How to Build a Terrarium, So It’s Always Gardening Season
As winter closes in, there’s at least one place where plants will still grow: a terrarium. Here’s how to get started on yours.,
During the months when you can’t be outside working in the garden, what could be better than a miniature landscape that sits in your living room?
Just remember, as you put the finishing touches on your first terrarium and celebrate by cuing the chorus of “It’s a Small World (After All)”: This is a tiny garden, not a scaled-down theme-park installation where the scene is picture-perfect, day after day.
“It’s not a diorama, and these are not plastic plants,” said Patricia Buzo, a terrarium designer who owns Doodle Bird Terrariums, in the suburbs of St. Paul, Minn. Mrs. Buzo’s terrariums are living gardens that she plants with narrow tongs and then prunes with shears more appropriately sized for manicures than hedge trimming.
The same rules that apply to tending your garden outside also apply here: Choose the right plants and put them in the right place. Or else.
Your subjects should be selected not just for their good looks, but for their compatibility with the environment you’ll prepare for them — inside a container of a particular size and shape — and with one another.
Shallow containers may make good homes for open-dish gardens that are more forgiving, like a bowl of succulents on a sunny windowsill. But in a conventional terrarium — a vessel that has a lid and is closed at least some of the time, or has a narrow opening — the conditions are different.
For one thing, the environment is more humid. It was that heightened humidity that allowed the Victorians to cultivate orchids and ferns at home in otherwise inhospitable environments, conjuring diminutive versions of the dreamy splendor inside the grand climate-controlled conservatories of the era.
A terrarium allows you to expand your indoor plant collection far beyond typical houseplants — most of which aren’t suited to terrariums, anyway.
“The plants that do well in terrariums are the ones that are not your common houseplants,” Mrs. Buzo said. “The best choices don’t tolerate the conditions out in the house, but really need that high humidity.”
Within those little glass walls, it’s possible to cultivate mosses, miniature tropical orchids and even certain carnivorous plants.
Mrs. Buzo, a former mural painter, began her terrarium business in 2008. “I work tiny now, and in 3-D,” she said.
She takes her horticultural cues from the real-world environments where she seeks visual inspiration: the steep, rocky slopes of a picture-postcard mountain retreat, perhaps, or the shores of Blue Springs in Florida, a vacation spot she invoked in one design. Or a familiar scene from her home state: a pasture surrounded by pine forests.
Add tiny horse figurines, and it becomes a glass menagerie.
When she sees a closed jar planted with cactus on Instagram, or succulents combined with moss, she can predict the outcome: “Even if it looks nice now, it isn’t going to last.”
Mrs. Buzo, the author of “A Family Guide to Terrariums for Kids,” offered this advice: “If you’re mixing plants, pick ones with similar needs.” (Note to grown-up beginners: The book’s how-to basics apply no matter what size your hands are.)
Maria Colletti, the owner of the Westchester-based Green Terrariums and the author of “Terrariums: Gardens Under Glass,” agreed. “Let’s not just plop some plants in a jar,” she said. “I wouldn’t put palms in a forest scene.”
Apart from the visual incongruity, there’s a bigger problem: Tropical plants and temperate woodland types won’t cohabit happily, said Ms. Colletti, who began making terrariums around 2006, when she was the shop manager at the New York Botanical Garden, and now teaches how-to workshops.
Whether the desire to grow certain plants or to show off a spectacular container comes first, the two must be aligned.
Again: What works within the humid environment under a bell jar or inside a narrow-mouthed lab flask is quite different from what flourishes in an open bowl or an uncovered fish tank. (It’s also worth considering the relative degree of difficulty in planting containers with openings so narrow you can’t fit your hand inside.)
Another consideration: What is the available light where you will display the terrarium?
Bright, indirect light is generally most suitable, because many terrarium plants come from tropical understory habitats. But even terrarium plants that prefer direct light — certain carnivorous species, for instance — would cook inside a closed or narrow-mouthed vessel on a sunny windowsill. Using an LED grow light that emits little heat would be a smarter choice.
Preparing a terrarium is not like filling a traditional flowerpot. It is part terrain-shaping and part creating a rooting zone — all supported by a base layer of pebbles, as a terrarium has no drainage hole. Pebbles create a reservoir for excess moisture, so the roots don’t rot.
Next comes the planting medium, chosen to suit what is being grown. Most tropical plants do fine in a peat-based houseplant potting soil. Some terrarium designers spread activated charcoal between the drainage and soil layers; others mix a little into their potting soil, which should be packed down once it’s added, and then repacked, where needed, after the plants are put in place.
Tricks of the Trade (Hint: Neatness Counts)
Another difference between terrariums and potted plants, Ms. Colletti reminds her students: In glass terrariums, all the “underground” layers will be visible.
Ms. Colletti sometimes uses colored aquarium sand in the drainage layer to complement a design. But whatever media she layers into her creations, she places construction paper cut to fit the container’s shape between applications of stone, sand and soil, to keep them from seeping into one another.
“You don’t see it,” she said. “But it’s a subtle thing that makes a difference in the final product. It’s all about the small details — the small steps — with a terrarium.”
Washing your hands between each step is another detail that yields positive results (note: glass shows everything). So is Ms. Colletti’s delivery method for the base media: With a small coffee scoop she scored at a cooking shop, she pours sand or soil into a funnel bought at an auto-supply store. She positions the funnel’s extra-long spout just so, allowing her to target where the material goes.
“When you pour from too high, it gets everywhere,” she said. “This way, you can be very precise with the placement in the landscape.”
Another of her repurposed terrarium tools: chopsticks, used to gently move aside a plant when placing another nearby, or to create little contours in the terrain.
Moss: A Good Place to Begin
Many terrarium landscapes become dreamier with the inclusion of moss, which loves a humid, closed environment.
Or moss can be the whole story. It’s forgiving in low-light conditions, and has no roots to rot. Another plus is its cold-hardiness, meaning mail-order sellers may ship it later in the year than other plants. (A homemade moss terrarium as a gift or a holiday centerpiece, perhaps?)
In her moss creations, Mrs. Buzo uses coconut coir as the planting medium, over the usual base of drainage pebbles.
Good candidates include pincushion moss (Leucobryum), which grows in pillowlike tufts “like tiny, grassy hills,” Mrs. Buzo said. Rock cap moss (Dicranum) is a little taller; at the back of terrariums it simulates miniature pine trees. Sheet mosses like fern and feather moss (Hypnum and Ptilium) grow in dense mats, resembling so many little ferns.
And while we’re on the subject of ferns: Although they might seem like obvious terrarium subjects, many get too tall for terrarium containers. Besides the fern moss, another appropriately sized look-alike that Mrs. Buzo uses is a fern ally called spike moss (Selaginella), which despite its common name is not a moss.
Some Reliable Tropicals
Small-scale tropical plants may be familiar looking, but they are nevertheless showy potential inhabitants for traditional terrariums and can coexist with moss.
For the look of ground covers or vines, Mrs. Buzo recommends creepers like variegated creeping fig (Ficus pumila Variegata), oakleaf fig (Ficus pumila Quercifolia), Pilea glauca Silver Sparkle and string of turtles (Peperomia prostrata).
If you’re looking for more upright plants, try Emerald Ripple Peperomia (P. caperata), strawberry begonia (Saxifraga stolonifera), friendship plant (Pilea mollis), nerve plant (Fittonia albivenis) with pink- or white-veined foliage, or the jewel orchids (Ludisia discolor, Macodes petola and Ludochilus species).
To make certain carnivorous plants happy, you’ll need a grow light. But their otherworldly qualities more than repay the investment.
Daniela Ribbecke, the assistant manager at California Carnivores, a specialty mail-order nursery in Sebastopol, Calif., recommends starting with one kind of plant before you try mixing genera in a design. Carnivorous plants like to grow lean, so be sure any medium you use does not contain fertilizer.
The butterworts (Pinguicula), especially the Mexican species, make newcomers to the carnivorous world gasp.
With “the Pings” — which Ms. Ribbecke described as “kind of succulent-looking” plants “that then also flower profusely” — use sandy or sandy-and-rocky planting mix in an open container, like a fish tank. Leaving the top off suits these drier-growing species.
Like the Mexican butterworts, various sundews (Drosera) crave high light, best supplied artificially. The sundews should be grown in a medium of four parts fertilizer-free peat moss to one part perlite. But they favor a wet, boggy environment, so a closed container is to their liking. (In a lower-light spot, three species of Australian sundews, sometimes referred to as the three sisters, are more adaptable: Drosera prolifera, adelae and schizandra.)
Boggy, closed conditions can foster mold, so weekly upkeep is critical. Check for and remove any faded leaves, and wipe off any buildup you see starting on rocks or glass — a good practice with any terrarium. Mrs. Buzo uses a peroxide-dipped cotton swab for the task.
Although Ms. Ribbecke advises beginners to avoid hardy native pitcher plants (Sarracenia) and Venus flytrap (Dionaea), which require an actual winter, Mrs. Buzo can’t resist them. The lower-growing species Sarracenia purpurea is a particular favorite of hers, and it remains terrarium-size, so Mrs. Buzo learned to simulate the needed season of chill from Ms. Ribbecke.
The recipe for a faux winter: Remove the plants from the terrarium, rinse their roots and wrap the roots in moistened long-fiber sphagnum moss. Place the entire plant in a plastic bag and put it in the refrigerator for about two months.
‘Targeted Watering’ and Other Aftercare Tips
How often do you need to water a terrarium? That depends on the evaporation rate, which is influenced by the terrarium’s size, whether it is closed or open, and by various environmental factors.
Closed units can go months or longer without watering. More water may be required in open containers during indoor heating season or in arid climate zones. And some terrariums that do fine when they’re open during humid months may do better partially or completely closed in drier months.
The water you use is at least as important as how often you apply it, because the chemical additives and high mineral content in tap water can build up, causing havoc in small, closed environments. Mrs. Buzo recommends using distilled water; spring water or collected rainwater are backup possibilities.
Each designer has her own watering strategy. Although Mrs. Buzo uses a spray bottle on the mist setting to water moss, for other plants she prefers a miniature watering can with a narrow spout “that won’t uproot everything,” or even a small cup.
To water the roots of plants without disturbing their foliage, Ms. Colletti targets her actions, using a spray bottle on the stream setting.
Ms. Ribbecke’s targeting is designed into her terrariums. In a corner of each one, she builds a rocky channel, like a riverbed, to provide a safe landing pad for the incoming stream of moisture. She learned why that was necessary the hard way.
“You took all this time to make this perfect little world and then you ruin it — and never make that mistake again,” she said.
Maybe the most important advice: Try to conjure a place you’d like to be.
One such spot from long ago continues to inspire Mrs. Buzo, who grew up near Como Park Zoo and Conservatory in St. Paul, with its century-old greenhouse.
“When I was little, there was one room inside it that I loved: the fern room. It made me feel safe — and like ‘Alice in Wonderland,'” she recalled. “I would go in there and think, ‘This is a fairy tale.'”
The goal when you look at your finished terrarium?
“You get the ‘I want to live here’ feeling inside your chest,” Mrs. Buzo said. “I try to create something like that inside each tiny glass container.”
Margaret Roach is the creator of the website and podcast A Way to Garden and a book of the same name.