Decide what kind of container you want to use. Inexpensive glass jars and bowls are often available at thrift stores and garage sales. Old aquariums work very well, too. A leaky aquarium can still make a good terrarium. Missing or broken glass can be replaced with plastic or screens. The aquarium may even have a matching light fixture that you can install a grow light bulb in.
Once you have your container, go shopping with the kids at your local greenhouse. The staff will be able to help you with choosing plants appropriate for the container, based on the size of the container and whether it has a lid or is open to the air. Young and small plants of all kinds can be planted in a terrarium, but to maintain a long-lasting terrarium, use plants that mature as small plants.
Larger containers are easier to maintain. If the opening is too small to allow a hand into the container, it will be difficult to install and maintain, which is fine for some people who like a challenge. A closed terrarium will have high humidity that tropical plants will thrive in, but you need to watch out for disease problems that can develop rapidly in high humidity.
The type of plants used will determine the type of soil needed in the terrarium. A cactus and succulent terrarium will require sandy soil and an open-topped terrarium. For most tropical plants, a peat moss and perlite soil mix works well. Tillandsias don't require soil for roots, while Venus flytraps require very wet soil.
Since there are no drainage holes in a terrarium, water circulation and drainage must be considered. In a closed system, the water evaporates and is transpired by plants into the air. It then condenses on the container and drains back into the reservoir. In an open system, the drainage system keeps the roots from drowning.
We build the terrarium in layers. The bottom area is the reservoir layer, which will be large gravel or marbles to allow the water to accumulate in the reservoir. The reservoir needs to be deep enough to hold the water necessary to create the humidity and water circulation for the plants being used. A dry soil terrarium will have a smaller reservoir than a wet soil one. We then place a screen to keep soil particles from clogging the reservoir. The next layer up is a layer of activated charcoal that filters the water going into the reservoir. Place another screen on the charcoal to keep it from getting clogged. Next is the soil mix for the type of plant we are growing. It can be a sandy mix for succulents, a peat moss mix for tropical plants, or an orchid mix for some kinds of orchids.
Finally, on top of the soil mix, we can install decorative items, including pretty sand, pebbles, crushed glass, rocks, crystals, shells, and assorted knickknacks.
Sealed terrariums need to stay out of direct sunlight, or they may overheat and cook the plants. Even open terrariums need to be watched carefully for a while to see how they react to the overall environment.
Terrariums are a nice way to create a pretty centerpiece. They do require some maintenance, and occasionally they may need to be rebuilt, but that is part of the fun of terrariums.
I have added a video on the basics of making a terrarium that is now available on the Greener View YouTube channel. Don't forget to go to GreenerView.com to get a link to the Greener View Gardening book and use the promo code "newspaper" to get half off the PDF version of the book.