The surface of the greenhouse cladding should be sprayed with 20 – 40 g.m-3 lime. The lime can remain on the surface for at least 6 months depending on climatic factors such as wind and rain intensity. For better adhesion to the plastic greenhouse cladding, the lime should be mixed with a wetting agent such as dish washing liquid at a rate of 5 ml per 20 L. Whitewashing has several disadvantages, these are:
- There is no control over the shading density. The application thickness is determined by the person applying the whitewash.
- The transmissivity cannot be adapted to suit a specific crop.
- Environmental factor such as wind, rain, dew and radiation reduces the life span and shading density of the whitewash.
- Considerable time and labour is required to remove the whitewash.
- The transmissivity of the greenhouse cover is reduced after the whitewash is removed. The lifespan of the cladding material is also reduced by detergents and other chemicals during whitewashing (Fernandez-Rodriguez, Vadillos, et al., 2000).
Thermal screens are placed inside a greenhouse just above the crop. In most cases the screens are placed so that a person can stand upright under the screen and can also open and close the screen by hand. The cables on which the screens are tied to, are usually fixed to the gables of the greenhouse.