The greenhouse is an ancient invention, dating back 2,200 years. Modern ‘active’ greenhouses are distinguished from older ‘passive’ ones by the fact that they allow for the adjustment of air and soil temperature. Previously, the first such greenhouses were believed to have been built in Germany in 1619, using a stove to regulate air temperature, and later developed in England in 1691.
The first ‘active’ greenhouse to be built, as was discovered by chance in 2001 in a historical document, was in 15th century AD Korea. On the basis of the surviving document, entitled ‘Growing Vegetables During Winter’, a reconstruction of the greenhouse was made, and it was found to combine traditional heating technology with sophisticated heat retention features and control of condensation.
The greenhouse described in this book was designed to regulate the temperature and humidity requirements of plants and crops in a very precise manner. Heat was supplied by means of a kudle (구들), a traditional Korean method of heating. The oiled hanji paper made it possible to raise the inner temperature and control ventilation and humidity. However, the extent to which these methods alone could regulate the temperature was limited. The additional inflow of steam from the cauldron had the effect of raising both the temperature and humidity.
One feature of the greenhouse that particularly impressed horticulturalists and academics is the process by which the kudle would automatically raise the temperature of the soil as the boiling water from the cauldron warmed the air.
Vegetables were planted in the new greenhouse, the temperature and humidity levels of the greenhouse were monitored over a space of 20 days. At first, the subterranean temperature outside was 8.6ºC, while the soil inside the greenhouse was at an ideal temperature of 26ºC. The radish and lettuce sprouted after three days, the other vegetables soon afterwards, and after two weeks they had all grown to full size.
The soil above the kudle always remained at 20ºC or higher, and the air temperature at 10ºC minimum. During the night, the soil temperature inside and outside differed by more than 25ºC, proving that the greenhouse was generating adequate heat. Although humidity was as low as 40% at the hottest time of the day (1 p.m), it was otherwise kept at 70%, a level suitable for growth.
This remarkable control was made possible by the oiled Hanji paper used for the walls of the greenhouse, as well as the heating system. Hanji paper when oiled has a high transmission ratio for incident solar rays, and low one for infrared rays. It also diffuses a large proportion of the transmitted rays, and its tensile and tear strength are also high. It retains the droplets of condensation that naturally form on the surface of the walls, allowing them to flow down to the floor. This is necessary to prevent them falling directly on to the plants, and potentially causing damage or introducing impurities.
Thanks to the discovery of a single volume in the corner of an old bookshop, forgotten for hundreds of years, the horticultural world has discovered a missing chapter in its history!