Dr. N. B. Ward was a 19th century London physician who also loved growing plants. He was interested in establishing 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 development of the terrarium soon followed.
The terrariums 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 morefrequent watering, but danger of disease buildup is reduced.
A suitable container is critical to the establishment of a successful terrarium. 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, here 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 very 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 usually 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 “jewels 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 (but not hot) temperatures. A night temperature of 65 F degrees is ideal for this type of plant; day temperatures normally should be about 10 degrees higher. A few terrarium plants prefer cool temperatures and fit well 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.
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. Charcoal also helps to eliminate potentially toxic by-products in closed terrariums. It is most effective if placed in a half inch layer above a 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 sufficient 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.
REVISED: October 23, 2012