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How to Fix Mold in a Terrarium ― Do THIS!

How to Fix Mold in a Terrarium ― Do THIS!

The internet has a lot to answer for. All those picture-perfect terrariums splattered across social media and on the covers of digital magazines never show the true story. 

The green algae climbing the glass of terrariums. The fogged glass that happens every day at some point. And the occasional fuzzy mold growth that needs to be controlled before it takes over. 

As easy as terrariums can be to grow, they are not a no-maintenance setup. 

They can be minimal maintenance. Not while the mold is there though. 

Read on to discover the quickest fixes and the ongoing maintenance needed to keep your terrarium free from mold. 


How to fix mold in a terrarium 

Mold grows when there is a lack of airflow, excess moisture, and high humidity. It grows on decomposing organic matter. In closed terrariums, springtails, and isopods break down organic matter, preventing mold problems. In an open terrarium, cut back on watering to lower moisture and humidity. 


The cause of mold in terrariums

Mold is an unavoidable consequence in terrariums. The spores of it are everywhere because they are airborne but invisible. 

The only time you see them is when those tiny spores germinate to form mold. 

Spores germinate in climates that are warm and humid. Sound familiar? It is the exact manmade climate inside a terrarium.


Closed terrariums must be bioactive 

Bioactive just means adding insects. In a bioactive vivarium, which is used to keep animals and plants alive, insects are added to control fungal and bacterial growth. 

The same setup is needed in a closed terrarium. The type of plant determines the type of ecosystem that will be needed. 

For example, moss is used is in open terrariums as a decorative ground covering. It will need to be replaced. Clubmoss (Selaginella kraussiana) is one species of moss that dies back fast in an open terrarium. 

Moss is a suitable ground covering for any type of terrarium. Some moss species will stay alive with consistent misting, but without high humidity, moss will not grow.

Closed terrariums are how to ‘grow’ moss in a terrarium. Moisture keeps the moss alive in open terrariums. It does not promote growth. 

Moisture + humidity = mold spore germination. 

This is where insects come in. 


Picking the right janitors (the clean-up crew) 

Just like in the garden, there are good bugs and bad bugs. The same ecosystem happens inside terrariums, just on a microscopic level. 

Instead of the bugs eating bugs, or feeding dead bugs to plants in a carnivorous terrarium as an organic diet booster, terrarium janitors keep the decaying material to a minimum, preventing mold from germinating.

Nematodes can be herbivores, detrivores, bacterivores, algivores and fungivores.

All of these are beneficial in the sense that they feed on dead organic matter. Herbivores are the only no-go nematodes for terrariums. Herbivores eat live plant matter. 

The most common terrarium clean-up crews (CUC’s) to use are: 

  • Isopods
  • Springtails
  • Millipedes 

Among those three, springtails are the most beneficial for controlling mold growth, mainly because they make quick work of the already decomposing organic matter. 

Where mold growth is heaviest, drop some springtails into the terrarium. 

Those are fast breeders. They have a reputation for eating mold. What they prefer to eat is dead plant debris that has decomposed to the point of almost liquifying. 

They make it look as though they eat the mold but in reality, they are just starving the mold out. They can eat the sugars out of decomposing plant matter faster than mold can use to grow on. The mold just dies back.


Isopods have a slower reproduction rate

Isopods are another detrivore, but being bigger than springtails, they make better work on decomposing wood. 

For mold that is growing mostly on driftwood (a common problem in terrariums), isopods are the better option. 

Being bigger, they make quick work at eating down moist wood, draining it of the sugary contents that the mold would use to grow. 


Millipedes help by increasing soil aeration

Millipedes are more suited to larger terrariums because they can be too big for smaller ecosystems. Like a glass bottle terrarium. 

In a terrarium with compacted soil, the plants will die. Dead plants in an enclosed space with heat and high humidity are how mold starts. 

The more of your plants you can keep alive, the less debris there is for mold to germinate. 


Airing the Terrarium Stops Mold Spreading

Airing a terrarium is the fastest way to drop the humidity levels and let the substrates dry out. 

Some plants may suffer a little, however, most are tolerant to an occasional drop. In a closed terrarium, once the substrate is misted and the lid is replaced, humidity increases quickly. 

Even faster when there is a heat mat under it. 

Speaking of heat mats, be careful with those. If the extra heat is not needed, it can be the reason for mold spores spreading. Mold spores seek out warm and humid climates. 

Mold is more likely to occur in closed terrariums than open ones. In a closed terrarium, the tank needs to be aired on occasion. Open terrariums naturally get more airflow contributing to mold prevention. 

In an open terrarium, the cause of mold will be the moisture levels being too high rather than a lack of airflow.

In an open terrarium, cut back on watering to let it dry out. 

In a closed terrarium, take the lid off to let it air out a little. 


Pruning or plant replacement 

Numerous plants grow ferociously in terrariums. Ferns are one of them. The bigger the green leaves are in enclosed space, the higher the moisture levels are. 

Plants breathe. That respiration pushes the airflow and moisture content up. Big plants crowded together will cause condensation. 

The higher the moisture content is will set the path for mold to germinate. 

Crowded terrariums rarely look good. They are inviting to fungi though. Mold will set up home in a crowded terrarium.

To prevent that from happening, keep plants in the terrarium pruned to a reasonable size. 

Any plants that die need to be removed or they just become a breeding ground for mold. 

Especially if the terrarium has no insects to get rid of the dead plants, which is more of a concern in open terrariums. Those will need to be pruned more frequently. 


The need for sterilization – on everything

Mold spores are in the air and tend to settle in terrariums because of the high humidity. 

Anything you put into an open or closed terrarium needs to be sterile. Rocks should be washed with bleach and the soil sterilized before using it. 

Any new plants should also have the roots sprayed with rubbing alcohol (70% isopropyl) to kill off any mold spores present. 

If you are maintaining an active bioactive terrarium with isopods and springtails, they may need the occasional feeding of leaf litter. Those should be sterilized before dropping into the terrarium too. 


Frequently Asked Questions related to fixing mold in a terrarium


Can I remove mold with a Q-tip dipped in hydrogen peroxide? 

Wiping mold with a Q-tip dipped in hydrogen peroxide (3%) is a spot treatment method. Use this when you see mold start, then air the terrarium to prevent more germinating. Once it spreads, it takes ages (and a lot of Q-tips) to remove.


What is the best clean-up crew for closed terrariums to eat mold? 

Clean-up crews technically do not eat mold. They starve it. The type to use depends on what decomposing matter is in the terrarium. For decaying leaves, use springtails. For decaying wood, use isopods (pill bugs). For plants experiencing high leaf drop, add some millipedes to keep the soil aerated.