A terrarium is a delightful way to grow an indoor garden of small plants in an enclosed glass container. With proper care, a terrarium will create a humid atmosphere that protects tender, tropical plants that are difficult to grow in the normally dry atmosphere of our homes.
The concept of a terrarium as we know it was discovered quite by accident by an English physician and botanist named Nathaniel Ward. He was trying to raise a moth in a glass jar and found that the jar also provided ideal conditions for fern spores and plant seeds to sprout. He then devised the Wardian case, a glass box used to transport plants collected in distant countries back to the British Isles. Within the glass enclosure, he could grow plants in conditions almost like those of their natural environments and protect them from the harsh conditions onboard sailing ships. Because the plants inside the glass enclosures were decorative, the idea was soon adapted for home interior decorating.
Today, terrariums can be made of almost any type of clear glass: old-fashioned candy jars, fish bowls or tanks, goblets or bottles. However, cloudy or colored glass filters out too much light for good plant growth. While the containers should have covers or lids, clear plastic film may be used to cover open containers. Some kind of cover is necessary to control the moisture and humidity inside the terrarium. The size of the container is optional, but the opening should be large enough to permit manipulation of soil and plants.
The size of the container will determine the size of the plants and accessories that you may use. For example, small evergreens and deciduous tree seedlings can be used in large containers, such as aquariums and dish gardens.
Since terrariums have no holes for water to drain out, the gardener must provide other ways for the excess water to escape. A layer of moss on the bottom can serve as a drainage layer in very small containers. For large containers, broken pieces of clay flower pots or charcoal, or a layer of sand or fine gravel may be added before the moss layer.
A soil mixture of one part sand, one part peat moss and one part loam should be used in terrariums. Ordinary garden soils used alone are too heavy for plants to grow well. One level teaspoon of 5-10-5 fertilizer should be added to a 6-inch potful of the soil mixture.
Many types of plants may be grown in a terrarium, including native and tropical plants; however, it is not a good idea to combine both types of plants in the same terrarium. Cacti and succulents are about the only plants that are not well suited to terrarium culture.
After planting, wet the soil and the plants with a fine mist until water begins to seep through the moss at the bottom. Wipe off the inside of the glass with a tissue or soft cloth. To prevent the moisture from escaping, cover the terrarium, preferably with a glass cover. A temporary cover can be made from cellophane or plastic wrap attached with a rubber band or cellophane tape.
Don't let water stand in the bottom of the terrarium. If there's too much water in your terrarium, remove the cover for several hours per day until the excess water evaporates. Add water only when the soil surface becomes dry, and then add just enough to moisten the soil. Moisture condenses on the sides of the glass and drips back into the soil, where the plants reuse it.
Terrariums should be placed in bright light but not in direct sunlight. If plants become too tall, pinch them back and remove any plant that crowds the others.
For further information and lists of suggested plant species, see Purdue Extension publication HO-13 "Terrariums," available at http://www.hort.purdue.edu/ext/HO-13.pdf .