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 on creating and maintaining a closed terrarium. The sides of a 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 the 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. When creating a closed terrarium, you will be creating a miniature landscape with its own unique rain cycle. Your plant palette is a little more limited. But once created, your terrarium will require minimal effort to maintain.

Step by Step: Choosing Items and Building the Terrarium

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 allow enough light to reach the plants for them to flourish. You may use a container with a narrow opening, but it will be more challenging for you to plant and tend to. 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. Select a container that complements 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 adding the potting soil to 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.)  It is no longer thought that a layer of rocks improves drainage. 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 closed 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, due to their affinity for 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 thrives in the room, use plants for your terrarium with the same or brighter light requirements as the houseplant.

You can purchase dedicated terrarium tools or create your own from items 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 clippers for cutting wayward, damaged, or yellowed leaves.

The fun part, of course, is 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, 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 viewed from only one direction, the tallest plant is placed 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. Avoid choosing too many variegated plants or those 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, a 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 do so gently. 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, which rolls down and waters the soil. The goal is for the condensation to resemble a light fog. Anything heavier and you'll need to remove the lid for a day or two. Diseases typically become established within the first few weeks. Be diligent about removing any infection. You can choose to use a diluted fungicide.

Maintenance

Once the rain cycle is established, your terrarium is almost maintenance-free. Some experts suggest removing the lid for up to 15 minutes once a week to allow fresh carbon dioxide to enter. 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 rotate the container as the plants begin 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.

Switching It Up Now and Then

I have had terrariums that survived and were healthy for nearly a decade. Without a refresh now and then, even a well-designed closed terrarium can become stale and boring. After its first year, you may want to freshen up the design with a new plant or non-living feature. Another fun design option is to change up the terrarium for your favorite holidays throughout the year – for Halloween, Thanksgiving, Christmas, Valentine’s Day, Independence Day, Flag Day, and your birthday, among others. A less involved effort would involve a seasonal change. A simple choice, and one that is less intrusive for the terrarium’s microenvironment, is to add holiday-themed ribbons to the nob on the lid or the base stem. Items can be placed inside the closed terrarium alternatively or in addition. Make sure that the object is in scale with the terrarium and plantings. Every introduced item must be clean so as not to introduce pathogens. Non-porous figurines (made of glass, resin, or even porcelain) are perfect because they can be scrubbed and can endure the moisture in the terrarium. Wooden items should be avoided or used for a very short time period. Skip valuable objects altogether. 

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. Pay attention to the maintenance. By mixing it up from time to time, you can enjoy your closed terrarium year-round and for many years to come.