Creating a Closed Terrarium

In the depth of winter, most gardeners believe their efforts are restricted to armchair activities—reading books, reviewing catalogs and planning for the spring garden. Not so! Through designing, creating and maintaining a terrarium, a gardener is afforded the opportunity to apply gardening skills to a new pursuit. You'll be able to shop for plants, get your hands in the dirt and create a beautiful, even intriguing, green vignette that will enhance any room in your home or workplace.

The focus of this article is in creating and maintaining a closed terrarium. The sides of closed terrarium are higher than the plants and it has a lid. This is distinct from an open terrarium, which has no lid, or a dish garden where the sides of container are lower than the plants. Open terrariums and dish gardens are a bit easier to create and their available plant palette is almost unlimited. In making a closed terrarium, you will be creating a miniature landscape with its own rain cycle. Your plant palette is a little more limited. But once created, your terrarium will require minimal effort to maintain.

The first step is to choose a container. It must be clear glass or plastic with a good 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 repurposed 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 décor of your home or office. Etched, ornate terrariums can be the centerpiece for a dining room table (mine are). A cookie jar will work well in a kitchen. An apothecary jar could be a nice addition to a bathroom.

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. A purchased potting mix is a good choice and most are sterile. Before the potting soil is placed in the container, you will need to 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.

In times past, it was thought that a layer of rocks or pebbles should be added to the bottom of the container for drainage. (It can be a design choice for a clear dish garden or open terrarium.)  This is no longer the case. 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 you plant, start with the smallest plants first.

When it comes to choosing plants, start with the adage that all gardeners know: right plant, right place. A large number of plants will work in a closed terrarium. Most fall under the category of either moist woodland or tropical. Succulents and cacti do not make good terrarium plants. Save those for your open terrarium or dish garden. Choose small plants—slow growing or naturally dwarf plants—that will fit into your container with room to spare. They should be healthy specimens. Remove any dead or yellowed leaves before introducing the plant into its new home. Moss, lichen and ferns are natural choices for a terrarium, with their fondness of moisture and their ability to thrive in dim settings. Choose plants with similar light and moisture requirements for the same container. This, of course, will depend on the planned placement of the terrarium in your home or office.

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. Some of these plants need very bright light. Others will do well in a dimmer room. Let your knowledge of light requirements for houseplants guide your decision here. If a houseplant does well in the room, then use plants for your terrarium with the same or brighter light requirements as the houseplant.

You can purchase dedicated terrarium tools or make your own out of things you have around the house. Clean hands are my favorite tool. 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 other tool you may need is a scissors or very small garden clipper for cutting wayward, damaged, or yellowed leaves.

The fun part is, of course, creating the design. The simplest terrariums may house but a single specimen and yet 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 (the canopy), 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, then the tallest plant goes in the back. You may even decide to slope the soil so that it is deeper in the back. Firm up your design outside of the terrarium. It is much harder to adjust the design within the confined space 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. It's wonderful to add a stone or twig or a curved line of pebbles to designate a path. But a few can quickly become too many. You can also create a miniature landscape with undulating soil and paths complete with buildings, small people, animals or fairies. You are only limited by your imagination and what you may have on hand. Just remember to clean any item that you put in the container.

When your installation of plants and non-plant items is complete, you will want to clean off any dirty leaves and the inside glass of your 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. 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 on the lid on the terrarium. Once lidded, there should be condensation on the inside which 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. Diseases usually get established in the first few weeks. Be diligent about removing any infection. You can choose to use a diluted fungicide.

Once the rain cycle is established, your terrarium is almost maintenance-free. Some experts suggest taking the lid off for up to 15 minutes once a week to let in some fresh carbon dioxide. This is more important if the lid is extremely tight fitting. Water only if condensation has stopped, the plants start to droop, or the soil feels dry. And then, of course, go gently. If the lid is especially tight-fitting, watering may be necessary only once every four to six months. I check mine by poking my clean finger into the soil. I must water one of my terrariums every three weeks. The rest require a little supplemental water every six weeks to two months. Occasionally, you may need to turn the container as the plants will start to lean toward the light source. Prune out any leaves that have yellowed, are touching the glass, or are growing out of bounds. You will need to dust or clean the outside, but avoid getting any commercial cleaner fumes into the inside of your terrarium. After a year, you may want to fertilize with an organic water-soluble fertilizer at ¼ the recommended rate, but not during winter. Spring is an appropriate time to fertilize. After a year, you may want to freshen up the design with a new plant or non-living feature.

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.