The technical progress created the prerequisites for the rapid spreading of protected growing of vegetables.
In the beginning of 1968 the total greenhouse area designed for vegetable production exceeds by far 10,000 ha.
The glass greenhouses in the various countries take up approximately the following areas (in hectares): Holland - 5,200, England - 1,500, the German Federal Republic - 800, Belgium - 750, France - 500, the German Democratic Republic - 400, Italy - 300, the USSR - 300, the United States of America - 300, Rumania - 130, etc.
In recent years the areas of plastic greenhouses considerably increased.
In Japan they covered an area (in hectares) of about 6,500, in Italy -3,500, the USSR - 2,000, the USA - 1,800, Spain - 450, Hungary - 350, France - 350, Rumania - 280, Bulgaria - 80, etc.
Greenhouse vegetable growing in this country has a relatively brief history.
The first stage in greenhouse construction was principally orientated to the use of the hot water of mineral springs.
By 1956 the area of ferro-concrete, glass and wood greenhouses reached 4,8 hectates and their gabarits were not suitable for mechanized growing practices.
The second stage in greenhouse construction comprises the period of 1957–1963 when the hot water of industrial enterprises was utilized.
During this period the Greenhouses State Economic Corporation set up, undertook the construction of the large greenhouse enterprises at the Dimitrovgrad Thermoelectric Station - 12,5 ha, and at the Sofia Thermo-electric Station - 4,8 ha.
New types of greenhouses were adopted -MZG 0/56 and Venlo.
Those structures were much better.
The creation and maintenance of the microclimate therein was easier and labour was considerably more productive.
The construction of comparatively large blocks of 0,5 to 3,5 hectares made it possible to mechanize certain processes.
The use of hot water from industrial enterprises and from thermo-electrical stations in particular as a source of heat proved to be not very felicitous since it necessitates the construction of greenhouses to be on sites which in many cases do not meet the requirements of this production.
The basic inconvenience is that the glass is strongly sullied by the smoke and other gases which sharply deteriorate the light conditions in the greenhouses.
The third stage of construction was initiated in 1964 when the building of iron-glass greenhouses was undertaken, having independent boiler house equipment.
They are at the contemporary technological level with automated regulation of temperature regime, ventilation and irrigation.
The adoption of independent thermal stations, using liquid fuel, made it possible to undertake rapid construction of greenhouses on large areas.
As a result of this the greenhouse area augmented from 36,3 hectares in