A cactus terrarium is its own mini ecosystem. Those who live in the north, where cactus cannot be grown outside, can bring a bit of the desert into their own homes. You can add decorations to your cactus terrariums such as small rock and pebbles and even a small plastic lizard to give it an authentic desert look.
Cactus terrariums can be made in just about any glass container. You can try a round fish bowl, a rectangular fish tank, a large brandy snifter or a fancy wide bottom bottle. If you have safety concerns about using glass, there are choices that are made out of plastic that will do just fine. Make sure it is clear. Your cactus will need a lot of sunshine and colored glass, will stop the sun and your plants will die. You can choose to have the top of the terrarium open or closed and each one has good and bad points. Ir you choose to have a closed terrarium, you will need to pick plants that can take a lot of humidity. Containers without covers will need to be watered more. Open containers do have one advantage. The fact that air circulates around the plants reduces the risk of diseases.
Not all cactus are suitable for a terrarium. You need to pick varieties that will not grow that large. Acanthocalycium glaucum is a short flat top cactus that has stalks that grow as high as 6 inches with a bright yellow or red/yellow flower at the top. Tiger's Jaw is another cactus that will grow to about 6 inches height, but it will spread out to about 9 inches. Tiger's Jaw has a bright yellow flower and has small sharp spines that do look like teeth. Sand Dollar Cactus is a round ball shaped plant without spines. It has a yellow flower with a red circle in the middle.
Use organic potting soil that drains well. The bottom few layers will provide drainage. The first layer should be gravel, either pea or aquarium. On top of that, about ½-inch of planting charcoal and on top of that, a layer of Sphagnum moss. The top layer is the organic soil and it should be from 1 ½ to 2 inches deep. Decide where you plants will go with larger ones in the back and smaller in the front. Plant each plant to the same depth it was in the pot it came in.
You can use regular household items to help you plant your cactus terrarium. A soup spoon, or a teaspoon for smaller containers, make a perfect spade. Chop sticks or wooded kabob sticks are used to move the plants around. A spray mister is the best for watering.
Care begins before you put in the plants. Make sure the terrarium is sterile. Wash it in hot water using dish soap or put it in the dishwasher. Do not put the terrarium in direct sunlight. The plants will dry out too fast. Do not over water. Cactus do not like wet feet. Make sure the water level at the bottom of the container stays at about 1/4-inch. Cactus have a root system that goes deep and they will be able to reach it. A closed terrarium will only have to be watered from 2 to 3 times a year. Do not fertilize, at least not for the first year, as it will make the plants grow too much. Cactus only need fertilizer if they begin to look yellow and even then, only a very little bit.