The Greenhouse Industry


Greenhouses (or glasshouses) date from roman times where wealthy people grew plants in small enclosures, which allowed for spring flowers in the winter and fruit out of season. This desire spread across Europe, helping to create the largest greenhouse industry in the world in the Netherlands. The initial structures were built so that during the day when the sun shone brightly, the glass could structures that could be opened were called cold frames by the English. To add heat and make it a hot frame, decaying manure was buried about 0.5 m (18 in.) under the structure. Both the cold and hot frames were constructed to allow a person to reach in and to the plants. These units developed into pit houses, similar to today’s greenhouse, which were constructed with a packed roof, doors at each end, and a center walkway cut about 1 m deep to accommodate a standing person.

The modern greenhouse with its high sides and glass roof was developed later. Greenhouses were constructed to allow the rays of the sun to enter the glass roof perpendicularly in the winter but partially reflect them when the sun was high during the summer. Heat was supplied with circulating hot water and, at a much later date, with steam. The sash bars were usually made of thick wood, especially in areas with great snowfalls, to provide structural support. Lumber resistant to rot such as cypress and redwood was found to be ideal, but because these woods are not particularly strong, the girth of the sash bars had to be large, creating many shadows and thereby reducing the light intensity.

The even-span greenhouse was the most popular glass greenhouse design for many years. Even-span greenhouse connected along the eave are termed ridge-and-furrow greenhouse. For decades greenhouses did not vary much in design except that the width of the glass increased as the technology of making better glass developed. With improvements in techniques for extruding aluminum bars, the wide wooden sash bars were replaced with narrow aluminum bars, which were very strong. Narrow sash bars and wide glass allowed more light to enter during the winter when it was needed most.

The use of plastics as greenhouse glazings led to the development of Quonset-style and gutter-connected greenhouse (two or more Quonset-style greenhouse connected, which are less expensive to build than even-span greenhouses today with design changes that continually optimize the growing of plants.