POSSIBILITIES OF APPLYING PLASTIC FILMS IN GREENHOUSES AND THE INCREASING OF EARLINESS OF VEGETABLES
Bearing in mind the results obtained so far, it may be stated that plastic greenhouses are suitable in the following cases:
- when the film is used to cover the plants temporarily so as to protect them from adverse atmospheric influences with a view to affecting earliness and the quality of the yield;
- for crops the growing of which does not entail great outlays;
- on areas intended for temporary use in vegetable growing on account of which large investments are not justified.
Plastic films made of polyethylene or polyvenylchloride of a thickness of 0.15 to 0.2 mm are suitable for greenhouses.
In order to be profitable plastic greenhouses should be simple. Such a greenhouse may be with or without a supporting construction. Greenhouses without a supporting construction are airproof. They consist of polyethylene film, 0.15 mm thick, dug in the soil. A balloonshaped, cylindrical structure, which is maintained by a light, unnoticeable super-pressure inside the greenhouse, is produced by inflation with a pneumatic ventilator. A large-mesh steel net covered with perlon or plastic, stretched over the film and fastened with clamps, takes the stress and does not allow tearing of the film (fig. 1 and 2).
Greenhouses with a supporting construction may be procured ready or may be erected on the spot. They are constructed from easily transportable, assemblable parts. The shed construction is also suitable. The plastic film is stretched over a wooden or metal construction. The film may be placed over or between large-mesh plastic or wire net or between the net and the supporting construction. When reticular films are used no supporting net is necessary. They are stabilized by filaments of hard polyethylene, perlon or polyester. Ventilation is effected by means of a ventilator. During storms the ventilators are turned off and the doors are shut. The ventilator is reswitched and by drawing off the air a small vacuum is produced in the greenhouse, the film adheres tightly to the supporting net and is protected from damage (fig. 3 and 4).
The use of plastic films results in enhanced earliness of vegetable crops. In recent years the plastic plant-growing structures increased. Thus, for instance, in 1965 in the region of Vorderpfaltz these areas accounted for about 5 hectares, in 1968: 40 hectares and in 1969: 60