GARDEN HOTLINE | Bringing the outdoors in with terrariums

Rather than search for a genie in a bottle, why not settle for the magic of a contained landscape? 

Derived from the Latin word terra, meaning “earth,” terrariums are essentially mini-greenhouse ecosystems where plants can grow in an environment that recycles all inputs and outputs. 

These sustainable enclosures are more than just decoration; they’re a great option for gardeners with little to no outdoor space or time for maintenance, who are concerned about indoor air quality or who crave some Zen greenery to perk up their mental health and make feng shui flow. 

The history of the terrarium

Originally used for transporting newly discovered plants across seas for study, these mini landscapes have survived interest for more than 100 years. 

London medical doctor Nathaniel Bagshaw Ward created a “natural environment” to study a Sphinx moth chrysalis and was excited to discover a fern and some grass having sprouted from the soil, especially since most of his outdoor ferns had died from factory pollution. 

Dr. Ward’s curiosity of how long the fern would survive in this sheltered environment led him to invent what was originally called the Wardian Case. This fancy terrarium was used to transport and display rare plant specimens from around the world for study and protect them from the salt spray of the ocean and the abrupt change to England’s chilly, polluted environment. They quickly became the rage of the Victorian era and housed exotic plant collections of wealthy enthusiasts.

How it all works

Sunlight is allowed in through the glass or plastic for the plant to use the process of photosynthesis to create food for itself. An initial watering is usually enough to last the course of the terrarium. Water molecules will evaporate from the heat of the greenhouse effect and collect on the walls (in essence, make clouds) and slide down (rain) to the soil surface, a recycled-water effect mirroring the Earth’s own atmosphere. Because this is a small space, the moisture will make for a very humid environment. 

When it comes to respiration, plants that do not require a lot of energy to grow have no problem surviving on less carbon dioxide in a terrarium. Much like mammalian lungs inhale and exhale a combination of gases — not just oxygen in and carbon dioxide out — plants release a variety of gases and will not entirely choke themselves out in a closed environment, though growth may be stunted.

Make your own

For a closed terrarium, choose plants that like moisture and humidity, such as mosses, orchids and ferns. 

Begin with a layer of pebbles on the bottom of the terrarium for drainage, add activated charcoal on top of the pebbles to help filter bacteria and then top it all with a couple of inches of organic potting soil.  

Avoid potting soil mixed with synthetic fertilizer or “water-saving” beads because the terrariums are designed to be self-sustaining environments and need the least amount of intervention to thrive. 

Starting with transplants, remove the pot from your plant and make a small hole just big enough for the roots. If your plant is excessively leafy, trim off some of the leaves at the stem before planting to delay crowding. 

Water the plants until the soil is uniformly moist, but not too soggy. Open the lid once a week to prevent mold from growing on the sides of the terrarium. Some closed terrariums make great environments for cuttings and seed starts to keep them sheltered when they start to take root. 

If you begin with a clean glass enclosure and new potting soil, there is no need to worry about insects or diseases entering a closed terrarium. 

Good plant choices include:

•Baby Tears (Soleirolia soleirolii);

•Creeping fig (Ficus pumila);

•Maidenhair fern (Adiantum spp.);

•Polka dot plant (Hypoestes phyllostachya);

•Asparagus fern (Asparagus aethiopicus); and

•Aluminum plant (Pilea cadierei).

Place in a warm, sunny windowsill or covered yet bright patio.

Don’t forget a tiny bench for the garden fairies to sit upon and smooch!

For open terrariums, some gardeners choose to keep the walls but shatter the glass ceiling. This allows excess moisture to escape and creates a warm, dry environment better suited for plants like succulents. 

As above, add a layer of pebbles for drainage, activated charcoal and then cover with a potting soil specially designed for succulents, such as a cactus mix. Gently place a few plants in the soil, making sure to choose species that fit the parameters of your terrarium. 

Good plants include:

•Hens and Chicks (Sempervivum spp.);

•Sedums (Sedum spp.);

•Jade (Crassula ovate);

•Echeveria (Echeveria spp.);

•Aloe (Aloe spp.); and

•Air plant (Tillandsia spp.).

Watering is only necessary once a month, with a tablespoon full of water at most.

To learn more about choosing the right plants, water conservation and sustainable plant systems, contact the Garden Hotline at (206) 633-0224 or www.gardenhotline.org. 

MELISSA McGINN answers the Garden Hotline for Seattle Tilth (www.SeattleTilth.org).