Closed terrariums are, in my opinion, the easiest type of terrarium to care for – if they’ve been put together properly! A closed terrarium consists of a glass or clear plastic container such as a fishbowl, with some kind of stopper or lid. The idea behind a closed terrarium is that a mini ecosystem is created. A really good closed terrarium will have layers of pebbles, sphagnum moss, horticultural charcoal and potting medium. These are all important ingredients for terrarium success.
How does a closed terrarium work?
So, very briefly, plants process sunlight and carbon dioxide. This is called photosynthesis. Photosynthesis produces oxygen and plant food essential for a plant’s growth. As a terrarium is usually made of glass, sunlight can get through, allowing plants to photosynthesise and grow.
A terrarium is really its own little world with its very own water cycle. The moisture from both the potting mix and plants has nowhere to escape through evaporation, so it condenses on the sides of the terrarium and plants and re-waters the soil keeping all the little plants happy and hydrated. For this reason, closed terrariums only require traditional watering every couple of weeks.
What plants are suitable for a closed terrarium?
Firstly, lets look at what’s not suitable. SUCCULENTS. CACTUS. All those beautiful popular plants being shoved in terrariums with lids are being sentenced to a rotten, moldy death. These plants like warmth, air flow and minimal water. If you’ve got these plants in a closed terrarium, open the lid!
Terrarium plants should be anything small that loves a moist and humid environment. Mosses are perfect, as are little ferns, miniature violets, dwarf syngoniums… and my personal favourite, fittonia (or nerve plant).
Caring for your closed terrarium
Terrariums are beautiful worlds that need care, observation and interaction. Water your terrarium when it looks like the potting mix is starting to dry out. Never let the potting mix completely dry out. Have a look at your layer of sphagnum moss. If that still looks nice and hydrated you probably don’t need to water. If it’s dry, give your terrarium a good water. You may only need to water your terrarium once a month, but this is something you will work out through observation.
Water with filtered water or rain water if you can. Our South Australian water is terrible to drink and plants don’t like it much either.
You will need to wipe off condensation occasionally, and it also helps to open the terrarium for an hour or two weekly to let in fresh air and discourage mold. At the first sign of mold, take off the terrarium’s lid. You need to cut down on watering too as this is a sure sign of too much water being added to your terrarium. If there’s only a little mold on a plant, remove the moldy bit and put it in the bin. Keep a good eye on the plant over the next few days for signs of mold appearing. If the mold is covering the plant, I’m afraid the plant has to go. Nothing moldy should stay in your terrarium as mold grows. Replace yucky moldy plants with new plants (wash your hands or gloves after taking out the moldy plants – you don’t want to transfer spores to a healthy plant) . It is also good to get a piece of wire or satay stick and start poking some holes in your potting medium. This will introduce some air into the soil so that it is less likely to produce mold spores.
Slugs – ugh – it doesn’t matter how careful you are, you’ll always end up with a sneaky slug or two in your terrarium. Look out for tracks in the condensation, then hunt down those critters and remove them from your terrarium.
Other maintenance and pruning tips are the same for open terrariums, we have a guide for them too!