top of page
Card Back.png

Terrariums and Mold

Mold is essentially a collection of fungal cells, known as hyphae. Under the right conditions, these cells interconnect, forming a noticeable mass termed as mycelium. This growth typically occurs on decaying organic matter and is particularly fond of warm, moist settings. Fungi serve an essential function on our planet, aiding in the decomposition of organic materials and supporting plant growth. Given the consistently organic and often humid environment of a closed terrarium, it's no surprise that mold can be a challenge for terrariums. However, remember that not every fungal growth indicates a problem for your mini-ecosystem. Nearly every new terrarium will experience some mold growth, especially as the ecosystem comes into balance.  But in a confined space like a terrarium, unchecked mold growth can quickly dominate. Usually, mold issues boil down to introducing unsterilized components into your terrarium or overwatering. While there's no guaranteed method to completely prevent mold, we’ve incorporated some strategies that have kept our terrariums nearly mold-free.

How to prevent Mold


Ensure Clean Glassware 

Think of it as preparing for homemade jams or sauces; always start with immaculate glassware. Never assume your containers are free from contaminants. Tiny fungal spores or bacteria might lurk within your containers, making cleaning essential.


Our preferred method is a good alcohol wash.  Isopropyl alcohol, particularly in solutions between 60% and 90% alcohol with 10 – 40% purified water, is rapidly antimicrobial against bacteria, fungi, and viruses. We dilute with 30% water and spritz the vessel inside and out with a clean sprayer. Wipe down with a paper towel and let it air dry. Follow up with purified water to remove any traces of the alcohol.  If you don’t have access to alcohol or a sprayer, the next best method is a good wash.  Give your vessel a good clean inside and out using hot water and soap.  For vessels with tricky designs, fill them with hot water for 10 minutes before rinsing thoroughly to ensure you remove any residual soap.


Sterilize Your Gravel

For many people, the gravel drainage layer becomes mold's breeding ground. This often results from using un-sanitized drainage materials.

Steps for Boiling Gravel:

1 - Wash your gravel thoroughly to remove any unwanted dirt and debris.

2 - Fill a sturdy pot with your gravel.

3 - Add water and simmer on the stove, ensuring it boils without overflowing.

4 - After 15 minutes, drain and let the gravel air-dry.

Ensure the gravel has adequate time to cool before you touch it.


This process, akin to baking, helps get rid of unwanted microorganisms.


Use Activated Charcoal

Hazardous chemicals might be present in your terrarium's soil, substrates, and watering source, posing threats to your plants and moss. This, in turn, can lead to mold. Activated charcoal, or activated carbon, has a vast, porous structure and is a magnet for toxins and harmful chemicals, mitigating odors and fostering a thriving ecosystem. You can incorporate it as a distinct layer or mix it within the drainage layer.



Ordinary charcoal is effective but not as potent as its activated counterpart. For longevity, opt for the activated variety.

Charcoal is available online and in many aquarium stores.


Use Sphagnum Moss

We use a layer of Sphagnum moss in all of our terrariums between the drainage and soil.  Besides retaining moisture for roots, sphagnum moss offers multiple advantages:

Its phenolic compounds create anaerobic, low-PH soil conditions, hindering mold growth.

It protects plants from excessive moisture, safeguarding against root rot.

Its airy structure aids in humidity regulation, further minimizing mold chances.


Sterilize Your Soil

Every terrarium layer has its purpose. And for a plant-centric closed terrarium, the nutrient-rich substrate is critical. Unfortunately, dormant spores can lie in wait in the soil.

To Sterilize Soil:

Oven Method: For bulk amounts, preheat your oven to 180-200F / 82-93C. Place the soil in an oven-safe dish, cover with foil, and bake for 30 minutes.

Microwave Method: For smaller batches, put the soil in a microwave-safe container (avoid foil) and microwave for 45 seconds per pound of soil.

Be Selective About Your Sources

Plants and moss can carry fungal spores, but that's not the only reason for sourcing them cautiously. If foraging, it's vital to ensure you're not unintentionally damaging your terrarium.

Foraging Tips:

Choose Your Moss Spot: Evaluate where you're sourcing your moss. Moss from sidewalks might not be as pristine as that from tree bases.

Clean Your Moss: A preliminary soak can help shed unwanted inhabitants. If any critters are present, they'll likely float, allowing easy removal.

Assess Health: Make sure to inspect the health of any plants or moss you collect. Weak or dying plants can become fodder for mold.


By adhering to these guidelines, you'll enhance the longevity and health of your terrarium, reducing the risk of mold invasions.


If despite all your best-efforts mold starts to overtake your terrarium, we recommend the following:


Recruit a Cleanup Team


For long-term health, we consider introducing some friendly critters that serve as a natural cleanup crew to be essential. This isn't just about mold mitigation but cultivating a self-regulating ecosystem. While the idea of bugs may be unappealing to some terrarium enthusiasts, these tiny allies can play a vital role. A terrarium is not likely to have long term success without these beneficial helpers.



A favorite among many, Springtails are primitive, wingless microarthropods, closely related to insects. They’re among the oldest of terrestrial animals, with a 400-million-year-old Collembola fossil discovered in Scotland. The name ‘springtail’ comes from a lever-like hinged spring, the furcula, located on the underside of their abdomen. This appendage is held under tension, and once released, snaps against the ground propelling the springtail up to several times its own body length Their minuscule size means they'll largely go unnoticed in your terrarium. They are very good at regulating their population, thriving when mold and bacteria are plentiful and dying off when food is scarce. Springtails can be purchased from most pet stores and online. A healthy population in your terrarium will go a LONG way to preventing an overgrowth of mold.



Another great addition, Isopods are commonly known as “Roly-Polys” and are not insects but are crustaceans like lobsters. They breathe through gills and consume debris such as dead plant material. This is why they make great additions to the terrarium. They will convert dead and dying foliage to Nitrogen and Oxygen to feed the ecosystem.  While Isopods prefer rotting plant matter, they WILL eat live plants if no other food source is available. Care should be taken to not overburden the terrarium with too many ispopods for the dying material to support of they will start to eat up your live plants.


If all else has failed and your terrarium is being overrun by mold, there is a chemical solution that will not harm your new terrarium.


Hydrogen Peroxide


For established terrariums showing mold signs, hydrogen peroxide can be great last alternative. Hydrogen Peroxide is simply water with an extra Oxygen molecule. When it meets mold and bacteria, the extra Oxygen is released, breaking down protein and other matter. It can even be beneficial by oxygenating the soil and plants.


 Dab a cotton swab in a 3% hydrogen peroxide solution, usually available online, and gently clean the moldy sections.

For broader issues, spraying a diluted solution (1 part peroxide to 32 parts water) into the terrarium.

After application, make sure to let the extra moisture you’ve introduced evaporate out.

Remember: moderation is crucial. While hydrogen peroxide is generally safe, avoid excessive amounts.


In conclusion, don’t fear mold! Mold is part of every environment on earth and is essential to a thriving ecosystem. You have the tools you need to control it and keep your terrarium thriving and happy!

bottom of page