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Revised May 2002

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Contents

• Brief history
• Planning a terrarium
• Other uses for the terrarium
• Assembling the terrarium
• Care of the terrarium
• Favorable conditions for terrarium plants

Related publications

• G6510, Caring for Houseplants
• G6515, Lighting Indoor Plants
• NCR99, Care of House Plants

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

Terrariums

David Trinklein
Department of Horticulture

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 way to grow many plants with minimal care.

Brief history

Transparent containers for growing plants date back to at least 2,500 years ago in Greece. But in the United States, terrarium culture is believed to originate in New England, where housewives placed squawberry (partridge berry) in hand-blown glass bowls.

Invention of the terrarium as we know it is credited to Dr. N. B. Ward of London. 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.

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?

Open terrariums provide higher humidity for plants than do dish gardens, but closed terrariums retain more humidity than either of the other container types. The open terrarium and dish garden require more frequent watering, but danger of disease buildup is reduced.

Before beginning your terrarium, decide whether you prefer a woodland, tropical or desert theme. Keep in mind the temperature and light conditions in the location where you plan to place the terrarium. Select plants that suit the location.

Containers

A terrarium container must 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: empty fish bowls, fish tanks, brandy snifters, old glass jars, jugs, bottles. There also are containers specially designed for use as terrariums.

All 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 without high sides are suitable for dish gardens and need not be transparent.

Growing medium

The growing medium for terrariums must be clean, well drained and high in organic matter. Potting soils sold at garden centers and nurseries where plant supplies are sold are sterilized and ready for use. Prepackaged peat-like mixes also are excellent choices.

To prepare your own soil, mix one part peat moss with one part rich garden soil. Make sure the soil is moist before sterilization. Place it in an oven at about 200 degrees until the soil is thoroughly heated for 30 minutes. The exact time needed depends on the quantity of soil. Cover the soil with aluminum foil to prevent excess drying during heating. Spread the soil to cool on clean papers with clean tools. For planting, the soil should be moist enough to cling in a ball when it is squeezed tightly.

Adding fertilizer is not necessary, since plants in terrariums should not grow rapidly. If you know the soil used has exceptionally low fertility, light fertilization with a houseplant fertilizer may be done after plants are established.

Plants

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

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

lists some plants suitable for terrarium or dish garden use, along with some of their cultural characteristics. It is designed as an aid in selecting plants with similar cultural needs. They are listed alphabetically by common name. Since there is variation in common names, more than one may be given, but each plant is listed only once. The following points are described in the table:

Height
Since low plants are preferred, more emphasis is placed on small plants. They are divided into three size groups. Some of these are creeping or climbing vines that may grow tall, but with limited height in the terrarium they form a groundcover. Most plants listed in the "over 12 inches" category need occasional cutting back to be kept low.

Light
Most plants listed are in the "medium" light category and require good light near a window, or perhaps less light from a window supplemented with artificial light. The terrarium should be placed within several feet of a bright window, but not in direct sun.

Few plants tolerate "low" light for extended periods. Those listed for "low" light will tolerate a location no more than about 10 feet from a bright window.

"Bright" light means a location close to a window, often in direct sun, as for a grouping of cactus in a dish garden. Do not put closed containers in full sun.

Container
The true terrarium is tightly closed. Most plants require high humidity. Plants suitable for this condition are listed 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 planting in which a container is used 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
"Warm" in this case refers to normal home conditions. Most tropical plants are suited for common house temperatures. At night, 65 degrees is ideal; day temperatures normally should be about 10 degrees higher.

"Cool" is a designation that primarily fits woodland plants in woodland terrariums. These plants should have nighttime temperatures about 50 to 55 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.

Comments
This section lists only conditions or characteristics peculiar to the plant indicated.

Tools

Only a few tools are necessary for planting the terrarium. Long sticks, either bamboo or 1/4-inch dowling, will be most useful. The appropriate length depends on the height of the container to be planted. They can be used to dig holes, move items and support plants while they are being planted.

Household scissors will be helpful for pruning plants before they are planted.

A large kitchen spoon will be helpful in placing soil and drainage material in the container. If a container with a very small opening is used, make a funnel from paper or aluminum foil for placing soil into the container.

An atomizer or bulb-type sprayer will be useful for watering the terrarium. A kitchen bulb baster may be used for watering hard-to-reach spots.

A stick with a wire loop on the end will be helpful for lowering plants into large containers with small tops.

Accessories

Rocks, gravel and other natural materials provide pleasing accessories in designing terrariums. Sticks, wood, seed pods or bark are possible choices. Ceramic figures, frogs, mushrooms or snails suggest a natural setting. The materials added are a matter of individual taste. However, avoid too many accessories, or those with vivid, unnatural colors. Care should be taken not to introduce insects or disease with the accessories.

Other uses for the terrarium

Closed terrariums, with high humidity, make ideal propagation units for a few cuttings of houseplants. The idea also may be used to keep houseplants during a short vacation period. Plants can be placed in large containers or plastic bags with the top closed. They should not be placed in direct sunlight, or overwatered, but can normally be kept for 2 to 4 weeks in this way.

Assembling the terrarium

Design
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. 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.

Prepare the container
Before planting sterilize the container by washing it in hot, soapy water and rinsing thoroughly. Make sure the container is dry before planting. If you use a commercial glass cleaner, such as one containing ammonia, allow the open container to air for several days before planting.

Add drainage material and soil
In general, about one quarter of the terrarium's volume will be used for soil and drainage material. These can easily be added with a spoon, funnel or other convenient tool.

Drainage
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 1/2-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 soil from sifting into the drainage area.

Growing medium
Next, add the growing medium. It should be slightly moist so that dust is not stirred up, but not so moist that it is muddy and sticks to the sides. For most containers, a minimum thickness of 1-1/2 inches is necessary to provide sufficient volume.

Adding plants
Before adding the plants, arrange those selected for use in an open area about the size of the container. This will give you some 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 give a unified pattern.

If there are disease concerns, enclose the plants to be used in plastic bags for about two weeks before planting in the terrarium. If any diseases are present, they normally will become visible on the foliage or stems. It is essential that tightly closed terrariums not be exposed to diseases.

To assemble the terrarium, take the plants from their pots and remove extra soil 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.

Plants may be placed in a deep container 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, it is a common practice to wrap the plant in a piece of paper for protection before inserting it through the small opening. The plant is unwrapped and the paper removed once the plant is in the container. This also will help keep the inside of the container clean. Holes can be dug in the planting medium with a pointed stick before inserting the plants. After the plant has been inserted in the hole, fill in with soil and tamp it firm. A long stick with a cork fixed on the end makes a good tool for lightly tamping the soil. 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 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, no heavy watering will be necessary. 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 is thoroughly dried. Then, if the terrarium is the closed type, apply the cover.

Watch terrariums closely for the first weeks after planting. It is at this time that diseases often get started. Any leaves that die or plants that begin to rot should be removed promptly before the problem spreads to other plants. Rots often are associated with too much moisture. If rot develops 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
The closed terrarium normally will not need water for 4 to 6 months, or when fogging on the sides stops, or when plants wilt. 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. It is important that waterings always be light. Since there is no external drainage, heavy waterings result in standing water in the gravel and charcoal, which encourages root diseases. These materials may help overcome occasional light overwaterings, but frequent heavy watering will inactivate the system. When watering the 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
A terrarium, open or closed, should not receive direct sunlight. The dish garden, however, which contains plants needing bright light, may be placed in direct sun. Direct sunlight on a closed or tall, open container will cause heat buildup that will injure most plants. Most plants suitable for terrariums don't require extremely bright light, but do well in good light. If the terrarium is to be located where there is low light, add artificial light. A 100-watt bulb placed close to the terrarium or fluorescent tubes placed over the terrarium will be helpful. Where outside light is poor, add artificial light up to 16 or 18 hours each 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 shoots that fill out plants. Pinching out tips before plants become too tall results in better growth than severe cutbacks. Be sure to remove all vegetation trimmed from the terrarium.

Fertilization
Since plants in terrariums should not grow rapidly, 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.

Other care
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.

Favorable conditions for terrarium plants

G6520, revised May 2002