Terrariums are the equipment used for growing tropical plants indoors. They add a superior decorative touch. Placed under grow lights, they make a stunning focal point.
As terrariums are used for tropical plants, the key to success in a terrarium necessitates stable humidity levels.
Both open and closed terrariums need to be kept humid. How humid depends on the types of plants you grow.
If you’re learning about how to plant a terrarium with succulents, it is an open terrarium, which requires more air circulation, which lowers humidity levels.
Using sealed enclosures is how to set up a carnivorous plant terrarium, which requires higher humidity.
In both cases, growers need to know how to keep a terrarium humid, but at which levels are determined by what you grow.
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How to keep a terrarium humid
Three things contribute to humidity in a terrarium. Moisture, temperature, and air. Misting adds moisture, heat raises the temperatures and the air holds the water vapor. Maintaining humidity in a terrarium has a lot more to do than the plants you choose than any equipment you try to use.
Understanding Humidity
High humidity hydrates plants. The basics of humidity is that is a reference to invisible water vapor in the air.
It is measured as a percentage so the highest humidity is 100%. Rarely does it ever reach that level.
When it rains, the relative humidity at the clouds-in-the-sky level is maxed out at 100%. At land level, the air never reaches maximum capacity.
When you hear the weather reporter state the relative humidity is x percent, the higher that relative humidity number is, the more likely it is to rain.
But that is not absolute, because in a closed terrarium, you can create a rain cycle ecosystem with humidity under 100%. Humidity over 70% generally creates finer water particles that can be absorbed by plants.
It is dependent on the temperature. The hotter it is, the more water vapor the air can hold.
For that to happen inside a terrarium, you need to monitor two variants. Temperature and humidity.
Higher temperatures mean more water vapor can be stored in the air. Lower temperatures mean the humidity levels drop making the ecosystem damp and moist.
That is not good in an open or closed terrarium.
The 3 best ways to keep a terrarium humid
Place a heat mat under the terrarium
As previously mentioned, the warmer the temperature, the more water vapor the air can hold.
In all types of terrariums, the base layer is a rock bed where the water pools under, gradually evaporating into the air.
The easiest way to boost humidity is to raise the temperature at the base. A heat mat does that.
Placing your terrarium on top of a heat mat warms the water at the base layer, and raises the temperature inside the terrarium, boosting humidity as it does.
Add more tropical plants
All houseplants increase humidity in the home. It is cause and effect in action. Transpiration and respiration. They do the same in terrariums, however, most are not acclimated to growing in such humid habitats.
The best types of houseplants for terrariums are tropical species that adapt well to low light conditions.
That fits the bill for a host of plants.
Some plants to consider adding to a terrarium include:
Ficus plants
Among the hundreds of Ficus species, there are a few that can go in a terrarium.
The Ficus Benjamina (Ficus tree) prefers the high humidity levels terrariums provide.
Indoors, it is a slow grower. It grows even more slowly when it is not fed a fertilizer. High humidity, warm temperatures, and plenty of indirect light keep this alive. It will not grow fast in a terrarium, but the leaves sure help boost humidity. It will removed when it grows too big for the enclosure.
The Ficus Pumila (Creeping Fig) is a trailing or climbing vining plant. The vines produce aerial roots and those cling to the glass of the terrarium. They can grow aggressively fast, so keep a pair of scissors nearby to regular cut them back.
These should only be used temporarily. If kept in the terrarium for too long, they will take over.
Another broadleaf trailing plant is the Golden Pothos, which similar to the creeping ivy, will require its growth to be kept in check to prevent it from ruling the land.
Bromeliads
There are over 2,500 bromeliads (a type of air plant), each of them requiring no soil, yet still assisting with humidity.
In a terrarium, these can be planted near the top of the canopy.
Attach them to pieces of wood, or rock, or if you lack a top canopy, gluing a suitable anchor material, even if it is a piece of foam stuck to the glass with a hole cut out to hold a pot, bromeliads will grow in terrariums.
They just need to be kept away from the ground soil. Bromeliad care is slightly easier in a terrarium, but they do need access to light.
How much light they need depends on the species you use.
Leaves change color when their light requirements are not being met. That is the part that makes bromeliads a little easier to care for than most other plant species.
Tillandsias
Tillandsias are another type of air plant.
The difference between bromeliads and tillandsias is that bromeliads have cups or tanks in the center of their leaves. That holds the water it needs.
Watering bromeliads just needs the tank to have water in it and for the water to be changed periodically to prevent it from stagnating.
Tillandsias do not have tanks to store water. Instead, these are true air plants that get nutrients through trichomes on their leaves.
A popular colorful variety for open terrariums is the Tillandsia Aeranthos.
Always remember that bromeliads are tillandsias are epiphytes and air plants. They need to be kept out of the soil and cannot survive in a closed terrarium because they need access to air.
Fern plants for terrariums
Some of the smaller fern plants suited to terrariums include the Plumosa fern (Asparagus fern) and the Lemon Button Fern (a dwarf variant of the Boston fern).
In an open terrarium where temperatures are cooler, Hart’s Tongue fern can work.
Ferns can also be an integrated natural (and free) alternative to using a hygrometer to monitor moisture levels. When humidity drops too low, fern leaves curling up is when you know to mist it. It beats misting when moss starts to turn brown.
Use moss to fill spaces
Moss is extremely absorbent. The most common type to use as a moisture barrier is sphagnum moss (where peat moss comes from). However, all moss grows through spores on the leaves. Not the roots in the soil.
The purpose of moss in terrariums is to hold moisture and slowly release it to the base layer where it then evaporates. Moss controls moisture levels, which in turn, support humidity in the terrarium. That is why it is so easy to grow moss in a terrarium.
The more moss you have, the more water can be absorbed, preventing water from sitting stagnant under the gravel base.
Take a look around your terrarium setup to see where all the nooks and crannies are. Between rocks, on top of stones, inside the shells of seashells, or atop some natural terrarium ornamentals.
Sheet moss and fern moss are often used as soil coverings.
Clumping varieties grow vertically and are suited to filling in tiny little crevices.
You may be surprised by how much more moss your terrarium can accommodate when you fill in spaces.
Mist more frequently or add more water
The warmer the temperature, the more water evaporates into the air. Expect open terrariums to need a light misting frequently.
To decrease the amount of misting required, use a bowl to place water inside the terrarium without it wetting the soil.
Misting will moisten the soil, but if it is already at capacity, it can cause the soil mix to become waterlogged, in which case water gets trapped in the soil instead of evaporating into the air raising humidity.
The workaround is to place a small amount of water inside the terrarium.
For a large terrarium, a saucer might fit. In a smaller terrarium, you may need to get creative by using smaller holders like a shot glass or a few sewing thimbles.
In an open terrarium, after misting or adding water, covering the opening decreases air circulation, which will also raise humidity temporarily, until you remove the cover from the opening.
Frequently Asked Questions about how to keep a terrarium humid
Do terrariums need automated misters?
Automated misters and foggers are most beneficial in vivariums (plants and animals in a closed terrarium). In a terrarium with only plants, the frequency of misting does not need as much accuracy. For plant terrariums, they are not needed but can be helpful for the forgetful.
How do grow lights impact the humidity in terrariums?
Grow lights are a heat source. In any terrarium, open or closed, increasing the light intensity warms the air, which speeds up water evaporation, which in turn increases humidity. Too much light can cause leggy growth and burn leaf foliage, so be careful with light intensity.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.