What is a terrarium?

By definition a terrarium is a transparent sealed container in which plants are grown in a moisture-balanced." In the terrarium a water cycle is established so that moisture condenses on the sides and top of the container, provides high humidity and trickles back into the soil, thus eliminating the need for watering. When you make a terrarium you create a visually pleasing arrangement of plants in an environment requiring minimum care. It can be a simple or an elaborate creation. It can be tropical or woodland.

A woodland terrarium is not discussed in this column because often the species chosen are on the endangered or threatened plant lists and so should not be subjected to the uncertainties of indoor gardening efforts. Please don't try to make a woodland terrarium.

To make your own tropical terrarium you will need the following basic supplies and equipment.

A clear glass or plastic container.

Gravel or coarse perlite, and charcoal.

Unmilled sphagnum.

Soil mix, sterilized.


Your container can be any shape or size, with or without a lid. It may be a fishbowl, candy jar, brandy glass or apothecary jar, even a mayonnaise jar; or you may prefer to purchase one from the vast array especially made for terrariums. A recycled adquarium can become a large landscaped terrarium.

It is best to start with a wide-mouth container that is easy to put your hand into. Bottles with small openings require special tools for placing plants in the soil mix - and a great deal of patience and dexterity.

Start with a clean container - washed with soap and water, rinsed and dried. Put a layer of moist gravel or coarse perlite in the bottom of the container. Add a sprinkling of charcoal to help prevent a sour condition. An inch of gravel is enough for most containers. This provides a drainage area for excess water.

Cover the pebbles with a thin layer of unmilled sphagnum to keep the soil from sifting down into the pebble layer.

Add an inch or more of barely moist sterilized potting oil or a mix, such as Black Magic Terrarium Soil, prepared for terrarium use. The combined depth of gravel and mix should be about 20 per cent of the height of the container to maintain good proportion between size of container and its contents.

The number and size of plants chosen for the terrarium depends on the size of the container. It is important to select plants that grow well together - are compatible - and do not need to be replaced often; that is, slow growers. Choose plants that are relatively small even when full grown. Plants should be free of diseases and pests.

Use plants sparingly; they do grow, often luxuriantly, in the enclosed environment. Plan the placement of plants in the terrarium by setting the plants on a mound of soil on the table in front of you; it is better to arrange them this way than to have to rearrange them after they have been planted.

Take the plants from their pots and gently remove most of the soil from the roots. With a spoon or other small digging tool, scoop planting holes in the soil mix. Set each plant in its place and gnelty tamp the soil around its roots. Give the soil surface a mulch of gravel or bark chips, if you like.

Use warm water in a spray bottle, atomizer, or food baster to wash stray bits of soil or moss from plants and inside of the container. This watering should provide adequate moisture.

Allow foliage to dry before putting the top in place, if yours is a covered terrarium. The cover may be a piece of glass or Saran Wrap or similar film.

It is possible to have a successful terrarium without a cover. The rounded sides of fish bowls, brandy glasses, candy jars and other containers with relatively small openings tend to confine moisture in the atmosphere immediately around the plants so that humidity in the terrarium interior remains fairly stable without the need for a cover. Enjoyment of a bowl of small flowering plants, such as miniature Sinningias and Gesnerias is thus enhanced; some of the leaves of a begonia like "Robert Schatzer" can even extend above the rim of the bowl.

Set the planted terrarium in low light for several days. Gradually increase the light until the amount suited for the chosen plants is reached. Avoid direct sunlight.

Condensation on the inside of covered containers indicates whether too much or too little moisture is present. If plants are obscured by the condensation, remove the cover to allow excess moisture to evaporate. This step may need to be repeated several times until there is only slight condensation - a thin film.If there is no condensation, the environment is too dry and a very small amount of water should be added. In the first few weeks you should be watchful until you are sure you have established the required moisture balance.

It is a good idea to remove the cover once a week to give the plants a change of air and introduce some carbon dioxide (needed for plant growth) by breathing on or speaking to the plants. Be sure to close it after a few minutes.

If you have made a terrarium without a cover, you will need to be much more observant about the moisture situation from day to day. The soil mix appears light brown when it becomes dry. Water can be added, using the baster and dribbling it around the sides of the container until the soil darkens.