Why does mold grow in a closed terrarium?

Closed terrariums can become infested with mold. Mold is the biggest threat to a stable everlasting terrarium and can steal all the nutrients from your plants in no time. This causes your plants to wither, your ecosystem to topple over, and your closed terrarium to become nothing more than a brown mass and more like compost. In this article, we'll look at why mold develops in a closed terrarium and how you can prevent and combat it.

Why does mold grow?

A perpetual terrarium can be the perfect breeding ground for mold. As you may know from home, mold grows especially where there is a lot of moisture accumulation over a long period of time. In a perpetual terrarium, there is naturally a lot of moisture to keep the water cycle working properly. If puddles form from all that water, mold can quickly form around them.

In addition, in a closed terrarium is always a very comfortable temperature. Most mold species will also grow in cold, dark corners, but a nice warm room temperature (or slightly above) will not harm them at all. Due to the greenhouse effect, it will always be slightly warmer in your closed terrarium than in your room. Also the humidity is higher because warm air can absorb more water than cold air.

Mold grows much, much faster than most plants and can easily double in size in the course of a day. Small patches of mold are not dangerous for terrariums, but can quickly grow larger and overgrow everything.

Mold on fungus closed terrarium bottle garden
A mold growing on a mushroom. Source: Anna (sapphyre) at MushroomObserver.org, CC BY-SA 3.0

Types of mold in a closed terrarium

In a terrarium, most of the same types of mold appear that you can usually find at home on the windowsill, wallpaper or bread. Among them, simply put:

  • Yellow mold
  • Red mold
  • White mold
  • Green mold
  • Black mold

Regarding these forms of mold, it is important to note that they can be different species, but they are apparently not really distinguishable from each other. Most often in your terrarium you will probably encounter the green mold Aspergillus fumigatus, which grows particularly well on food and soil. However, black mold is not unlikely either. If you find black mold in your terrarium, you should do something about it quickly, as it can be very toxic to humans. Black mold triggers diseases such as kidney disease or stachybotryosis, among others.

Black mold closed terrarium bottle garden
Black mold on the lid of my bottle garden

How can you prevent mold?

Mold can be prevented in a closed terrarium in several ways. You will not be able to avoid high humidity, but you can ensure good drainage of water. You have to prevent standing water in any case. On the one hand you can do this by using the wrong soil. If you have a loose layer of stones under your substrate, excess water can run there and not cause any damage.

You can read about how to build a false bottom for a closed terrarium here.

A false bottom in a closed terrarium
A false bottom in a closed terrarium

However, in order for the water to drain well into your false bottom, you should provide a loose substrate with a lower average surface area. You can reduce the surface area of your substrate by mixing in larger objects such as rocks. This will also provide better aeration of your substrate, allowing your plants to take up more oxygen.

In addition, activated carbon can prevent the effects mold. Mold can colonize other places by dispersing fungal spores in the air. However, if there is activated charcoal in your substrate, many of these spores will be bound by the activated charcoal and will no longer be able to fly through the air. By the way, this can also be a great advantage for open terrariums and potted plants. Activated carbon can also bind so-called mycotoxins (toxins) and thus prevent us from inhaling them and becoming ill as a result.

Read on here to learn more about substrates for closed terrariums.

How can you fight mold?

If you already have mold in your closed terrarium or bottle garden, it won't be too easy to mix activated charcoal into your substrate or retrofit a false bottom. However, there are other tricks that can help you.

First, make sure your closed terrarium isn't too wet. There is a relatively simple trick to find out the right amount of water. Place your closed terrarium in a sunny spot and let it sit there for a day. In the morning, the glass walls should be misted or wet with water droplets. In the afternoon to evening, however, the glass wall should be completely dry. If it is still wet in the evening, you have too much water. Here you can read again in detail how much water a closed terrarium needs.

Another option is to clean with vinegar, but you have to be very careful not to get too much vinegar on your plants. You should really be sparing here and only hit the mold if possible. Because of its low pH, vinegar can be as damaging to your plants as it is to mold. Alternatively, you can try hydrogen peroxide. This is much more effective, but all the more risky. You can dab it on the appropriate areas with a small piece of cloth or a cotton swab.

One sure way to combat mold is with active cleanup creatures like springtails and woodlice. Although both springtails and woodlice prefer to eat dead plant material (detritus), they are not averse to fungi. In principle they can eat almost everything. Some springtails can even digest heavy metals. Mold doesn't bother them at all there. You can cultivate springtails very easily yourself or buy them cheap online.

Three springtails
Different kinds of springtails. Source: AJC1 on Wikimedia Commons, CC BY-SA 3.0

If you follow all of these tips, mold should be a thing of the past in your closed terrarium or bottle garden.

Summary

Mold can develop in closed terrariums and bottle gardens due to excessive humidity. The high humidity and temperature provide the best breeding ground for various types of mold. Especially in stagnant water, mold can quickly become a problem. To prevent mold in a closed terrarium, install a false bottom, loosen the substrate well, provide the optimal amount of water if possible, and if necessary, use springtails and woodlice that can eat mold.

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