Terrariums – Stylish Miniature Glass Gardens, A Step-by Step Guide

Terrariums – Stylish Miniature Glass Gardens
By Dr. Sue Hamilton, Former Director – UT Gardens

I am excited to see terrariums becoming popular again. I credit these beautiful glass plantings that were popular in the ‘70’s as one of my inspirations for choosing horticulture as my career.  They are fun to build and use in decorating your home. So what exactly is a terrarium? It’s 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 such as ferns and mosses. When properly planted and located, they provide a novel way to grow many plants with minimal care.

A brief history

The use of transparent containers for growing plants dates back at least 2,500 years in Greece but 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 the development of “Wardian cases,” which were large, glass 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.

Creating a terrarium

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 too high humidity over an extended period can cause disease problems. I love the closed terrarium look and to solve the humidity problem, I just remove or crack the container lid a bit each week to allow the moist air to escape.

·       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, or bottle. Containers specially designed for use as terrariums are readily available and can be fun to shop for. Of course the wider the terrarium opening, the easier it is to plant. Any container you choose should be clean and disinfected before using. 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.

·       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. Be sure that larger plants fit the scale of your container. You can always prune and cut back plants if they begin to grow too large. 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. Do not mix desert plants with moisture-loving tropicals.

·       Temperature - Many terrarium plants are tropical in nature and well-suited for normal house temperatures. Avoid placing a closed terrarium in full, direct sunlight due to heat build-up. Most all terrarium plants thrive with bright, indirect sunlight. 

·       Accessories – Use rocks, gravel, and other natural materials — such as sticks, wood, seedpods and bark — to create a miniature landscape when designing your terrarium. Ceramic figures of frogs, mushrooms, or snails can help to suggest a natural setting. The accessories added are a matter of individual taste. Avoid using too many accessories.

·       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.

·       Growing medium - The growing medium used in terrariums must be clean, well- drained and high in organic matter. A pre-packaged 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. These types of mixes already contain fertilizer to get your new planting off to a great start. In general, about one quarter of the terrarium’s volume should be used by the growing medium and drainage material. Drainage material is important to add to the bottom of the terrarium container since it has no drainage holes.                               

Activated charcoal and pebbles are ideal drainage materials and they should be placed in the bottom of the container before any growing medium is added on top. Charcoal is usually effective in eliminating chemicals that could prove to be toxic to plants if placed in a 1/2-inch layer on top of the layer of pebbles, 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 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. 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 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 - 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 the terrarium closely for the first few weeks after planting. Any 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.

Terrarium 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.

·       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. 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. 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 to do well.  Plants receiving light from a window gradually will grow in 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.