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How to Make the Perfect Terrarium (and Keep It Alive!)

Even if you don't have much of a green thumb, terrariums act as small, self-sustaining ecosystems all on their own. But maintaining one is as much a work of art as it is an experiment in science. To help, senior garden editor Todd Carr shares five of his secret techniques for growing a garden under glass.

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Think of it as an ongoing science experiment. Here are some of the basic materials you'll need and the steps to follow in order to build one that will thrive:

 

First, choose a container. Maybe you already have a container on hand you would like to use in creating your terrarium. Keep in mind, however, that not all containers are suited for being repurposed into terrariums. Ideally, a closed container with a lid will encourage the humidity that helps your plants thrive. Search your attic, basement, and cupboards for unique vessels: clear soup tureens, kitchen jars, and empty bottles.

 

Whatever you choose, be sure that it is smooth, clear glass. "Colored glass will block the sunlight your plants need to thrive," Carr says. And besides, wouldn’t you want your plants in full view?

 

As for plants, choose varieties that thrive in high humidity and low light. We recommend plants like tropical ferns, mosses, Selaginella, Peperomia, Cryptanthus, and air plants. From there, you can add a personal touch with small garden ornaments: Just be sure that your plants have enough room to breathe.

 

Now you're ready to build your terrarium. Deposit a base of pebbles or gravel about an inch thick (this is extremely important for drainage), followed by a thin layer of horticultural charcoal, then a thicker layer of potting soil. Nestle the plants neatly into the soil so that the roots are covered and the leaves, petals, and stems do not touch the sides.

 

Water with caution. The worst thing you can do is deluge your plants with water. This is one of the most common mistakes made by beginner terrarium-keepers, Carr says. All you need to do is lightly spray your plants until the soil is evenly moist about an inch down. There should never be standing water pooling at the bottom.

 

Terrariums act like tiny greenhouses, resulting in condensation on the inner walls. Clean the inside once you begin to see water spots or algae buildup; this is typically once a week. Reaching the corners of a container isn’t always easy, which is why Carr offers these makeshift gardening tool ideas (incorporating items you likely already have at home).

 

Pour with a pastry bag: When creating the drainage base, there's no need to pour pebbles, rocks, and soil into the bottom of the container haphazardly. For precision and even distribution, use a pastry bag with no tip to add fine soil or gravel to your terrarium.

 

Think long: The narrow neck of some containers can make reaching around on the inside an absolute impossibility. The easy homemade solution: "Use long-handled tweezers and scissors to either pick up dead leaves or prune leaves off the plant or edge of the glass," Carr says. "Chopsticks can work as well for picking up debris."

 

Be wily with wire: When even a pair of chopsticks won’t do the trick in getting to those out-of-reach corners, a wire hanger will work: "Use a wire hanger to create a little hook or loop to move leaves and plants out of the way to gain access and see what you’re maintaining beneath."

 

Two tricks to keep a lid on it: Keeping the terrarium closed is what keeps the humidity in your terrarium and keep your plants happily thriving. Therefore, a little ingenuity is called for: "If there is no lid on your terrarium, you can use a glass plate on the top," Carr suggests, "or if it’s a narrow-necked bottle, use a lightbulb to block the opening and create extra humidity."

 

Scrub down: Wire a small piece of sponge to the end of a chopstick, a wooden spoon, or thin bamboo with floral wire to wipe and clean the inside of your terrarium.

Curious to learn more? In this video below, Martha shows how you can make your own tools, using ordinary household items like forks, spoons, and brushes attached with wax twine to handles made from bamboo garden stakes.

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