What is a terrarium?
When physician and amateur botanist Nathaniel Ward of London, England, put a moth cocoon and a bit of dirt into a glass container to watch it hatch in the late 1820s, a small fern incidentally sprouted and grew beautifully. Shortly thereafter, Ward hired a carpenter to build special glass boxes just for plants. These became known as Wardian cases; today we call them terrariums (transparent enclosures for keeping plants indoors that often, but not always, have transparent tops).
Gardeners throughout Europe—and eventually North America—were thrilled to be able to grow plants in their homes, something hitherto thought nearly impossible owing to the widely fluctuating indoor temperatures in winter, which dropped to below freezing at night, then rose to near-baking levels as the wood stove was stoked.
Fast-forward some 200 years and, while house temperatures are now more constant, indoor air is much drier than it used to be (a side effect of central heating). This causes the leaves of many houseplants to turn brown and the soil to dry out. Inside a terrarium, however, the air is always humid, so little water is lost to evaporation—in fact, a completely enclosed terrarium may need watering only once a year. (Note: If persistent condensation builds up on the cover, the soil is a bit too moist; remove the cover for a day or two to allow for some evaporation.)
Even an open terrarium—being exposed to dry air only from above—rarely needs watering more than once or twice a month. It’s the ultimate low-maintenance indoor garden.