ASK MARTHA: Creating a Healthy Terrarium

Like a ship in a bottle, a terrarium is a miniature plant kingdom seemingly frozen in time. Lift the glass lid and breathe in the warm wood smells of the forest floor.

Question: Hi Martha, What is a terrarium? Are they hard to make?

Martha: Captured like a ship in a bottle, a terrarium is a miniature plant kingdom seemingly frozen in time.

A terrarium can represent a place you know, a place you’ve visited, or one you’ve only dreamed about. It can be a tropical world of orchids and other epiphytes, a temperate woodland forest, or a tranquil Zen retreat. Collect an intriguing patch of moss on a backyard walk and place it in a bottle filled with soil. Add small stones and young ferns, and it becomes a nature study. In a larger container, such as a pet-shop aquarium, soil can be graded to form hills and valleys, then planted with small ming aralias or miniature palms to emulate trees at higher elevations, while dwarf begonias on lower slopes form a jungle canopy protecting delicate Selaginella creeping below.

The landscape above was formed by mounding up the soil at the center to create a sense of topography. Ferns and moss-covered sticks were collected on a walk (if you don’t have these on your own land, take them only with permission) and transported to their new environment to be planted in fresh soil. Mosses collected from nature contain seeds that are eager to sprout in a terrarium, providing unexpected surprises.

The principle behind the terrarium is simple. It is an enclosed environment, where weather, temperature, and humidity fluctuations outside do not affect the plants inside. When properly balanced—with the right soil mixture, just enough light and moisture, and the appropriate plants—it acts as a self-contained ecosystem. A balanced terrarium will exhibit a continuous buildup of moisture on its glass sides, caused by water evaporating from the plant leaves in a biological process known as transpiration. The condensation trickles down, replenishing the soil and keeping the plants watered. A continuous exchange of oxygen and carbon dioxide, needed for the plants’ survival, occurs due to respiration and photosynthesis.

Although some people believe that terrariums, like lava lamps, are a product of the sixties, they actually have their roots deeper in history. The concept dates from 1829, when Dr. Nathaniel Ward, a curious young London surgeon, wanted to observe an adult sphinx moth hatch from a chrysalis. He placed soil and a cocoon into a glass jar. When grass and a fern sprouted unexpectedly, he lost interest in the moth and turned his attention to the plants, which were thriving with no additional water or fresh air. He had discovered the possibilities of cultivation under glass. Ward went on to experiment with hundreds of fern species and developed the principles behind what became known as the Wardian case. Beyond their practical applications, Wardian cases were also quite beautiful and soon caught on for decorative use in the home. 

Terrariums can take root in a vast range of containers, from flea-market finds to brand-new fishbowls and aquariums. A clear-glass container is best since colored glass filters out light. Wide-necked containers are easiest to work with; it takes skill and some patience to plant in a narrow-necked jar, and maintenance requires handmade tools designed specifically for maneuvering in tight quarters. A clear-glass lid that fits snugly in place is also essential; a terrarium will most likely flourish in an airtight container. But keep an eye on it, particularly at first, to make sure the moisture level is balanced.

Terrariums fail most often from overwatering. If you can’t see your plants through the fog of intense condensation, give them a breather by removing the lid for a day. Conversely, if the soil appears parched, moisture may be evaporating, and the terrarium should be gently rewatered and given a new lid.

A terrarium is an exquisite addition to any room, but placement isn’t merely an aesthetic decision. Plants have varying light requirements. Diffused light is best for most terrariums. Direct light is usually too intense, scorching the plants and raising the temperature. Where natural light is not available, artificial sources, like cool and warm fluorescent lights, can be used.

And avoid placing a terrarium directly on a heat source. Prune away leaves that are dead, yellowing, or growing against the sides; contact with the walls encourages bacterial growth and could contaminate everything. Remove plants that are not doing well or have outgrown their space. Maintaining a terrarium is a process, like all gardening, and requires some experimentation to get it right.

Creating a terrarium is a chance to elevate and appreciate the small wonders of nature. Atop a table or pedestal, your tiny conservatory becomes a place to escape to, where sometimes you will be calmed or surprised, but always struck by awe.

Article by Martha Stewart, as part of the Growing with Martha Stewart partnership.