Greenhouses are frames of inflated structure covered with a transparent material in which crops are grown under controlled environment conditions. Greenhouse cultivation as well as other modes of controlled environment cultivation have been evolved to create favorable micro-climates, which favors the crop production could be possible all through the year or part of the year as required. Greenhouses and other technologies for controlled environment plant production are associated with the off-season production of ornamental and foods of high value in cold climate areas where outdoor production is not possible. The primary environmental parameter traditionally controlled is temperature, usually providing heat to overcome extreme cold conditions. However, environmental control can also include cooling to mitigate excessive temperatures, light control either shading or adding supplemental light, carbon dioxide levels, relative humidity, water, plant nutrients and pest control.
Classification of greenhouse based on suitability and cost
a) Low cost or low tech greenhouse
Low cost greenhouse is a simple structure constructed with locally available materials such as bamboo, timber etc. The ultra violet (UV) film is used as cladding materials. Unlike conventional or hi-tech greenhouses, no specific control device for regulating environmental parameters inside the greenhouse are provided. Simple techniques are, however, adopted for increasing or decreasing the temperature and humidity. Even light intensity can be reduced by incorporating shading materials like nets. The temperature can be reduced during summer by opening the side walls. Such structure is used as rain shelter for crop cultivation. Otherwise, inside temperature is increased when all sidewalls are covered with plastic film. This type of greenhouse is mainly suitable for cold climatic zone.
b) Medium-tech greenhouse
Greenhouse users prefers to have manually or semiautomatic control arrangement owing to minimum investment. This type of greenhouse is constructed using galvanized iron (G.I) pipes. The canopy cover is attached with structure with the help of screws. Whole structure is firmly fixed with the ground to withstand the disturbance against wind. Exhaust fans with thermostat are provided to control the temperature. Evaporative cooling pads and misting arrangements are also made to maintain a favourable humidity inside the greenhouse. As these system are semi-automatic, hence, require a lot of attention and care, and it is very difficult and cumbersome to maintain uniform environment throughout the cropping period. These greenhouses are suitable for dry and composite climatic zones.
c) Hi-tech greenhouse
To overcome some of the difficulties in medium-tech greenhouse, a hi-tech greenhouse where the entire device, controlling the environment parameters, are supported to function automatically.
1. Less expensive greenhouse without fan and pad Rs.300 to 500/m2
2. Medium cost greenhouse with pad and fan system Rs.800 to Rs.1100/m2
3. Expensive greenhouses with fully automatic Rs.2000 to Rs.3500/m2
The greenhouse can also be classified based on type of structures, type of glazing, number of spans, environmental control etc. The various types are as follows.
Classification as per type of structure
a. Quonset type
b. Curved roof type
c. Gable roof type
Classification as per glazing
a. Glass glazing
b. Fiberglass reinforced plastic glazing
i. Plain sheet
ii. Corrugated sheet
c. Plastic film
i. Ultra violet stabilized low density poly ethylene
Classification based on number of spans
a. Free standing or single span
b. Multispan or ridge and furrow or gutter connected
Classification based on environmental control
a. Naturally ventilated
b. Passive ventilation
The crops grown in open field are exposed to vivid environmental conditions, attack of insects and pests, whereas the polyhouse provides a more stable environment. Polyhouse can be divided in to two types
a) Naturally ventilated polyhouse
These poly house do not have any environmental control system except for the provision of adequate ventilation and fogger system to prevent basically the damage from weather aberrations and other natural agents..
b) Environmental controlled polyhouse
This type of poly house helps to extend the growing season or permits off-season production by way of controlling light, temperature, humidity, carbon-dioxide level and nature of root medium.
Carnation under high-tech greenhouse
Shade houses are used for the production of plants in warm climates or during summer months. Nurserymen use these structures for the growth of hydrangeas and azaleas during the summer months. Apart from nursery, flowers and foliage which require shade can also be grown in shade houses. E.g. Orchids, These shade structures make excellent holding areas for field-grown stock while it is being prepared for shipping to retail outlets. Shade houses are most often constructed as a pole-supported structure and covered with either lath (lath houses) or polypropylene shade fabric. Polypropylene shade nets with various percentages of ventilation are used. Black, green, and white colored nets are used, while black colors are the most preferred as it retains heat outside.