Just like any plant your closed terrarium needs water. The amount of water is largely dependand on the size of your container, the kind of substrate you're using and especially the kinds of plants in your closed terrarium. How can you see if it's too wet? There's no simple answer. It really is a balancing act. In this article I'll show you some tips and tricks to reach the right amount.
Why do we even need water?
Water is one of the main building blocks of life. Although you only need to water your closed terrarium once until you close it up forever, it sadly isn't that simple. Too much water and your plants will quickly drown. That's more like a bad aquarium. Too little water and your plants won't be able to transport enough nutrients from the substrate into their leaves and can no longer maintain photosynthesis. Watering a closed terarium is quite a balancing act. Luckily there are tips and tricks you can use to reach exactly the right amount.
Your closed terrarium has too little water
Let's first look at the case of your closed terrarium not having enough water and being too dry. On one hand you'll see your substrate being dry superficially and on the other hand your plants will look quite weak and unstable. That's since the water pressure inside them is low. You can think of it like a flat bicycle tire. Dry plants cannot transport their nutrients as well and thus cannot maintain photosynthesis. In your closed ecosystem, photosynthesis isn't only important for any single plant alone but also for all of the animals living there.
The oxygen cycle in a closed terrarium
Animals and humans alike breathe in oxygen and convert it into carbon dioxide. Plants revert this process as long as they get enough sunlight. Using water and carbon dioxide they produce sugar (glucose) and oxygen. As soon as it's dark plants also breathe in oxygen and produce carbon dioxide. This closes the oxygen cycle. If your plants cannot maintain photosynthesis, soon all of the oxygen in your closed terrarium will be used up and everything will asphyxiate.
A closed terrarium really doesn't need that much water. I used a bottle cap to water my very first closed terrarium in a jar. I filled the cap up five times and distributed the water evenly. I quickly realized that even such a smal amount of water was too much for my closed terrarium. It was fogged up all day and therefore too wet. If your own closed terrarium is too dry I recommend using two bottle caps of water to begin with. It lastly depends on how big your container is. If your closed terrarium is much smaller than mine, consider using a spray bottle. Just make sure that there is no residue of soap inside. Keep in mind that your closed ecosystem is, well, closed and any soap or detergent cannot escape.
Maybe your closed terrarium has too much water
We'll now look at the opposite problem. Your closed terrarium has too much water and you need to get rid of it or else your plants will drown. In a healthy not too wet substrate, small bubbles of air can form which your plants' roots use to get oxygen. If your soil is too wet, it has a look less of these air bubbles. There are a few signs indicating that your closed terrarium has too much water.
A fogged up closed terrarium
The first sign of too much water is your jar being fogged up so that you cannot really see you plants and animals anymore. If this condition stays constant over multiple days, your plants will start to become a little yellow-ish. Now you'll have to take action. Open up your closed terrarium so the excess water can evaporate. To speed things up, use a paper towel or a lint-free cloth to wipe the inside of your container. As soon as all droplets of water are gone, close it back up and wait for the next day. If it's still fogged up the entire day, repeat the process.
A well watered closed terrarium is fogged up in the morning and seems completely dry in the late afternoon. If there's still water the entire day, repeat the process we just covered. At some point you'll definitely have the right amount of water. Now you can close it up and will likely never have to open it again.
My first closed terrarium in a candy jar was dried the same way. Since then it's been thriving flawlessly. I didn't put in any more work since December 2020. Important! If your closed terrarium isn't entirely airtight you'll have to water it from time and time again. That's not a problem if you're using a sealing ring though.
It's obvious that your closed terrarium needs water. You can ensure the right amount by carefully observing and tactically ventilating your closed terrarium. If your closed terrarium is fogged up the entire day you have too much water and should open it in the afternoon for the water too evaporate. Too little water will be obvious by a dry substrate and weak plants. In this case you can water it with a bottle cap.
Did it work?
By the way: If you've built your closed terrarium and want to share it with the world, send me a photo to firstname.lastname@example.org or on Instagram @terrarium.blog. I'll gladly share nice photos and link your profile. You'll also be on the homepage of the Terrarium Blog!