Humidity is a key part of what makes a terrarium such a valuable horticultural tool.
The ability to trap moisture is fundamental to creating the tropical environment that allows us to grow such interesting and exotic plants.
But, humidity is a fickle mistress…
She’s as quick to change as the weather, and when it comes to plants she can quickly go from ideal to problematic.
After all, humidity is dependent on many factors, and maintaining an even balance can be tricky as we progress through the different seasons.
In this article, we’re going to break down the concept of humidity into digestible chunks that are super helpful to know, so you can learn to control it to fit the situation.
Let’s dive in.
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Understanding Terrarium Humidity
Humidity is simply a figure that’s used to describe the amount of water vapor in the air.
It seems like a straightforward concept but it can be expressed in a variety of ways, and if you’re working it out yourself the calculations can be a little confusing…
That’s why we’re keeping it simple (using tricks and tools to measure the humidity for us) and only discussing what we need to know for terrariums.
Usually, when we hear humidity levels on the morning weather report, we’re being told a percentage figure. That % figure is a relative humidity level, and it’s pretty much the only one we’re interested in.
It’s a reflection of how close the air is to becoming saturated (with 100% being the maximum).
As temperatures change, the amount of water vapor the air can hold will also change. Relative humidity accounts for that, which is why we use it.
Most high humidity terrarium plants are going to be looking for above 50% relative humidity.
This isn’t too hard to achieve in a terrarium if you have enough warmth, and considering tropical plants typically need a reasonably high temperature too – it kind of comes with the territory.
So, by understanding how relative humidity is affected by different factors, we can use them to our advantage to control the humidity in our terrariums.
How to Increase Humidity in a Terrarium
When we’re talking about ways to humidify a terrarium, they tend to fall into two different categories.
- Processes that can help you saturate the air.
- Processes that can drive up the potential relative humidity figure.
The type you need to employ will depend on your home and terrarium conditions.
For example, if the temperature in your home is typically warm and highly regulated, you’ll probably just need help in further saturating the air (as the capacity of that air to hold moisture is already high).
Whereas, if the temperature in your closed terrarium is low or prone to change a lot, you may need help in increasing that saturation threshold. After all, there’s not much value in saturating the air if the maximum it can hold is too low.
#1 Increase Heat
As we’ve discussed, warmer air can hold more moisture. So, it stands to reason that the most obvious way to increase humidity is to warm up the air. Simple!
In fact, it works two-fold, as the extra heat also makes water more readily evaporate.
There are lots of ways to increase the temperature inside your terrarium.
- The most balanced way is to simply crank up the central heating. Warming up your terrarium and the room it’s in, will balance condensation levels too by equalizing temperatures across the glass.
- Move the terrarium to a brighter spot (but still out of direct sunlight) to absorb more of the Sun’s rays. Take care with this one though, as the trapped heat can quickly build up.
- Pop your terrarium on top of a plant heating mat. This was my solution through the winter months, and it works particularly well if you have a drainage layer and a reservoir of water at the bottom.
#2 Increase Water
Adding more water into the system would appear to be the next obvious step, and it works… up to a point.
Having extra water available in the system can indeed fuel the evaporation process if temperature conditions are suitable, but it can’t drive the potential maximum humidity up beyond what the conditions allow.
That being said, having a drainage layer in place at the bottom of your terrarium can help a lot, as that reservoir of water can further feed the system.
Just note, that a deeper drainage layer isn’t more effective than a shallow one when it comes to evaporation.
Water will only evaporate from the surface, so wider containers with more surface area exposed to the air should evaporate more, but deeper water won’t make a difference.
#3 Use the Right Substrate
Terrarium substrates with a high potential for water retention can significantly boost ambient humidity where there’s sufficient warmth and moisture (see our terrarium substrate guide for more info).
Plus, your system can hold more water overall without waterlogging your plants.
When it comes to growing tropical plants in a terrarium, you’ll almost certainly benefit from a substrate that retains moisture well. So, I would look to include these materials in your substrate mix anyway.
#4 Add More Plants
It’s easy to forget that plants play a key part in this living water cycle too.
More plants = More transpiration = More water evaporation
If water is the fuel for your terrarium, then your plants are the engine to create humidity. So, the more full your terrarium is with plants, the easier it is to maintain that high humidity level necessary for tropical plants.
See our Terrarium Plant Index if you need help filling your terrarium with plants!
Due to its large surface area and density, moss is particularly good for increasing the amount of plant biomass in your terrarium without having to stuff it to the brim with plants (part of the reason moss terrariums are so easy to look after).
As if you need any reason to add more moss, right?
#5 Add A Misting System or Fogger
Finally, there’s the artificial way to increase humidity.
Terrarium humidifiers come in a variety of options. Both mechanisms will periodically spray the terrarium with a set amount of moisture, it’s only really the aesthetic that changes when you’re choosing between a fogger or mister.
The mechanism is going to periodically spray water into the terrarium from an external reservoir. It’s super helpful in larger ventilated terrariums where humidity can be lost, but it’ll quickly oversaturate a smaller or fully sealed terrarium.
How to Decrease Humidity in Terrariums
This probably isn’t going to be quite as common a problem, but for more temperate terrariums full of plants that don’t like high humidity, there are ways to reduce it.
Most of them are the exact opposite of the above suggestions, but also one very easy additional method.
- Increase ventilation – opening up your terrarium periodically (or permanently if your plants allow it) will allow water vapor to escape, and very effectively prevent the build-up of humidity.
- Cool your terrarium – reduce the air’s capacity to hold water and reduce evaporation rates at the same time.
- Use a coarser substrate – the less water your substrate can retain, the less water you can maintain the system at.
How to Check Humidity in a Terrarium
When it comes to checking the humidity levels in your terrariums, There’s the scientific way and the visual way.
The quickest way is to simply observe the amount of condensation on the glass.
If water is condensing, you can assume the air is near its saturation point. Of course, there are extra factors to consider here – as the cooler glass surface causes condensation – but you can read more about that in the terrarium condensation guide.
You can at least tell that your terrarium has a reasonable level of humidity via condensation.
If you want exact figures, you’re going to need a hygrometer (essentially a terrarium humidity sensor). They’re fairly inexpensive, and I picked up one from Amazon for very cheap.
The Automated Option
For those who love a technical solution, there are fully automated terrarium temperature and humidity controllers on the market.
For example, when combined with a mister and a fan, the REPTI ZOO EZ Smart Thermo-Hygrostat Timer Controller seems to be capable of regulating most terrarium conditions.
It’s probably overkill for most plant terrariums, but for vivariums with animals or large plant terrariums that need significant regulation then I can see the benefit. I guess it gives you peace of mind too.
Over to You
Do you have any issues maintaining humidity? It can seem like a difficult thing to wrangle but there’s always a solution.
If you need more guidance getting all of the elements of your terrarium just right – check out our Essential Guide to Tropical Terrariums!