Ann Whitman, who works in our Burlington, VT, garden center, shows a pair of terrariums she created on a snowy afternoon.
I love planting colorful and unusual tropical plants inside glass containers to create small, fascinating worlds. They invite you in for a closer look and provide a perfect escape from the wintery weather outside. Terrariums are easy to care for and don’t require any special skills for success.
You may already have a container that you’d like to showcase; if not, you can find large glass containers — either open at the top or closed — at craft stores.
Closed containers hold more humidity and create a jungle-like atmosphere. Open containers are ideal for cactus and succulent gardens, as well as other plants that prefer less humidity. Clear, smooth glass offers the best view of the plants.
When choosing plants, select varieties that will thrive where you plan to display your terrarium. How much light will the plants will receive? South- and west-facing windows provide strong, bright light. East windows are medium. Fluorescent office lighting and north-facing windows provide low to medium light.
After light requirements, I consider the suitability of plants for life in a container. Ideally, the plants will grow slowly or remain small. I look for a mix of textures, colors and growing habits. Be sure to choose pest-free plants; even snails and slugs can wreak havoc in a terrarium.
The first step in assembling the terrarium is to soak the planting medium. I use Coir Bricks, which are made from coconut husk fiber. Coir has a loose, airy texture and it holds water without getting soggy. Its neutral pH, natural color and resistance to decay make it ideal for a closed environment.
Soak the coir brick in a bucket with plenty of warm water for about 30 minutes and break it up with a trowel as it expands.
I add an inch or two of expanded clay pellets, such as Aqua Pebbles, to the bottom of the container. These ensure good drainage, and absorb and hold water. They’re often used for hydroponic and orchid growing. If you can’t find expanded clay pellets, you can use large-diameter aquarium gravel. Avoid shells, limestone and other materials that might change the pH of the soil.
The next step is to size and cut a piece of fiberglass window screening to completely cover the clay pellets. The screen keeps the soil from falling into the clay pellets. It’s important to use fiberglass rather than metal screening, because it won’t rust.
Next, put moistened coir into the container, on top of the screen. Add at least 2″, and even 3″ if the container is deep enough. I like to step back and make sure that the proportion of soil within the container is pleasing; usually, the deeper the container, the better it looks with more soil. If I plan to view the terrarium from one side, I may slope the soil so that it’s deeper at the back to make the landscape more interesting.
Before setting plants into the terrarium, move them around on the tabletop to decide on the best combination and arrangement.
Starting with the largest plant, gently remove as much of the soil and white perlite as you can from the root mass. Trim overly long roots to help fit them into the container. I like to place the tallest plant toward the back or off-center to create an asymmetrical design.
Some pots may have more than one plant in them. You can divide these and place the smaller plants around the terrarium to create unity or rhythm in your design. Selaginella or spike moss can be broken into smaller bits because it grows roots along its stems, creating a vibrant carpet as it creeps over the soil.
Accessorize! You can add moisture-resistant objects to your design. I’m tying a tillandsia or air plant to a piece of driftwood with monofilament fishing line.
Terrariums don’t require much more than occasional watering and trimming. I use a small watering can or a little cup to water down the inside surface of the container, so that I don’t disturb the plants. It’s important to keep the water level just below the screen so that the soil doesn’t get saturated. Add water as needed to keep the clay pellets wet.