INDIAN FLORICULTURE

Greenhouse Importance & Benefits

by Dr. Abhay Gaikwad, Trainer, HORTICULTURE TRAINING CENTER, PUNE

The greenhouse effect is a natural warming process of the earth. When the sun's energy reaches the earth some of it is reflected back to space and the rest is absorbed. The absorbed energy warms the earth's surface, which then emits heat energy back towards space as long wave radiation. This outgoing long wave radiation is partially trapped by greenhouse gases such as carbon dioxide, methane and water vapour, which then radiate the energy in all directions, warming the earth's surface and atmosphere. Without these greenhouse gases, the earth's average surface temperature would be about 33 °C cooler.

The above phenomenon is used in the greenhouse to grow plants in the optimum conditions by controlling the climate, which is one of the most important aspects of protected cultivation. Growing plants under roof in closed area makes it possible to control the inside environment or to create the suitable environment to grow the specific crop. Today, it is possible to grow most of the seasonal crops throughout the year because of protected cultivation, which enable us to control and manipulate the environment.

A Polyhouse or greenhouse is actually the same thing. Traditionally the greenhouses were constructed on wooden frames where glass was used as a covering material. On the advent of plastic technology, it became possible to replace glass with plastic material. Polythene material, being the most popular, the greenhouses came to be known as polyhouses in recent times. Around this time, even timber availability became a crucial issue. It became possible to use G. I. or M. S. to replace timber. Thus the modern greenhouses normally are built on a steel frame and are covered with plastic, which is fixed to the frame with aluminum grippers.

In general, flowers, vegetables, and fruits are produced in a greenhouse. Greenhouse is a framework structure covered by a transparent material in order to protect the crops from wind, rain, radiation, precipitation etc. It creates a microclimate surrounding the crop that helps the crop in its maximum growth. Greenhouses also provide higher concentrations of CO2 to increase the production to its maximum level. The greenhouse consists of steel structure, covering material, gutters and ventilation systems as well as climate control systems.

Structures for Protected Cultivation :

Protected cultivation comprises every form of physical protection of the plant. The modern, more advanced form of protected cultivation is not just concerned with the protection of the crop, but also controlling growth and development of the crop and with that the production processes through use of available technology.

Often the term 'protected cultivation' is specifically used for cultures, which utilize tall structures (structures under which one can walk) which are covered with plastic or glass and which are used for the entire growth period.  This form of protected structure is called greenhouse.  The greenhouse is always a compromise between cost, durability, mechanical strength, transmission of radiation, thermal insulation, ventilation capacity and serviceability among other factors. It is for this reason, that there are many different types of greenhouses.

The use of a cover, whether it is glass or plastic, creates an artificial environment.  This will cause the inside temperature to be higher than the outside temperature, due to limited ventilation capacity and of the 'greenhouse effect'. Thus in the greenhouse, five climate factors must be controlled because these are related to two important functions, namely photosynthesis and transpiration, which highly determine production and quality. These factors are:

1)        Solar radiation (light) 
2)        Temperature 
3)        Carbon dioxide 
4)        Humidity 
5)        Air movement.

The control of these in a greenhouse is called climate control.

Scope  :

There is good scope for protected cultivation. Advance research as well as new technical developments is taking place. This will help to achieve higher, uniform and quality production which consumer is demanding. Advance cultivation techniques will also help to have efficient resource use and management practices. This will be also environment friendly.

Improved infrastructure, lowering of trade barriers, refrigerated transport etc. has resulted in increased International trade. This is leading in expansion of protected cultivation worldwide. Cultivation under plastic is increasing in subtropical and semitropical areas. Protected cultivation makes it possible for developing countries to enter the world market with products which have a high need for labour and a high added value, through which they can earn the foreign currency which they need desperately for their national economies.

There has been visible progress in protected cultivation in these areas, resulting in better production at lower cost. If cultivation under glass can compete with cultivation under plastic, shall depend on many unpredictable factors, such as economy, politics, energy prices, labour costs, transportation facilities, market developments etc. Probably there will be a constantly shifting balance of periods and products, which will allow both methods of cultivation to continue.

The greenhouses are classified as under :

i)  According to the shape of structure

1.        Saw tooth type- Cost  Rs. 450/sq.mt
2.        Tunnel type-  Cost  Rs. 150/sq.mt
3.        Maxi vent type- Cost  Rs. 1200/sq.mt
4.        Gutter opening- Cost  Rs. 400/sq.mt
5.        Shadow Hall- Cost  Rs. 125/sq.mt

II)  According to the material of structure

1.        GI structure [some parts with Aluminum.]
2.        MS structure
3.        Wooden polyhouse

III.)  According to covering material

1.        Flexible covering material- eg: polythene
2.        Rigid covering material- eg: glass, FRP sheets.
3.        Shade house- Shade net

Greenhouse cooling

In most parts of India the climate is sub-tropical. The summers are hot while winters are mild, hence, greenhouse cooling is one of the important aspects of climate control. The goal of greenhouse cooling is to prevent increase of temperature. Different methods used are:

1.        Ventilation: with the help of vents
2.        Shading: with the help of screens
3.        Natural cooling: due to crop and soil
4.        Artificial cooling: with the help of Misters, Roof sprinklers, Pad and fan system etc

4.1 Ventilation

Greenhouse ventilation involves removing air from inside the greenhouse and replacing it with outside air. It influences the heat, water vapour and CO2 concentration inside the polyhouse. The ventilation may be (1) natural--caused by wind and temperature forces--or (2) mechanical or induced --accomplished by using fans. The vent design determines the air exchange and airflow in the polyhouse. The cold air enters from the side vents and warm air escapes from the top vent.

The ventilation capacity is determined by the structure of the greenhouse and growing conditions of the crop. If a crop is grown in summer and a low temperature is desired, a large ventilation capacity is required. Generally the side vents should be open when the temperature inside the greenhouse starts increasing while they are closed in the evenings. In rainy season, when the RH must be controlled accurately, a well controlled small ventilation capacity is desirable. The exhaust fans can be used to remove extra humidity.

4.2 Shading

To prevent an increase in temperature, shading with screen can be done. Shading can be done in the following ways:

1.  Inside screen- Horizontal screen below the roof. Eg: Aluminum net
  
2.  Outside screen- Screen over the polyhouse, it affects the incoming radiation in the polyhouse. Eg: Shade net
  
3.  White washing- White distemper or Lime can be applied on the polythene so that the sun radiation can be reflected reducing the intensity.

Screens :

Screens are used for shading, photoperiod control, and energy saving/cooling. The different screen materials are in use. Every fabric has its own specific properties and influences the greenhouse environment differently. The most important properties are:

1.        Light transmission
       2.        Insulating properties
       3.        Permeability to moisture

Screen materials are made of foil or woven fabrics. As the permeability of foil is less than the woven fabric, under a foil the RH will be higher under a woven fabric. Therefore, the foil is often used to raise RH in newly planted vegetable crops.
For some crops (for eg. chrysanthemum) photoperiod control for flowering is required, the material of screen must be opaque, black foil or black woven fabric. When the screen is closed well, there is little air exchange between top and bottom; it creates great temperature differences created locally. This can result in condensation at cold places. So, as a precaution, the screens are not completely closed- temperature and humidity gap.

4.3 Natural Cooling

The natural cooling mostly takes place in the greenhouse due to leaf area of the crop and evaporation from the surface of the greenhouse.

4.4 Artificial Cooling:

The artificial cooling can be done in the following ways:

a.   Spraying of Paths: In summer season the paths of the greenhouses should be sprayed with water, which creates humidity thus cooling the greenhouse.

b.   Foggers: Misting in the greenhouses helps in increasing the humidity. The foggers are placed over the head or in the paths. Misting helps in decreasing the temperature around 4-5 °C. It is generally done for 4-5 times in the afternoon for 5-10 minutes. It also depends on the crop stage, humidity of outside air,inside temperature etc.

c.  Roof Sprinklers: The over the roof sprinklers can be installed to reduce the heating of the polyfilm. The temperature difference of 3-4 °C can be achieved through this method.

Pad and Fan System: Evaporative cooling is a process that reduces the temperature of air by the evaporation of water into the air stream. As water is evaporated, energy is lost from the air reducing its temperature. Two temperatures are important when dealing with evaporative cooling systems. The dry bulb temperature of the air is the temperature that we usually think of as air temperature. A second air temperature is the wet bulb temperature. The wet bulb temperature is the lowest temperature that can be reached by the evaporation of water. The wet bulb temperature is the temperature you feel when your skin is wet and is exposed to moving air. The dry and wet bulb temperatures can be used to calculate the relative humidity. The equipment used to evaporate water, move cooled air through the greenhouse, and exhaust warm air from the greenhouse comprises of the pad and fan cooling system.

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