-by-step terrarium build for open and closed vessels — drainage layer, plant picks (fittonia, fern, succulent), pet safety, and 12-month care.
How to build a terrarium — open vs closed, plants, drainage
A terrarium is the most beginner-friendly indoor ecosystem you can build, and a natural companion project to a moss-ball kokedama or a glass-tube propagation station once you want more living-display projects on the shelf. A sealed glass jar with the right plants, drainage, and moss recycles its own moisture for months — you can ignore it for entire seasons and it keeps going. An open succulent bowl is even simpler: a shallow dish, gravel, sterile mix, and three or four small succulents that ask for nothing but a sunny windowsill. The catch is matching the plants to the vessel. Closed terrariums kill cacti within weeks (rot); open terrariums fry ferns within days (dehydration). This guide is the build, the plant-picks-by-vessel matrix, the pet-safety check, and the 12-month care plan. Plant toxicity verified against the ASPCA non-toxic plant list, May 2026.
Try Growli: Snap a photo of your terrarium in Growli. The app identifies each species inside, sets a unified watering reminder, flags any plants that are mismatched to your vessel type, and tracks humidity if you put a small sensor in.
Open vs closed — pick the vessel first
The single decision that determines everything else. Open and closed terrariums are completely different ecosystems.
Closed terrarium
A sealed glass vessel — a clear jar with a cork or glass lid, a large bell jar, or an aquarium with a clear top. Moisture from the soil evaporates, hits the glass, condenses, and rains back down. Humidity inside sits at 80 to 95 percent. Self-sustaining for weeks if balanced correctly.
Best for: Tropical understory plants that evolved in rainforest floors — high humidity, low to medium light, no air movement.
- Fittonia (nerve plant) — non-toxic per ASPCA
- Baby tears (Soleirolia soleirolii) — non-toxic
- Baby fern, button fern, maidenhair fern — most true ferns non-toxic (avoid asparagus fern, which is toxic to cats)
- Prayer plant (Maranta) — non-toxic
- Peperomia (any variety) — non-toxic
- Polka dot plant (Hypoestes phyllostachya) — non-toxic
- Small mosses (sheet moss, cushion moss, mood moss)
Vessel ideas: Mason jar, apothecary jar, hurricane vase with glass lid, IKEA Sockerärt, terrarium-specific bell jars from Etsy or H&M Home, repurposed pickle jar.
Open terrarium
An uncovered glass bowl, fishbowl, low dish, or jar without a lid. Air circulates freely. Humidity inside matches room humidity (40 to 60 percent in most homes). No self-sustaining water cycle.
Best for: Drought-tolerant plants that need bright light and dry air — cacti, succulents, air plants.
- Haworthia (zebra plant) — non-toxic per ASPCA
- Echeveria — non-toxic
- Hens-and-chicks (Sempervivum) — non-toxic
- Small cacti (mammillaria, rebutia, gymnocalycium) — most non-toxic (spines mechanical hazard only)
- Air plants (Tillandsia) — non-toxic, mount on driftwood inside the bowl
- Lithops (living stones) — non-toxic
Avoid jade plant (Crassula ovata) in open terrariums even though it looks the part — jade is toxic to cats and dogs per the ASPCA, causing vomiting, lethargy, and ataxia. If you have pets, swap for haworthia or echeveria instead.
Vessel ideas: Wide-mouth fishbowl, shallow ceramic dish, hurricane vase without lid, candy jar, repurposed brandy glass.
Materials list — what you actually need
A complete kit costs $25 to $60 depending on whether you buy plants or use cuttings from existing houseplants.
Universal materials (both vessel types)
- Glass vessel — 1 to 5 litre capacity for a starter project. $5 to $30 from IKEA, target, H&M Home, charity shops, or Etsy.
- Drainage stones — small aquarium gravel, pea gravel, or LECA (lightweight expanded clay aggregate). $5 to $10 a bag. LECA is the cleanest-looking option.
- Activated charcoal — horticultural grade, not BBQ charcoal. Filters water and prevents the soil from going anaerobic and stinking. $5 to $8 a bag from Amazon or a hydroponics shop. Brands: ProRep, Josh's Frogs Activated Charcoal, Hoffman.
- Sphagnum moss — long-strand, sold dry. Used as a barrier between drainage and soil so the soil doesn't fall through. $5 to $10. Brands: Besgrow Spagmoss, Mosser Lee.
- Sterile potting mix — for closed terrariums, a peat-free houseplant mix with extra perlite. For open terrariums, a cactus and succulent mix. $5 to $10 per small bag.
- Long tweezers, chopsticks, or a wooden spoon — for placing plants in narrow-necked jars. $3 to $10.
- Spray bottle for initial watering. $3.
Closed terrarium extras
- Sheet moss, cushion moss, or mood moss for the surface. Live moss from a terrarium specialist (Pistils Nursery US, Botanica UK) or harvested ethically with permission from your own garden.
- Small decorative elements (smooth stones, driftwood, a tiny ceramic figure if you must). Boil decorative pieces for 10 minutes to sterilise before adding.
Open terrarium extras
- Decorative top-dressing — coloured gravel, white sand, river stones. $3 to $8.
- Tiny grow light if your sunniest window is north-facing. See our indoor garden setup guide for grow light recommendations.
The five-layer build — step by step
The same five-layer structure applies to both open and closed terrariums. The difference is plant choice and what you do after.
Layer 1 — Drainage (2 to 5 cm)
Pour aquarium gravel or LECA into the bottom of the clean vessel. Aim for 2 cm in a 1 litre jar, up to 5 cm in a 5 litre bell jar. This is where excess water collects so the soil above never sits sodden.
There is an ongoing debate about whether drainage layers actually help in closed terrariums (one argument: water still saturates the soil before reaching the layer; another: the layer becomes anaerobic). The practical consensus from terrarium specialists in 2026 remains in favour of a drainage layer for vessels without a drainage hole, particularly closed ones where excess water has nowhere else to go. We recommend including it.
Layer 2 — Activated charcoal (1 to 2 cm)
Spread a thin even layer of activated horticultural charcoal across the drainage. This filters the recirculating water in closed terrariums and keeps the system from smelling. Skip this layer in open terrariums if you want — air circulation handles the smell on its own — but it does no harm.
Layer 3 — Sphagnum moss barrier (1 cm)
Soak dry sphagnum moss in water for 5 minutes, squeeze most of the water out, and lay a continuous mat across the charcoal. This stops the potting mix above from washing down through the drainage and turning it to mud.
Layer 4 — Potting mix (5 to 10 cm)
For closed terrariums, fill with peat-free houseplant mix + 30 percent perlite. For open terrariums, fill with cactus and succulent mix. Slope the surface — higher at the back, lower at the front — for visual depth.
The soil layer needs to be deep enough for the deepest-rooted plant in your collection plus 1 to 2 cm. For most starter plants (fittonia, peperomia, haworthia) 5 to 7 cm is plenty.
Layer 5 — Plants and decoration
Plant the largest specimen first, working from back to front and tallest to shortest. Use long tweezers or chopsticks for narrow vessels.
- Tip each plant out of its nursery pot. Gently tease the root ball apart so the roots can spread.
- Use a chopstick to dig a hole, lower the plant in, and press soil around the roots.
- Leave at least 2 cm between each plant so they have room to grow into each other rather than fighting on day one.
- Add decorative stones, driftwood, or moss carpet on top of the soil.
- Mist the soil and leaves lightly with a spray bottle. Do not pour water in — closed terrariums need only the moisture in the soil and a light mist.
Step 6 — Settle and observe
Leave the lid off for the first 24 to 48 hours. Wipe condensation off the glass with a paper towel. You want some condensation in the morning, clearing by midday — that is the right water level. Heavy condensation all day means too wet; no condensation means too dry. Adjust accordingly before sealing.
For open terrariums, water the soil lightly until the top 2 cm is damp, then leave on a bright windowsill.
Plant selection by vessel — the matrix
| Vessel | Light | Humidity | Best plants | ASPCA pet-safe |
|---|---|---|---|---|
| Closed jar (sealed) | Bright indirect | 80 to 95 percent | Fittonia, baby tears, peperomia, polka dot plant, baby fern, prayer plant | All listed non-toxic |
| Closed bell jar | Bright indirect | 80 to 95 percent | As above + small moss cushions, club moss, selaginella | All non-toxic |
| Open bowl (succulent) | Bright direct | 30 to 50 percent | Haworthia, echeveria, hens-and-chicks, small cacti | All non-toxic |
| Open bowl (cactus) | Bright direct | 20 to 40 percent | Mammillaria, rebutia, gymnocalycium, lithops | Non-toxic (spine hazard) |
| Air plant display | Bright indirect | 40 to 60 percent | Tillandsia ionantha, T. xerographica, T. caput-medusae | Non-toxic |
For broader pet-safe options across the house see pet-safe houseplants.
Care after the build — 12-month plan
Closed terrarium care
- Daily for week 1: check condensation. Wipe excess off the glass; lid off for an hour if soil is visibly wet.
- Weekly: open the lid for 10 to 15 minutes for fresh air. Closed terrariums can develop mould if airflow drops to zero.
- Every 4 to 8 weeks: lift the lid and check soil moisture by finger-test through a vent. If the top 2 cm is dry, mist 5 to 10 sprays. Most closed terrariums need watering only every 1 to 3 months.
- Quarterly: trim back any plant that has outgrown its position. Compost trimmings or use cuttings to start a new terrarium.
- Light: bright indirect. Direct sun on a sealed glass jar cooks the contents — temperatures rise 10 to 15 degrees inside.
- No fertiliser for the first 12 months. Closed terrariums are intentionally low-nutrient — fertilising triggers overgrowth and algae.
Open succulent terrarium care
- Watering: every 2 to 3 weeks in summer, every 4 to 6 weeks in winter. Pour water gently around the roots (not over the rosettes) until the top 2 cm of soil is damp.
- Light: bright direct. South or west window, or a grow light. Insufficient light = stretching (etiolation) within 4 to 6 weeks.
- Air movement: open terrariums benefit from gentle airflow. Don't sit them in a still corner.
- Fertiliser: half-strength cactus feed once in spring and once in summer. None over winter.
- Top dressing refresh: brush off dust from the gravel layer every few months for visual freshness.
For broader succulent care, see best soil for succulents and how often to water succulents.
Troubleshooting common terrarium problems
Condensation never clears in a closed terrarium. Too much water. Leave the lid off for 24 hours, dab moisture off the glass with paper towel, then reseal. Recheck after a week.
No condensation at all. Too dry. Mist 10 to 20 sprays and reseal. Should return to morning-condensation pattern within 2 to 3 days.
Mould on soil or moss. Common in week 2 to 4 — usually white fuzzy patches. Lift the lid for 48 hours and increase indirect airflow. Springtails (tiny soil arthropods) are a permanent fix — add 10 to 20 to the soil; they eat mould and decaying plant matter without harming live plants.
Yellowing leaves on fittonia / prayer plant. Too wet at the roots. Hold off misting for 2 weeks and lift the lid for 24 hours daily until soil dries.
Cacti stretching tall and thin. Not enough light. Move the open terrarium closer to a bright window or add a small grow light.
Algae growth on glass (closed). Too much direct sun. Move to bright indirect light. Wipe glass with paper towel inside (clean only — no soap).
Smell from soil. Anaerobic decay. The charcoal layer should prevent this, but if you skipped it, open the lid for several days and consider rebuilding with charcoal next time.
Advanced moves — once you've nailed the basics
- Bioactive terrariums: add springtails (Folsomia candida) and isopods (Porcellio scaber) to a closed terrarium. They eat decaying matter and mould, creating a true self-sustaining ecosystem that can run for 5+ years untouched. Buy live cultures from Josh's Frogs (US) or The Spider Shop (UK).
- Layered colour design: in open terrariums, use coloured sand or aquarium gravel in visible stripes for the drainage layer. Skip in closed terrariums where the design gets obscured by condensation.
- Driftwood and stone scapes: position rough driftwood at the back and smaller smooth stones at the front before planting for a depth-of-field effect.
- Mood-moss carpets: in closed terrariums, lay a continuous carpet of cushion moss across the soil for an old-growth-forest look. Source from terrarium specialists, never wild-harvest from protected areas.
- Aquatic terrariums (paludariums): a step up — half soil, half water, with plants like cryptocoryne. Closer to a small aquarium than a terrarium.
Related
- Indoor garden setup — broader gear and 30-day setup plan
- Pet-safe houseplants — full ASPCA-verified list
- Best soil for succulents — open-terrarium soil deep-dive
- How often to water succulents — open-bowl watering rhythm
- Humidity for houseplants — why closed terrariums work
- Types of succulents — picking succulent species
- Types of ferns — closed-terrarium fern matrix
- Calathea care — humidity-loving plant care
- Prayer plant care — terrarium-friendly species
- Pot size calculator — sizing for terrarium plant divisions
Pet safety verified against the ASPCA toxic and non-toxic plant database. Terrarium structure and care cross-referenced with University of Missouri Extension publication G6520 (terrariums) and specialist sources Terrarium Tribe and Pistils Nursery.
Frequently asked questions
How do I know whether to build an open or closed terrarium?
Match the vessel to the plants you want. Closed (sealed jar with lid) suits humidity-loving tropicals — fittonia, baby tears, baby fern, peperomia, prayer plant, polka dot plant — and self-sustains for weeks once balanced. Open (uncovered bowl) suits dry-air plants — haworthia, echeveria, hens-and-chicks, small cacti, air plants — and needs occasional watering like a regular pot. Never put succulents in a closed terrarium (they rot within 2 to 3 weeks) or ferns in an open bowl (they crisp within days).
Do I really need an activated charcoal layer?
Yes for closed terrariums, optional for open. Activated horticultural charcoal filters the recirculating water in a sealed ecosystem and prevents the soil from going anaerobic and smelling. Skip it and a closed jar can develop a sulphurous smell within 2 to 3 weeks. In an open bowl, the air circulation handles the smell on its own, so charcoal is optional but does no harm. Use horticultural grade, never BBQ briquettes — those contain accelerants that are toxic to plants.
What is the best plant for a beginner closed terrarium?
Fittonia (nerve plant). It loves the constant humidity inside a closed jar, gives visible feedback (leaves wilt dramatically when thirsty, recover within hours after misting), and is non-toxic to cats and dogs per the ASPCA. Pair with a button fern at the back and a polka dot plant for colour. Avoid calathea — looks tempting but is too sensitive and signals stress unpredictably in beginner terrariums.
Are terrarium plants safe for cats and dogs?
Most popular closed-terrarium plants are non-toxic per the ASPCA — fittonia, baby tears, peperomia, polka dot plant, prayer plant, true ferns including bird's nest and Boston fern, and most mosses. Pet hazards to avoid: asparagus fern (toxic despite the name), pothos, philodendron, calla lily, and jade plant (toxic — common succulent terrarium pick to specifically avoid). Always check the Latin name against the ASPCA non-toxic list before buying. See our [pet-safe houseplants](/blog/pet-safe-houseplants) deep-dive.
How often do you water a closed terrarium?
Roughly every 4 to 8 weeks for a well-balanced sealed jar — sometimes less. The water inside recycles through evaporation and condensation, so the only loss is whatever sneaks past the lid seal. Watch the condensation pattern: morning fog clearing by midday means perfect balance; no condensation means too dry (mist 5 to 10 sprays); heavy condensation all day means too wet (lid off for 24 to 48 hours). Open terrariums with succulents need a light watering every 2 to 3 weeks.
Why is my closed terrarium growing mould?
Three causes: too much water at setup, no airflow, or organic debris (a fallen leaf) decomposing. Fix in this order. Open the lid for 48 hours to dry the system out. Pick out any visibly dead plant material with tweezers. If mould returns, add 10 to 20 springtails (Folsomia candida) to the soil — they are tiny harmless arthropods that eat mould and decay without harming live plants. Springtail cultures are sold by Josh's Frogs in the US and The Spider Shop in the UK.
Can a terrarium really last for years without intervention?
Yes, a properly balanced bioactive closed terrarium with springtails and isopods can run for 5+ years untouched. The famous David Latimer bottle garden (planted 1960, last watered 1972) is the canonical example. The recipe: sealed glass, drainage + charcoal + moss + soil + slow-growing plants like fittonia and baby tears, springtails as a clean-up crew, indirect light, no fertiliser, no opening except for the rare top-up mist if condensation drops to zero.
How does Growli help me look after a terrarium?
Photograph the inside of your terrarium and the Growli app identifies every species (fittonia, fern, moss, etc), checks they are compatible with each other and with your vessel type, and warns if anything is mismatched (a jade plant in a closed jar, or a fern in an open bowl). The app sets a unified watering reminder, tracks condensation patterns when you tag photos, and suggests when to lift the lid for airflow. The conversational AI also troubleshoots mould, etiolation, and yellowing leaves — upload a photo and it diagnoses the cause.