1. Choose a container.
Start with a clean container of either glass or plastic. As long as it’s translucent, almost any container can be used. A container with a cover or lid retains the most moisture, making it perfect for ferns and tropical plants. However, if you’re interested in plants that prefer drier conditions, such as succulents, a more open or bowl-style container is best.
2. Build the base.
A good rule of thumb is that a quarter of the terrarium should be made up of planting soil and drainage material. Mary Jane Lavin of Mother Earth Gardens in Minneapolis recommends a multilayered approach: Start with gravel mixed with charcoal (for drainage), then layer on clean potting soil mixed with a light peat. For succulents and cacti, Amy Bryant Aiello, author of “Terrarium Craft,” uses sand made out of ground hematite, garnet or white quartz.
3. Add your plants.
“The key to a terrarium is finding plants that all like the same conditions,” said Lavin. That may require some trial and error. At the garden center, ask for small plants that do well in terrariums, but know that you may have to replace one or two if they can’t adapt to the container.
For variety, Cindy Tong, a horticultural science professor at the University of Minnesota, recommends selecting “non-blooming plants with different leaf shapes and different textures.”
4. Dress it up.
When you’ve finished planting, surround your plants with pebbles, small stones, dried moss or bark. If you want to dress it up a bit, you can add twigs, shells or distinctive rocks, or go the fairy garden route and add miniature furniture, accessories and even figurines.
5. Take good care.
Place your terrarium in bright, indirect light. (Because the glass magnifies direct sunlight, putting a terrarium in a window that gets bright sunlight might heat the plants too much.)
Water infrequently, if at all. Tong said it’s not uncommon to water only every four to six weeks. Check for moisture by opening the lid of your terrarium and feeling the soil for dampness. “You never, ever want to overwater it, because if you do, you’re toast,” said Lavin.
Because the goal is to keep plants small, there’s no need to fertilize.
Gail Brown Hudson