Terrariums are miniature gardens in lidded, clear-glass containers. Their closed environment requires little care and only occasional maintenance.
The container can be a jar, a glass globe or an unused aquarium. Whatever size container you choose, just be sure it’s been thoroughly cleaned.
Select plants that either are naturally small and slow growing or that can be pruned to remain small. Plants not suited to growing inside the limited space of a terrarium will soon outgrow the container and overcrowd their companion plants. For best results, choose plants with similar light and humidity requirements.
Plants to consider include miniature ferns, such as maidenhair fern (Adiantum microphyllum) and button fern (Pellaea rotundifolia), the palm-like little tree plant (Biophytum sensitivum), polka dot plant (Hypoestes phyllostachea) and miniature African violets (Saint paulia). Carnivorous plants, such as Venus flytrap (Dionaea muscipula) or sundew (Drosera), will appreciate the humidity a terrarium provides. In addition, mosses can be the star of a tiny terrarium or serve as ground cover in a large one.
To provide a good foundation for plants to grow, begin with a 1- to 2-inch layer of coarse gravel or small stones. Use the thicker layer for a larger container. This layer provides necessary drainage since the terrarium’s container has none.
Next, add a thin layer of activated charcoal. The charcoal will help prevent odors and bacterial growth.
The top layer in which plants will grow is made up of two to three inches of light potting soil that contains perlite and/or vermiculite.
Plan the arrangement of plants using a plate or a piece of paper the size of the planting area within the container. Taller plants should be placed behind shorter ones. Allow enough room, particularly near the walls of the container, for plants to grow.
When satisfied with your design, remove the plants from their pots, working away excess soil and freeing roots. Make a hole in the terrarium’s soil and spread roots to accommodate the shallow soil depth. Gently press soil around the base of each plant.
Depending on the container, it may be difficult to use your hands to place plants in the terrarium. You can use a long-handled spoon, dowel, chopsticks, tongs and other items to place each plant and firm it in soil.
Water sparingly by misting or adding spoonfuls of water. Soil can be covered with sand, gravel, colored aquarium stone or a living ground cover such as moss. Consider adding shells, miniature figurines and other decorative accents. Finally, secure the lid in place to create a closed environment that will need minimal maintenance.
If excess moisture accumulates on the inside of the glass, briefly open the lid. Conversely, if the seal is not tight, the terrarium may need to be watered periodically. If you have a lid with a good seal, you may not need to add water for months.
As plants grow, they may need to be groomed periodically with pruning to remove unhealthy or damaged growth or to maintain shape or size.
Select a location for the terrarium with bright light but avoid placing in direct sunlight as the glass will magnify the sun’s rays and overheat the environment inside the terrarium, damaging the plants.
If you’d like a terrarium featuring cactus and succulents, which prefer far less humidity, you can accommodate them by making an open terrarium in this same manner. Omit the lid to allow air to circulate. Monitor and water as needed.
Large or small, terrariums are a wonderful way to enjoy a tiny, private garden even in the middle of winter.
(Deborah J. Benoit is a University of Vermont Extension master gardener from North Adams, Mass., who is part of the Bennington County chapter.)