Reviewed by David H. Trinklein
Division of Plant Sciences

A terrarium is a tightly closed, clear glass or plastic container filled with small plants (Figure 1). 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 novel way to grow many plants with minimal care.

Figure 1
A terrarium is a good way to keep plants indoors with a minimum of care.
 

Brief history

The use of transparent containers for growing plants dates back at least 2,500 years in Greece. In the United States, terrarium culture is believed to have originated in New England, where housewives placed squawberry (partridge berry) plants in handblown glass bowls.

The invention of the terrarium as we know it is credited to Dr. N.B. Ward, a 19th-century London physician. A plant enthusiast, Ward was interested in growing many types of ferns in his backyard but had not been successful. While studying a sphinx moth emerging from a 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.

Ward continued his observations with other plants in containers and, in 1842, published a book called On the Growth of Plants in Closely Glazed Cases. 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 terrariums most often used today are small ornamental versions of the Wardian case.

Planning a terrarium

Closed, open or dish garden?
The first step in planning a terrarium is to decide whether it will be open (no lid or cover) or closed. Closed terrariums retain the most humidity, followed by open terrariums and then dish gardens. Open terrariums and dish gardens require more frequent watering than do closed, but danger of disease buildup is greater in the latter because of higher humidity.

Containers
A terrarium container should be made from clear glass or plastic. Tinted or cloudy glass greatly reduces light transmittance and interferes with plant growth. As long as it is clear, almost any type of container may be used: an empty fish bowl, fish tank, brandy snifter, old glass jar, jug, bottle. Containers specially designed for use as terrariums are also available.

Closed containers should have transparent covers. Containers with small openings also are quite 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. However, open terrariums are drier and less subject to disease. Containers with low sides are suitable for dish gardens and need not be transparent.

Growing medium
The growing medium used in terrariums must be clean, well drained and high in organic matter. A prepackaged peat-lite mix (blend of peat moss, vermiculite and perlite) is an excellent choice. Potting soils sold at garden centers and nurseries where plant supplies are sold are sterilized and ready for use.

Growing medium also can be prepared at home. Mix one part peat moss with one part rich garden soil. Sterilize the mixture by moistening it, covering it with aluminum foil to keep it from drying out while being heated, and placing it in oven at 200 degrees F for about 30 minutes or until it is heated through. The exact time needed depends on the quantity of soil. Using clean tools, spread the soil on clean papers to cool. For planting, the soil should be moist enough to cling in a ball when it is squeezed tightly.

Adding fertilizer to the growing medium usually is not necessary because plants in terrariums should not grow rapidly. If soilless mixes are used or if the soil used is known to be exceptionally low fertility, light fertilization with a houseplant fertilizer may be done after plants are established.

Plants
Decide on a theme for the terrarium: woodland, tropical or desert. When making this decision, consider the temperature and light where the terrarium is to be located. Select plants that suit the location.

Many plants are suitable for growing in terrariums. Plants that have a low and dense growth habit usually are best. Larger plants may be used but must be kept small in terrariums by cutting back the tips.

Don’t mix plants requiring widely different light, temperature and moisture conditions. Succulent plants and cacti are less desirable for terrariums because moist conditions promote rot. Don’t mix desert plants with moisture-loving tropicals.

Table 1 lists some plants suitable for terrarium or dish garden use and describes some of their cultural requirements. Use this table as an aid in selecting plants with similar cultural needs. Plants are listed alphabetically by common name, but because of the variation in and duplication of common names in the plant world, scientific names are included, also. The following points are described in the table:

• Height
  Terrarium plants often are divided into three size groups: small (1 to 6 inches), medium (6 to 12 inches) or tall (over 12 inches). Because small plants are preferred in terrariums, Table 1 places more emphasis on them. Some small plants actually are creeping or climbing vines that may grow tall in nature, but their height can be limited in the terrarium to form a groundcover. Most plants listed in the tall category need occasional cutting back to be kept low.
• Light
  Most terrarium plants are in the medium light requirement category and need to be placed near a window with good light. If light from the window is low, supplement with artificial light. Ideally, a terrarium should be placed within several feet of a bright window but not in direct sun. Few plants tolerate low-light conditions for extended periods. For plants listed as low light in Table 1, a location no more than about 10 feet from a bright window should suffice. Terrarium plants requiring bright light should be located close to a window, often in direct sun. Cacti or succulents in a dish garden benefit from such exposure. But do not put closed containers in full sun.
• Container type
  A true terrarium is tightly closed, as was Dr. Ward’s original case. Many plants suited for terrariums require high humidity. Plants that prefer this condition are listed in Table 1 as closed. An open container is one with high sides, generally at least as tall as the plants contained. The opening at the top is not covered. Plants not needing high humidity can be used. Dish refers to a container with sides that are lower than the plants, so that the entire planting is subjected to normal conditions in the room. Plants tolerating low humidity are most suitable for this type of planting.
• Temperature
  Many terrarium plants are tropical in nature and well suited for normal house temperatures. Plants in Table 1 listed as warm respond well to night temperatures of 65 degrees F and day temperatures normally about 10 degrees higher. The cool designation primarily fits woodland plants in woodland terrariums. These plants should have night temperatures of about 50 to 55 degrees F. Locations with these temperatures may be difficult to find in the home, but in the winter, when plants are placed on a windowsill close to the glass with the drapes pulled behind them, a pocket of cool air will develop. Day temperatures also should be cool but are not as critical.
• Comments
  Conditions or characteristics peculiar to the plant are identified in the comments section.

Tools
Only a few tools are need to plant a terrarium.

• Long sticks, either bamboo or 1/4-inch dowel rods
  Use to dig holes, move items and support plants while they are being planted. The appropriate length depends on the height of the container.
• Household scissors
  Use to prune plants before planting them.
• Large kitchen spoon
  Use when placing growing media and drainage material in the container. A funnel made from paper or aluminum foil can be helpful for placing the growing media into a container with a very small opening.
• Atomizer or bulb-type sprayer
  Use when misting and watering the terrarium. A kitchen bulb baster may be used for watering hard-to-reach spots.
• Stick with a wire loop on the end
  Use for lowering plants into large containers with small openings.

Accessories
Rocks, gravel and other natural materials — such as sticks, wood, seedpods and bark — provide pleasing accessories in designing terrariums. Ceramic figures of frogs, mushrooms or snails can help to suggest a natural setting. The accessories added are a matter of individual taste. However, avoid using too many accessories or ones with vivid, unnatural colors. Also, be careful not to introduce insects or disease with the accessories.

Assembling the terrarium

Design
When arranging plants, variation in size, color and texture is desirable. Because terrariums usually are viewed from one side, the growing medium should be sloped for viewing from that side and plants arranged so that taller plants are toward the back. Use rocks, sand, wood and other natural materials to create cliffs, rock ledges, dry streambeds or lush tropical forests. Undulations representing hills and valleys will make the scene more interesting than a flat surface. Sketching a design of the terrarium before actually assembling it can be helpful.

Prepare the container
Before planting, clean and disinfect the inside of the container by washing it with hot, soapy water and rinsing thoroughly. Make sure the inside of the container is dry before planting. If a commercial glass cleaner is used, allow the open container to air for several days before planting.

Add drainage material and growing medium
In general, about one quarter of the terrarium’s volume should be used by the growing medium and drainage material. These can be added easily with a spoon, funnel or other convenient tool.

Drainage
Activated 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 in eliminating chemicals that could prove to be toxic to plants if placed in a 1/2-inch layer above the layer of gravel, crushed pots, marble chips or other drainage material. Charcoal is especially important in closed terrariums, which prevent the natural escape of chemicals. Sphagnum moss, placed over the layer of gravel and charcoal, prevents the growing medium from sifting into the drainage area.

Growing medium
Next, add the growing medium. It should be slightly moist so that it doesn’t stir up dust but not so moist that it is muddy and sticks to the sides. For most containers, a minimum depth of 1½ inches is necessary to provide sufficient volume.

Adding plants
Select only healthy, disease-free plants because closed terrariums represent an ideal environment for plant diseases to flourish. If there are disease concerns, enclose plants in a plastic bag and place in bright light for about two weeks before planting in the terrarium. If any diseases are present, they normally will become visible on the foliage or stems.

Before adding the plants, arrange them in an open area about the size of the container to get an idea of relative sizes and textural patterns. 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 create a unified pattern.

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 or damaged or that show any indication of disease or insects. Trim off some roots from plants that were extremely pot-bound.

Promptly place each plant in the container so that the exposed roots do not dry. In a 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 decay.

Plants may be placed in deep terrariums using long slender tongs or a stick with a wire loop on the end. Deep containers with small openings will require considerable patience and practice in planting. For such containers, a common practice is to wrap the plant in a piece of paper for protection before inserting it through the small opening. Once the plant is the container, unwrap it and remove the paper. This practice also helps keep the inside of the container clean. Before inserting the plants, dig holes in the growing medium with a pointed stick. After a plant has been placed in a hole, fill in with growing medium and tamp to firm it. A long stick with a cork fixed on the end makes a good tool for lightly tamping the growing medium. After the plants have been positioned, add gravel, sand, moss or other materials to give a finished appearance. Accessories also may be added at this time.

After planting. After planting, mist the plants to wash off growing medium that has stuck to leaves or sides of the container. If the medium was properly moist at planting, heavy watering will not be necessary. The water misted over the leaves is adequate to settle the medium. Don’t cover the terrarium initially. Instead, repeat the misting process after one day. Allow the container to remain open until the foliage is thoroughly dried. Then, if the terrarium is the closed type, apply the cover.

Observe terrarium closely for the first few weeks after planting. Diseases often appear at this time. Any leaves that die or plants that begin to wilt or decay should be removed promptly before the problem spreads to other plants. Root rots often are associated with too much moisture. If rots develop in a closed terrarium, remove the cover to allow more drying. If a fungus seems to be spreading from a plant through the growing medium, it may be beneficial to remove a portion of the medium in the infected area and replace it. Application of a general fungicide also may help to reduce spread of a disease.

In most cases, after a few weeks the terrarium is established and the threat of disease is reduced. Continue to watch for fallen leaves, however, or any plant parts that begin to decay.

Care of the terrarium

Watering
A closed terrarium normally will not need water for 4 to 6 months. The failure of condensation to form on the inside of the container or the presence of wilting plants indicates the need for water. Open terrariums need watering occasionally but not as frequently as other houseplants. A dish garden, unless it is the desert type, will need frequent watering. Waterings must always be light. Because terrariums have no external drainage, heavy waterings result in standing water in the gravel and charcoal, which encourages root diseases. The gravel and charcoal may help overcome occasional light overwaterings, but frequent heavy watering will inactivate the system. When watering a closed terrarium, don’t replace the cover until wet foliage has dried.

Caution
Never overwater. Excess water is almost impossible to remove. Better a little too dry than too wet.

Light
An open or closed terrarium should not receive direct sunlight. However, a dish garden that contains plants needing bright light may be placed in direct sun. Direct sunlight on a closed or a tall, open container will cause heat buildup that will injure most plants. As previously mentioned, most plants suitable for terrariums don’t require extremely bright light but do well in good light. If the terrarium is in a low-light location, supplement with artificial light. A 100-watt bulb placed close to the terrarium or fluorescent tubes placed over the terrarium will be helpful. Supplemental artificial light should be operated 16 to 18 hours a day.

Plants receiving light from a window gradually will face that direction. To keep the terrarium attractive from the desired view, turn it occasionally to keep the plants growing normally.

Pruning
Many plants in a terrarium will gradually outgrow their limited space. A little trimming quickly brings them into bounds and often promotes side-shoot growth that fills out plants. Pinching out tips before plants become too tall results in better growth than severe cutbacks. Be sure to remove all trimmed vegetation from the terrarium.

Fertilization
Because plants in terrariums should not grow rapidly, terrariums seldom need fertilizer. Do not fertilize for at least a year after planting. If after the first year the plants appear 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 remain on the foliage.

Other care
Although a terrarium is designed for growing plants indoors with minimum care, it is not an inanimate object. Some plants will thrive, and 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. Adding new problems is always possible when adding new plants.

Table 1
Favorable conditions for plants growing in terrariums.