Glass House On Greens
Cristoforo Kanoy
A greenhouse is a special structure that is designed to regulate the temperature and humidity of the environment inside. There are different types of greenhouses, but they all have large areas covered with transparent materials that let sunlight pass and block it as heat. The most common materials used in modern greenhouses for walls and roofs are rigid plastic made of polycarbonate, plastic film made of polyethylene, or glass panes.[1] When the inside of a greenhouse is exposed to sunlight, the temperature increases, providing a sheltered environment for plants to grow even in cold weather.
The terms greenhouse, glasshouse, and hothouse are often used interchangeably to refer to buildings used for cultivating plants. The specific term used depends on the material and heating system used in the building. Nowadays, greenhouses are more commonly constructed with a variety of materials, such as wood and polyethylene plastic.[2] A glasshouse, on the other hand, is a traditional type of greenhouse made only of glass panes that allow light to enter. The term hothouse indicates that the greenhouse is artificially heated. However, both heated and unheated structures can generally be classified as greenhouses.
Greenhouses can range in size from small sheds to industrial-sized buildings and enormous glasshouses. The smallest example is a miniature greenhouse known as a cold frame, typically used at home, whereas large commercial greenhouses are high tech production facilities for vegetables, flowers or fruits. The glass greenhouses are filled with equipment including screening installations, heating, cooling, and lighting, and may be controlled by a computer to optimize conditions for plant growth. Different techniques are then used to manage growing conditions, including air temperature, relative humidity and vapour-pressure deficit, in order to provide the optimum environment for cultivation of a specific crop.
Before the development of greenhouses, agricultural practices were constrained to weather conditions. According to the climatic zone of communities, people were limited to a select range of species and time of the year in which they could grow plants. Yet around 30 CE, the Roman Empire built the first recorded attempt of an artificial environment.[3] Due to emperor Tiberius's declining health, the royal physicians recommended that the emperor eat one cucumber a day.[3] Cucumbers, however, are quite tender plants and do not grow easily year-round. Therefore, the Romans designed an artificial environment, like a greenhouse, to have cucumbers available for the emperor all year. Cucumbers were planted in wheeled carts which were put in the sun daily, then taken inside to keep them warm at night. The cucumbers were stored under frames or in cucumber houses glazed with either oiled cloth known as specularia or with sheets of selenite (a.k.a. lapis specularis), according to the description by Pliny the Elder.[4][5]
The next biggest breakthrough in greenhouse design came from Korea in the 15th century during the Joseon dynasty. In the 1450s, Soon ui Jeon described the first artificially heated greenhouse in his manuscript called Sangayorok.[6] Soon ui Jeon was a physician to the royal family, and Sangayorok was intended to provide the nobility with important agricultural and housekeeping knowledge.[6] Within the section of agricultural techniques, Soon ui Jeon wrote how to build a greenhouse that was able to cultivate vegetables and other plants in the winter.[6] The Korean design adds an ondol system to the structure.[6] An ondol is a Korean heating system used in domestic spaces, which runs a flue pipe from a heat source underneath the flooring.[6] In addition to the ondol, a cauldron filled with water was also heated to create steam and increase the temperature and humidity in the greenhouse.[6] These Korean greenhouses were the first active greenhouses that controlled temperature, rather than only relying on energy from the sun.[2] The design still included passive heating methods, such as semi-transparent oiled hanji windows to capture light and cob walls to retain heat, but the furnace provided extra control over the artificial environment.[6] The Annals of the Joseon Dynasty confirm that greenhouse-like structures incorporating ondol were constructed to provide heat for mandarin orange trees during the winter of 1438.[6]
The concept of greenhouses also appeared in the Netherlands and then England in the 17th century, along with the plants. Some of these early attempts required enormous amounts of work to close up at night or to winterize. There were serious problems with providing adequate and balanced heat in these early greenhouses. The first 'stove' (heated) greenhouse in the UK was completed at Chelsea Physic Garden by 1681.[7] Today, the Netherlands has many of the largest greenhouses in the world, some of them so vast that they are able to produce millions of vegetables every year.
Experimentation with greenhouse design continued during the 17th century in Europe, as technology produced better glass and construction techniques improved. The greenhouse at the Palace of Versailles was an example of their size and elaborateness; it was more than 150 metres (490 ft) long, 13 metres (43 ft) wide, and 14 metres (46 ft) high.
When returning to Mount Vernon after the war, George Washington learned of the greenhouse built at the Carroll estate of Mount Clare (Maryland). It was designed by Margaret Tilghman Carroll, an industrious gardener who cultivated citrus trees in this orangery.[9]In 1784 Washington wrote requesting details about the design of her greenhouse, and she complied. Washington wrote:
The French botanist Charles Lucien Bonaparte is often credited with building the first practical modern greenhouse in Leiden, Holland, during the 1800s to grow medicinal tropical plants.[11]Originally only on the estates of the rich, the growth of the science of botany caused greenhouses to spread to the universities. The French called their first greenhouses orangeries, since they were used to protect orange trees from freezing. As pineapples became popular, pineries, or pineapple pits, were built.
The largest glasshouses yet conceived were constructed in England during the Victorian era. As a direct result of colonial expansion, the purpose of glasshouses changed from agriculture to horticulture.[12] The accelerated transfer of plants and horticultural knowledge between colonies contributed to the Victorian fascination with 'exotic' plants and environments.[13] Glasshouses became spectacles to entertain the general public. The curated environments in glasshouses aimed to capture "the Western imagination of an idealised landscape" and support the fantasy of the cultural 'other'.[13] As a consequence, the collection of plants are believed to be true reflections of the world, yet are actually stereotypical arrangements of 'exotic' plants to symbolize exactly where British colonies are and how far their authority reaches.[12] To uphold British hegemony, glasshouses became arguments of colonial power which flaunt the "absolute control of colonized environments and flora...[using plants] as a symbol of British Imperial power.[14]
Greenhouse structures adapted in the 1960s when wider sheets of polyethylene (polythene) film became widely available. Hoop houses were made by several companies and were also frequently made by the growers themselves. Constructed of aluminum extrusions, special galvanized steel tubing, or even just lengths of steel or PVC water pipe, construction costs were greatly reduced. This resulted in many more greenhouses being constructed on smaller farms and garden centers. Polyethylene film durability increased greatly when more effective UV-inhibitors were developed and added in the 1970s; these extended the usable life of the film from one or two years up to three and eventually four or more years.
Gutter-connected greenhouses became more prevalent in the 1980s and 1990s. These greenhouses have two or more bays connected by a common wall, or row of support posts. Heating inputs were reduced as the ratio of floor area to exterior wall area was increased substantially. Gutter-connected greenhouses are now commonly used both in production and in situations where plants are grown and sold to the public as well. Gutter-connected greenhouses are commonly covered with structured polycarbonate materials, or a double layer of polyethylene film with air blown between to provide increased heating efficiencies.
The warmer temperature in a greenhouse occurs because incident solar radiation passes through the transparent roof and walls and is absorbed by the floor, earth, and contents, which become warmer. These in turn warm up the surrounding air within the greenhouse. As the structure is not open to the atmosphere, the warmed air cannot escape via convection due to the presence of roof and walls, so the temperature inside the greenhouse rises.
Quantitative studies suggest that the effect of infrared radiative cooling is not negligibly small, and may have economic implications in a heated greenhouse. Analysis of issues of near-infrared radiation in a greenhouse with screens of a high coefficient of reflection concluded that installation of such screens reduced heat demand by about 8%, and application of dyes to transparent surfaces was suggested. Composite less-reflective glass, or less effective but cheaper anti-reflective coated simple glass, also produced savings.[18]
Ventilation is one of the most important components in a successful greenhouse. If there is no proper ventilation, greenhouses and their growing plants can become prone to problems. The main purposes of ventilation is to regulate the temperature and humidity to the optimal level, and to ensure movement of air and thus prevent the build-up of plant pathogens (such as Botrytis cinerea) that prefer still air conditions. Ventilation also ensures a supply of fresh air for photosynthesis and plant respiration, and may enable important pollinators to access the greenhouse crop.
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