Let’s Build a Terrarium

by Evin Morrison, Extension Master Gardener
Photos by Evin Morrison

Choosing a vessel

While just about any glass or plastic container can be turned into a terrarium, there are a few things to keep in mind. Terrariums have multiple layers that ensure plants have proper drainage while still benefiting from the moist environment. So, is the vessel tall enough to accommodate the layers, the root ball of your plant, and the foliage? If the plant will be close to the top of the terrarium from the beginning, once it begins to grow you will have to deconstruct and replant your project much sooner than if you start with a larger vessel or shorter plants.

How does the vessel close? Many store-bought terrariums have an open side, a door or even vents to allow you to have access for maintenance and for air flow. It’s important to remember that any opening, even the space around miniature hinges on the doors, will allow humidity to escape. For plants like mosses and some rare begonias, choose systems that are as closed as possible to really lock in the moisture — or you will need to check on your terrarium much more often to make sure the water hasn’t evaporated.

And lastly, before you add all your substrate layers and plant your terrarium, fill your vessel with about an inch of water and set it in the sink or bathtub. Rectify any signs of leaking now before you put in all the effort and perhaps ruin a piece of furniture with a leaky terrarium.

Let’s start building

Starting with a clean vessel, add your first layer. This is a drainage layer that will hold excess water and allow it to be pulled back up into the plants. You can use clean pebbles, LECA, or even lava rocks. Make sure you rinse your drainage layer to wash away any rock dust and small particles. You don’t want dust and debris to form an unsightly silt layer at the bottom of your pretty terrarium. Washing this silt down the drain can also cause havoc on indoor plumbing and pipes, so wash it off outdoors.

Next is the  barrier layer. This can be made from several supplies, like leftover window screening, hardware mesh, garden cloth, or even coffee filters (although they will eventually break down). The goal of this layer is to prevent the smaller particles in  the next layers from settling down into the rock layer.

Like in a layered dessert, you want to be able to see each distinct layer in your finished product, so this barrier helps hold everything above the rocks from washing down and filling in the gaps. Whatever you choose, make sure the openings are small enough to hold back the substrate particles but the barrier is permeable enough to allow water to pass through. Many weed-blocker materials are not water permeable and will block the flow of water through the  layers.

You might think now it’s time for soil, but not quite! We still have a few layers to go. The  next two layers help maintain the environment, especially in a closed system. Moisture retention is obviously highly important, but too much moisture can also lead to bacteria and fungal growth. So, the  third layer helps combat both issues. It’s a filter layer that also happens to be great at holding moisture. This is a combination of sphagnum moss and horticultural charcoal. Both materials have antimicrobial properties that will help ensure your terrarium system stays clean. Build a nest of moss, pressing it up to the edges and then add a small layer of charcoal within the nest. This makes it so the “pretty” moss is seen from the outside, but the charcoal is still mixed in the middle to help kill off bacteria.

Now it’s time for the  substrate mix. A combination of cocoa coir, perlite, worm castings, and orchid bark is a great option for terrarium soil. The coir is an eco-friendlier alternative to the peat moss found in many premixed bags of soil. The perlite and orchid bark help maintain airflow and structure in the soil, improving drainage. The worm castings are a gentle fertilizer option that will keep your plants fed while in the closed system.

Of course, there are pre-mixed terrarium soils on the market, and there is no rule against using standard houseplant mix. Just make sure they are fluffy and include perlite or add your own to avoid their settling and getting too heavy. Keeping a good structure that allows adequate airflow will ensure your plants have healthy root systems.

Plant it up

Now for the fun part. It’s time to add the plants. Terrariums are a great place to plant some of your propagations to help them transition from water into soil, or you can also use store-bought plants that like a humid environment. While many plants will thrive in a terrarium, just as many will struggle with the humidity level and will end up rotting—so do a little research before chucking just anything into your new growing space.

Some common tried-and-true terrarium plants include ferns, syngonium, and fittonias. Especially with store-bought plants, you might find that the root ball is too large to plant. Try knocking off as much of the excess soil as possible to make them fit. You can also divide many of these plants by gently teasing them apart and creating three or four smaller plants to spread out in your planting scheme.

To finish your terrarium, you can top dress the soil with orchid bark or moss, add decorations, or just leave it plain. Part of the fun of terrarium builds is to show off your personality and to make them blend in with your décor. Give it a go and be creative!

What about water?

The first time you water your terrarium, pour water from your watering can down the inside of the glass. This will not only add water but will help wash down all the debris that stuck to the glass during the building process. You want enough water to wet the substrate but not so much to have a standing puddle in the  drainage layer. Always err on the side of not enough because it’s much easier to add a little more water than to pour some out.

For closed systems, you will know you have the perfect amount of moisture when you can see a full water cycle. Fairly soon after closing your terrarium you should notice a light fog or mist on the glass in the morning or evening that clears up during the day. The condensation should build up and then drip back down into the substrate.

If there’s too much water, the condensation won’t clear and you won’t be able to see your plants. To rectify an overwatered terrarium, remove the lid and let some of the moisture evaporate. You can do this a few hours at a time to see if the water cycle starts to happen. You can also use a cloth or paper towel to wipe down the glass and absorb some moisture.

On the flip side, too little water is easily indicated because there no condensation will form at all. Instead of dumping in a large amount of water, try using a spray bottle to mist the plants once a day until condensation begins forming on the glass.

So, whether you have an old vase, an unused fish tank, or even a giant glass water jug, with a few extra supplies, you can create an easy and aesthetic terrarium to house all of your favorite humidity-loving plants.