Plants are grown as per seasonal conditions If we want to grow some vegetable item of summer in winter then we have to use greenhouse technique.

What is green house?
           You might have seen a  colored or transparent sheet covering a small portion in the plant nurseries or at farm houses. This is called a green house. What's the utility of this is to be discussed here.

A Green house

                First we understand what green house effect is! When you sit in a car and close all the side glasses, what do you feel? It is felt as hot inside -gradually increasing! This hotness is due to 'green house effect' at small scale.

Hot during cloudy weather?
            You might have felt hotness in the cloudy weather as well - why ? This is because Sun in continuously radiating heat energy which is reaching earth atmosphere and surface during day time. This radiation energy (heat) is then reflected back from the surface of the earth. If this energy is allowed not to escape back  and is tried to remain confined under some cover, then this heat energy will rise the temperature along with continuously coming energy from sun. During cloudy weather, clouds and atmospheric gases full of moisture play the role of this 'cover'i-e atmospheric gases and moisture absorb the heat energy and clouds don't allow all the energy to escape to the space, The combined effect of this then make us feel hotness.

How we utilize green house

Depiction of Greenhouse Effect

                        Also we know that summer season brings its own variety of vegetables and fruits. And it clearly means that these summer productions need high temperature to grow. so, if we want to grow some vegetable item say tomato in the winter season then it would grow if we succeed in providing summer season like humid and hot environment. For this we use these green house cabins.

              The sun energy is trapped in the cabin and the moisture from plants and water also traps heat and produces summer season like environment - which a tomato seed /plant is 'happy' to find -and it's growth starts there.

Provide moisture and stable temperature for longer time

Polythene sheets with holes spread
 over irrigated land area

               Another way by which cultivators utilize greenhouse effect is spreading a transparent polythene sheets over the land where seeds are recently sown. As the moisture tends to escape quickly in to the atmosphere, the same can be blocked if transparent polythene sheets are spread over the cultivated area - keeping the land wet for longer time and hence providing sufficient moisture for longer time - which otherwise would have required too many frequent watering by different channels to the land. Spread transparent sheet also act as green house - by trapping inside the solar heat once it enters beneath it, thus providing required temperature for the seeds to grow