I'm not sure why this wouldnt be considered gardening, but it is a very good question. I havent seen the advert-obviously we dont have pound coins--but it sounds very much like a terrarium type of growing system so I found this:
Terrarium Environment - A Self-Sustaining System
In many cases, a closed terrarium might be considered a miniature, self-operating greenhouse, and can be equated to a small ecosystem. The water from the soil is taken up by the plants and transpired into the surrounding air. In the closed container, the water vapor is trapped within the system, creating a desirable high humidity environment for certain shade-requiring mosses, ferns, and tropical plants. When the room temperature is lowered, the excess moisture within the glass enclosure condenses on the side of the terrarium and is recycled back into the soil. The moisture is again taken up by the plant roots and the process is repeated.
The terrarium is therefore a self-sustaining system. At night, plants use oxygen and release carbon dioxide. During daylight hours, plants use that carbon dioxide for photosynthesis, releasing, in turn, oxygen and water vapor. The photosynthetic process supplies energy for growth, and the oxygen is used again by the plants at night.
sounds pretty simple
