Introduction: Enclosed plant terraria have been in use for more than a hundred years but no scientific explanation for their long-term ability to sustain the plants with no water, low light and no air have been proposed. For the present study, we constructed an enclosed and sealed terrarium with a variety of broadleaf plants and measured daily ionization and absolute humidity levels for 1 month compared to a similar container without plants.

Methods: Twelve healthy, watered broadleaf plants were placed in a large 19-quart plastic container and sealed with snap closers. A similar but empty container was used as a control.

Protocol 1: The time to reach maximum negative ion counts determined by introducing an ion counter into the canister and a hygrometer to measure humidity were monitored daily for 30 days.

Protocol 2: After the 30-day study, the plants were removed from the canister. For the next 10 days, the same procedure was followed daily.

Results: The time (in seconds) to reach maximum negative ion counts (2999x103 ion counts/sec, for this instrument) and absolute humidity (90+%) in the first 10 days and the last 10 days of the month-long monitoring period were maintained at exceedingly high levels. Even when the plants had been removed from the container. All p values were >0.05.

Conclusions: In an enclosed and sealed terrarium with a variety of broadleaf plants, and measured daily ionization and absolute humidity levels for 1 month negative ion counts and humidity were maintained at significantly high levels for another 10 days, even when the plants were removed from the container.