In the depths of winter, most gardeners believe their efforts are restricted to armchair activities -- reading books, reviewing catalogs and planning for the spring garden. Not so!
A closed terrarium allows you to continue gardening on a very small scale. You'll be able to shop for plants, get your hands in the dirt and create a beautiful green vignette that will enhance any room in your home or workplace.
The first step is to choose a container. It must be clear glass or plastic with a tight-fitting lid or cover. Colored glass will not let enough light in for the plants to flourish. You may use a container with a narrow opening, but it will be more difficult for you to plant and tend. While there are specifically designed terrarium containers, other options include large glass cookie or candy jars, old fish bowls, fish tanks, half-gallon canning jars, old apothecary jars and lidded brandy sniffers. Choose a container that will blend with the decor of your home or office.
Through every step of the process, be careful not to introduce pests and pathogens into your terrarium, as this humid enclosed environment is a perfect breeding ground. Wash your hands or wear clean gloves while building and maintaining your terrarium. Start with a very clean container. Wash it with soap and hot water. Rinse well in cool water and air dry. If you feel the need to use a commercial glass cleaner, keep the lid removed and wait a few days before your initial planting. This will allow time for all fumes to escape.
You will need a soilless potting mix with relatively low fertility. Garden soil is much too heavy and will compact. Before placing the potting soil in the container, mix in some charcoal. Just a handful or two is all you need, depending on the size of your container. Charcoal will add organic matter and absorb any chemicals or odors. The tiny pores in the charcoal hold water and nutrients and later make them available to the plants.
In the past, people added a layer of rocks or pebbles to the bottom of the container for drainage. This is no longer the thinking. Place enough potting soil in your terrarium to equal the depth of the largest root ball of your chosen plants. This soil should be just damp, not soaking wet.
When it comes to choosing plants, start with the adage that all gardeners know: right plant, right place. Most that work best are either moist woodland or tropical plants. Succulents and cacti do not make good terrarium plants; save those for your open terrarium or dish garden. Choose small slow growers or naturally dwarf plants that will fit into your container with room to spare. Remove any dead or yellowed leaves before planting. Moss, lichen and ferns are natural choices for a terrarium, because of their fondness for moisture and their ability to thrive in dim settings. Choose plants with similar light and moisture requirements for the same container. Plant the smallest plants first.
Your terrarium must not be placed in direct sunlight. It will cook. The preferable location is in a bright room or near a bright window. Let your knowledge of light requirements for houseplants guide your decision.
You can purchase dedicated terrarium tools or make your own out of things you have around the house. A long-handled kitchen spoon is a good substitute for a shovel to create a pocket in the soil at planting time or to remove a plant later. A cork attached to a skewer or chopstick with a rubber band creates a great tool for tamping down the soil after planting.
The fun part is, of course, creating the design. A single specimen can be extraordinarily gorgeous when the right plant is paired with the right container. For terrariums with multiple plants, you'll want to choose plants of varying heights with different foliage, form and texture.
A consideration with plant placement concerns whether it will be viewed from only one vantage point or from every direction. As with a garden bed, if you will be viewing it from every direction, plant the tallest plant in the middle with lower ones near the sides and low-growing "groundcovers" (like moss or baby tears) blended in. If it will be viewed from only one direction, the tallest plant goes in the back. You may even decide to slope the soil so that it is deeper in the back.
Lay out your design outside of the terrarium. Do not choose too many variegated plants or too many plants with colored foliage. This is a small space -- more is not better. This is also a good rule for adding non-plant materials such as a stone or twig or a curved line of pebbles to designate a path.
When your installation is complete, clean off any dirty leaves and the inside glass of the container. A soft clean brush or paper towel may work, or you can use a mister. Your terrarium will need to be watered, but you must go gently here. The misting may be enough. It is easy to add a little more water later but difficult to remove any excess. Your goal is to establish a rain cycle within the terrarium. The first several days will require your attention. If there is water on the foliage, let it dry before placing the lid on the terrarium. Once lidded, there should be condensation on the inside that rolls down and waters the soil. The goal is that the condensation should look like a light fog. Anything heavier and you'll need to remove the lid for a day or two.
Now that you know how to create a closed terrarium, go dig in the dirt and have fun with it. Create a beautiful new living feature for your home or office that you can enjoy year-round.
Susan Marquesen is a Penn State Master Gardener and Penn State Master Food Preserver. Columns by master gardeners will sometimes appear in place of the Garden Q&A by Sandy Feather, a Penn State Extension educator.