Plant selection and combinations of plants in the terrarium need to be given thoughtful attention. Use plants sparingly; they do grow, often luxuriantly, in the enclosed environment. Choose plants that are relatively small even when full grown.
For a terrarium to be set in dim light, concentrate on foliage plants. Variety of leaf pattern and color create the total effect. The contrast of green with silver or white can be achieved by using Fittonia, Pilea cadierei (aluminum plant) and "Moon Valley" Pilea, Euonymus japonica, Pteris cretica albo-lineata or Pteris ensiformis 'Victoria'. Reds and pinks can be introduced by use of Saxifraga sarmentosa 'Tricolor' (strawberry geranium), the red-veined Fittonia, or Episcia varieties.
In bright light but not direct sun you can have flowering plants. Sinningia pusilla, and other miniature gloxinias, Begonia prismatocarpa and its 'Buttercup' form, Gesneria cuneifolia in several forms are colorful choices.
A tall container might feature Dracaena sanderiana or dwarf palm, Anthurium polyschistum and Sinningia 'Dollbaby.'
A new dwarf caladium, Caladium humboldtii, has found favor with terrarium lovers. It can be combined with a small begonia and the Boston Fern 'Fluffy Ruffles' or 'Duffii'.
For open top terrariums, suitable selections are miniature Sinningias such as 'Dollbaby,' or 'Freckles,' miniature African Violets, Begonia 'Robert Schatzer.'
In a single plant terrarium Begonia versicolor is a wonderful choice. Another is Begonia prismmatocarpa, which will be covered with yellow blossoms. Miniature orchids, such as Lepanthes or the terrestrial jewel orchid, Anoechtochilus sikkimensis, are well suited to the tropical terrarium.
Ground covers complete your composition by enhancing the natural appearance and adding another element to leaf form or texture. The most frequently used ground cover plants are Selaginella kraussiana and Helxine (baby tears). Also suitable are Ficus pumila minima and Pilea depressa.
Plants usable in terrariums but needing to be restrained are Plectranthus (Swedish ivy) - it will take over; Fittonia, which grows almost too well in a terrarium and may need to be pruned, wandering jew - too aggressive for a small terrarium. Selaginella and Helxine also are rampant growers.
Do not combine succulents and desert plants with species which require a humid environment such as are listed above.
A terrarium is a minimum-maintenance garden, once you have established the moisture balance. The interior of a sealed terrarium is protected against sudden temperature changes, drafts and dryness. Normal living room temperatures are satisfactory. Most terrariums will not need fertilizing, certainly not for the first year.
You should have chosen clean, pest-free plants at the start. The main problem likely to develop in a terrarium is fungus growth. This slightly fuzzy gray growth on the soil surface or on plants should be removed as soon as it is noticed. The best remedy is to remove whatever it is growing on. A small pair of tweezers or a similar instrument is a useful tool. Leave the container open for 24 hours to dry up and of the fungus you may have missed.
Terrarium building can become a great hobby. Anyone interested in terrarium building - selecting plants, buying supplies, constructing your own glass container - is referred to two informative and profusely illustrated books: "Successful Terrariums (A Step by Step Guide), by Ken Kayatta and Steven Schmidt, published by Houghton Mifflin Co., 1975; price $14.95; and "The Complete Book of Terrariums," by Charles Marden Fitch, published by Hawthorn Books, price $9.95. For guidance in selecting and growing mini-plants, see "Little Plants for Small Spaces," by Elvin McDonald, published by M. Evans Co., 1974; price $8.95.