Quick answer: A grow-in is the period between building your bioactive terrarium and adding your animal. Most keepers wait 3–6 weeks for tropical builds and 2–4 weeks for desert builds. During that time, plants root and adjust, springtails and isopods multiply, any initial mould bloom passes, and you dial in temperature, humidity and lighting. Your enclosure is ready when the plants are pushing new growth, the clean-up crew is visible and breeding, and conditions are stable for at least a week.
Why grow-in matters
A freshly built bioactive enclosure is unstable. Soil microbes haven't established, plants are recovering from transplant shock, and there's a burst of mould as fungi feed on fresh organic matter. Adding an animal on day one causes problems:
- Plants get trampled or uprooted before their roots anchor.
- The clean-up crew gets eaten or overwhelmed before it can reproduce.
- Humidity and temperature swing while you're still adjusting vents and misting.
- Any pesticide residue on plants has no chance to show itself before an animal is exposed.
A few weeks of patience gives you a stable, self-maintaining system that's far more forgiving long term.
Day 0: Build day checklist
- Drainage layer: clay balls in tropical builds. See drainage layer guide.
- Barrier: mesh or a layer of dry sphagnum between drainage and soil.
- Bioactive soil: Jurassic ABG Terrarium Mix (tropical), Jurassic ABG Semi-Arid Blend (desert) or Jurassic ABG Tarantula Mix (inverts). All are Canadian-made by Jurassic Reptile Products. Use our substrate calculator for quantities.
- Hardscape: cork bark, spiderwood or driftwood, stones.
- Plants: cleaned of nursery soil and pests.
- Leaf litter: a generous layer of leaf litter, which is food and cover for the clean-up crew.
- Clean-up crew: springtails and isopods suited to your build.
- Equipment: lights on a timer, heat, thermometer and hygrometer.
Week-by-week: what to expect
| Week | What's happening | What you should do |
|---|---|---|
| 1 | Plants settle; some leaves may yellow or droop. White fungal fuzz often appears on wood and soil | Mist consistently; don't panic about mould; check temps daily |
| 2 | Mould peaks, then begins to fade as springtails multiply. First new leaves appear | Remove only mouldy decaying plant matter; keep lights on schedule |
| 3 | Springtails visible on soil and glass. Isopods foraging at night. Roots anchoring | Adjust vents and misting for stable humidity; prune any plant that's struggling |
| 4–6 | Plants actively growing, mould mostly gone, conditions stable | Check readiness; add more leaf litter; plan animal introduction |
The early mould bloom is so common it's almost a rite of passage. Read mould in a bioactive terrarium: what's normal for details.
Seeding and feeding the clean-up crew
Add springtails and isopods on build day or within the first week so they can establish before your animal arrives. During grow-in there's no animal waste, so feed them lightly:
- A sprinkle of fish flakes, dried yeast or springtail food every few days for springtails.
- Leaf litter, plus occasional vegetables, a piece of cork bark to hide under, and a calcium source like cuttlebone for isopods.
- Remove uneaten fresh food after 24–48 hours to avoid mites and mould.
Full species advice: springtails and isopods: the complete clean-up crew guide.
Dial in your environment
The grow-in is the perfect time to fine-tune conditions without an animal being affected:
- Temperature: set up your basking and ambient gradient and log readings morning and evening.
- Humidity: work out your misting schedule and ventilation. Canadian winters dry enclosures out fast, so you may need to cover part of the screen. See humidity and ventilation guide.
- Lighting: set timers and watch how plants respond. See lighting guide.
Signs your bioactive is ready
- Plants are pushing new leaves and roots are holding firm.
- Springtails are visible in numbers, especially when you lift leaf litter or cork.
- Isopods are active at night and you've seen juveniles (mancae).
- The initial mould bloom is gone or minimal.
- Temperature and humidity have held within your target range for at least a week.
- No signs of pests like fungus gnats, mites in large numbers, or snails from plants.
Grow-in differences by build type
| Build | Typical grow-in | Notes |
|---|---|---|
| Tropical (dart frogs, crested geckos) | 4–6 weeks | Longer is better; dart frogs benefit from established microfauna |
| Semi-arid (leopard geckos, bearded dragons) | 2–4 weeks | Let succulents root; keep the lower soil layers moist for isopods |
| Invertebrates (tarantulas, scorpions) | 2–3 weeks | Ensure the crew is established before adding predatory inverts |
| Large snakes and lizards | 3–6 weeks | Heavy animals trample plants; robust plants and anchored hardscape are key |
FAQ
Can I skip the grow-in?
You can, but you'll lose more plants and the clean-up crew may not establish. If you must add an animal early, use hardy plants and add extra crew.
Should I keep the lights on during grow-in?
Yes. Run your normal photoperiod so plants grow and you can test temperatures.
My springtails disappeared. Is that normal?
They often hide in the soil. Lift a piece of cork or leaf litter. If you see none, mist more and add a little food.