Terrariums

The cold days of January force gardening enthusiasts to peruse their hobby indoors and terrariums represent an ideal way to accomplish that goal. A terrarium is a tightly closed clear glass or plastic container filled with small plants. It also has come to mean an open, transparent container for growing and displaying plants. Terrariums are most useful for small plants that do not adapt well to normal home atmospheres. When properly planted and located, they provide a way to grow many plants with minimal care.

Transparent containers for growing plants date back 2,500 years ago to ancient Greece. However, the terrarium as we know it is credited to Dr. N. B. Ward of London. Dr. Ward was interested in growing many types of ferns in his backyard but had not been successful. While studying a sphinx moth emerging from the chrysalis he had buried in moist earth in a closed bottle, he was amazed to see a seedling fern and some grass growing inside. He watched them grow for four years, during which time not one drop of water was added, nor was the cover removed. This led to development of "Wardian Cases," which were large, enclosed containers for growing delicate plants in the home or transporting precious plants over long distances.

The terrarium most often used today are small ornamental versions of the Wardian case. We recognize two different types of terrariums: closed and open. Closed terrariums are more traditional and retain more humidity than do open ones. The latter, however, do provide higher humidity for plants than do dish gardens. Open terrariums and dish gardens require more frequent watering, but danger of disease buildup is reduced.

Critical to the establishment of a successful terrarium is the selection of an appropriate container. A terrarium container must be made from a transparent material such clear glass or plastic. As long as it is clear, almost any type of container may be used: empty fish bowls, fish tanks, brandy snifters, old glass jars, jugs, bottles, etc. Additionally, there are containers specially designed for use as terrariums. All closed containers should have transparent covers. Containers with small openings also are satisfactory. Containers with large openings without covers can be used but will require more frequent watering to maintain the high humidity needed by some plants.

The growing medium for terrariums also is important. It must be biologically inert, well drained and high in organic matter. Prepackaged peat-like mixes sold at garden centers and nurseries or where plant supplies are sold are excellent choices. Adding fertilizer is not necessary, since most packaged mixes contain a starter charge of fertilizer; plus, plants in terrariums should not grow rapidly to keep from "outgrowing" their container. Light fertilization with a houseplant fertilizer may be done after plants are established.

Plants are the "jewel in the crown" of terrariums and many different plants are suitable for use. Plants that have a low and dense growth habit usually are best. Don’t mix plants requiring widely different conditions. Terrarium plants differ relative to optimum light and temperature conditions. Those plants requiring medium light should be placed in good light near a window, or receive supplemental artificial light. Terrariums with this type of plants should be placed within several feet of a bright window, but not in direct sun. Few plants tolerate low light for extended periods. Those terrarium plants that tolerate low light will tolerate a location no more than about 10 feet from a bright window. Plants requiring high light are not frequently used but should be placed close to a window, often in direct sun. Cacti and succulents are examples of the latter. Do not put closed containers in full sun.

Since most terrarium plants are tropical in nature they require warm temperatures. A night temperature of 65 F degrees is ideal for this type of plant; day temperatures normally should be about 10 degrees higher. "Cool temperature" is a designation that primarily fits woodland plants in woodland terrariums. These plants should have night temperatures about 50 to 55 F degrees. In the home these temperatures may be difficult to find, but placed on a window close to the glass with a drape pulled behind them at night, a pocket of cool air will develop during the winter. Day temperatures also should be cool but are not as critical. Table 1 lists some of the more popular terrarium plants along with their light and temperature preferences.

When designing the terrarium, combine plants for variation in size, color and texture. Since terrariums usually are viewed from one side, the soil should be sloped for viewing from that side. Plants also should be arranged so that taller plants are toward the back. A low, coarse-textured plant is often desirable for a dominant focal point near the front. Don’t build a collection of variegated or unusual plants. They compete with each other and don’t give a unified pattern. Use rocks, sand, wood and other natural materials to create cliffs, rock ledges, dry stream beds or lush tropical forests. Hills and valleys will make the scene more interesting than a flat surface.

Once all necessary materials have been obtained, it is time to construct the terrarium. In general, about one quarter of the terrarium’s volume should be used for the growing medium and drainage material. Charcoal and pebbles should be placed in the bottom of the container for drainage. These may be mixed together, but the charcoal usually will be most effective if placed in a half-inch layer above the layer of gravel, crushed pots, marble chips or other drainage material. Sphagnum moss, placed over the layer of gravel and charcoal, prevents the growing medium from sifting into the drainage area.

Next, add the growing medium. It should be slightly moist so as not to be dusty, but not too moist that it is sticks to the sides of the container. For most containers, a growing medium minimum thickness of one and one-half inches is necessary to provide suffi cient volume.

To assemble the terrarium, take the plants from their pots and remove extra growing medium to expose the roots. Trim off any leaves that are yellowed, damaged or show any indication of disease or insect damage. Trim off some roots from plants that were extremely pot bound. Promptly place the plant in the container, so that the exposed roots do not dry. In the closed container, try to keep foliage from touching the sides of the container. Leaves touching the glass will collect water and be more subject to rot.

After planting, mist over the plants to wash off any growing medium that sticks to leaves or sides of the container. If the medium was properly moist at planting, heavy watering will be unnecessary. The water misted over the leaves is adequate to settle the medium. Don’t cover the terrarium, and repeat the misting after one day. Allow the container to remain open until the foliage has thoroughly dried. Then, if the terrarium is the closed type, apply the cover. Watch the newly-planted terrarium closely for several weeks for signs of diseases or other problems.

As stated previously, plants in terrariums should not grow rapidly. Therefore terrariums seldom need fertilizer. Don’t plan any fertilization for at least a year after planting. If after the first year the plants are yellowish and seem to lack vigor without any other apparent problems, a light fertilization may be necessary. Use a water-soluble houseplant fertilizer at about one-fourth the rate recommended for normal houseplants. Do not allow any of this fertilizer solution to be left on the foliage.

Although a terrarium is designed for growing plants indoors with minimum care, it is not an inanimate object. Some plants will thrive, others may die. Occasionally it will become necessary to remove certain plants or add others. When adding plants, take all precautions described for planting the new terrarium. It is always possible to add new problems when adding new plants.

Table 1. Plants suitable for use in terrariums

Common name	Scientific name	Light	Temp
African violet	Saintpaulia spp	medium	warm
Aluminum plant	Pilea cadierii	medium	warm
Arrow-head plant	Syngonium podophyllum	medium	warm
Artillery plant	Pilea microphylla	medium	warm
Asparagus fern	Asparagus plumosus	med.- brt.	warm
Baby tears	Helxine soleirolii	medium	warm
Begonia	Begonia spp	medium	warm
Bird’s nest Sansevieria	Sansevieria trifasciata hahnii	any	warm
Bloodleaf	Iresine herbstii	bright	warm
Chinese evergreen	Aglaonema spp	low - med.	warm
Coral berry	Ardisia crispa	medium	warm
Creeping fig	Ficus pumila	medium	warm
Croton	Codiaeum variegatum	bright	warm
Devil’s ivy (Pothos)	Epipremnum aureum	medium	warm
Dwarf gloxinia	Sinningia pusilla	medium	warm
Dwarf pomegranate	Punica granatum nana	bright	warm
Emerald Ripple	Peperomia caperata	medium	warm
English ivy	Hedera helix	medium	cool
Flame violet	Episcia cupreata	medium	warm
Foam flower	Tiarella cordifolia	med.- brt.	cool
Gold dust dracaena	Dracaena godseffiana	medium	warm
Goldfish vine	Columnea microphylla	medium	warm
Heart-leaved philodendron	P. scandens oxycardium	medium	warm
Maidenhair fern	Adiantum cuneatum	medium	warm
Miniature peperomia	Pilea depressa	medium	warm
Neanthe bella palm	Chamaedorea elegans	medium	warm
Nerve plant	Fittonia spp	medium	warm
Oxalis	Oxalis spp	bright	warm
Piggy-back plant	Tolmiea menziesii	medium	warm
Podocarpus	Podocarpus macrophylla	medium	warm
Pink polka dot	Hypoestes panguinolenta	bright	warm
Prayer plant	Maranta spp	medium	warm
Rosary vine	Ceropegia woodii	med.- brt.	warm
Sander’s dracaena	Dracaena sanderiana	medium	warm
Satin pellionia	Pellionia pulchra	med.- brt.	warm
Spider Aralia	Dizygotheca elegantissima	med.- brt.	warm
Spider plant	Chlorophytum comosum	lowmed.	warm
Strawberry begonia	Saxifraga sarmentosa	medium	warm
Swedish ivy	Plectranthus australis	lowmed.	warm
Table brake fern	Pteris spp	medium	cool
Tahitian bridal veil	Gibasis geniculata	medium	warm
Var. ovalleaf peperomia	P. obtusifolia variegata	medium	warm
Venus fly trap	Dionaea muscipula	bright	warm
Watermelon peperomia	Peperomia sandersii	med.- brt.	warm

David Trinklein
Associate Professor Plant Sciences