How to Decrease Humidity in a Terrarium (And Keep It Down)

Terrariums, especially closed terrariums, have inherently high humidity. But it only becomes a problem once it gets excessively high and causes things such as mold growth, constantly fogged glass, and worst of all root rot.

To decrease humidity in a closed terrarium simply open it periodically and let the humidity escape. For an open terrarium, just reduce the watering frequency and the humidity will go down. Less water will result in less humidity.

Keeping it down and preventing excess humidity from happening in the first place is always better than having to deal with it once it starts causing problems. The rest of this article will teach you how to control humidity before it gets out of hand, and exactly what to do when it does.

For Closed Terrariums

The most common cause for excessively high humidity in closed terrariums is a bad initial setup and overwatering. Overwatering is something that most terrarium builders fall victim to, myself included.

The only way to get rid of the excess humidity in your terrarium is to open it up and wait. Some terrariums will need to be aired more often than others because of the shape of the container.

Glass bottle terrariums for example have a very narrow neck, and will need to be aired more often and for longer if there is excess humidity, compared to a terrarium with a wider neck like a fishbowl or jar.

For next time, when in doubt always water less. And always use a spray bottle and never water directly.

For Open Terrariums

Decreasing humidity in open terrariums is a lot simpler than in closed terrariums and it just comes down to less frequent watering and a temporary change in temperature.

High humidity and high temperatures go hand in hand. If you cool the air inside the terrarium, it will hold less water. You can do this by placing the terrarium in a colder room for an hour or two. As far as watering goes, it’s pretty straightforward, less water = less humidity.

A good initial setup with an adequate container will prevent humidity problems from arising. Typically containers with a narrow neck and many plants, or big plants, will retain a lot more humidity than containers with a wide neck and fewer plants.

How To Keep Humidity Down

Once you get the initial setup right, and your terrarium finds a healthy water cycle, you will not need to worry about humidity. The moisture inside will stay consistent indefinitely. The trick is to get it right the first time by appropriately watering and a proper setup.

the terrarium water cycle illustrated

If you are still fresh with building terrariums, don’t overthink these things. You’ll inevitably make mistakes. Everybody does. You’ll get the hang of it after a few terrariums if you are attentive and observant. Some general advice would be to do a bit of research about the plants you use. For example:

  • Mosses thrive in high humidity, so putting them in a closed terrarium with a humidity-retaining soil, like clay-based soils, is ideal
  • Succulents on the other hand don’t like high humidity, so you should only place them in open terrariums
  • Plants that have root systems and at the same time like high humidity, like the polka dot plant, require a drainage layer in your terrarium so that the roots don’t drown

And so on. You get the point. A little research can mean a lot.


Now you know how to decrease the humidity in both open and closed terrariums alike. It’s fairly simple and easy to do, but it can still be a headache, especially if you overwater the substrate.

The best way to lower excess humidity is to avoid it altogether by following the steps we talked about. If you do make a few mistakes, don’t overthink it, and don’t stress over it. Messing up is a part of building terrariums (wish it wasn’t).

Good luck, and happy terrarium building!

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