A greenhouse structure, complete with universal installations, suitable for all cultivations and microclimatic environments, does not exist. Therefore the grower has to make decisions, to create a suitable greenhouse for the Roses, with in mind the local climate (temperature, winds, sun light intensity, frequency of rain and hail-storms, heavy snow, typhoons etc) as well as the kind of market he wants to supply (local markets, national or international).
Globally speaking, the majority of greenhouses (about 90%) are built with a plastic covering. This is the case in South American countries (Colombia, Ecuador, Mexico, Brazil, etc...), most Asian countries (China, Thailand, Vietnam, South Korea, Taiwan etc), the Mediterranean areas in Southern Europe (Italy, Greece, Spain, Turkey etc) and in all African countries (Kenya, Ethiopia, Uganda, Zimbabwe, etc).
In the Northern European countries (Holland, France, Ukraine, Russia etc) Japan, in parts of Italy and in all those regions that are technologically developed, the greenhouses that are used, have glass coverings. There are various reasons why in some countries growers prefer to use plastic and technologically simple greenhouses, namely:
• Difficulty in finding the steel-glass structures in their countries;
• higher costs;
• Plastic requires simpler and lighter greenhouse structures which have lower transport costs.
The latest ranges of plastic have good light and thermal properties, allthough they do not last long because of the effect of UV rays. They also need to be replaced earlier, especially the plastic film.
Steel-glass greenhouses undoubtedly have their advantages because of:
• Higher resistance to wind and weight in general. Exceptions are very strong hail-storms.
• They offer a better interior climate for the crops and keep a more stable light transmission throughout the years.
Fundamental aspects to consider, when building a greenhouse from the beginning, are:
Location, Dimension and size, Orientation, Structure and covering material.
It is a legal requirement that the greenhouse is situated at a certain distances away from roads, houses, towns and trees (the later because of shade effect). It is also essential to consider:
• the slope of the land (level is preferred)
• the chemical and physical composition of the soil
• its colour (light coloured soil, in comparison with dark soil, reflects light more and therefore loses heat by radiation)
• the availability of electrical power
• the quality and quantity of (irrigation) water
These mainly depend on the external climatic characteristics (light). Because Roses need a lot of light (heliophilous plant) especially during winter, greenhouses are commonly built with the sectors (bays) or spans (a single unit of greenhouse) as wide as possible to minimise shading caused by the structure. The greenhouses require openings at the top (top-vents), to optimising air circulation and to minimising disease and pest incidence (e.g. insects, botrytis).
The gutter height has to be at least 3.50 m, while the roof curve is about 40-45% in northern European countries and about 20-30% in Mediterranean countries (as the curve increases, the light available to the plants increases; on the other hand, the greenhouse loses more heat to the external atmosphere as it has a larger surface).
Based on research carried out by Nisen on conditions with light in Belgium, the importance of the greenhouse orientation towards the sun and the shape of its roof, (figures 3, 4 and 5) were calculated. The figures, which relate to 2 equinox days and 2 solstice days, illustrate the direct effect of solar radiation on 16 shapes of greenhouse roofs during cloudless sunny days (i.e. ideal lighting conditions).
From the figures presented we can see how a regular curve-shaped roof makes the best use of the light available to the plant (n 15 circular). Also asymmetric shaped roofs uses the light very efficiently, however these are not generally recommended for greenhouses
(n 11,13,14,16). It also shows how the East to West orientation of the greenhouse brings certain advantages, i.e. it makes better use of the available sunlight in winter, and in summer can limit the amount of sunlight (in summer we need to cool the greenhouse).
This applies to solar radiation which enters the greenhouse at an angle greater than 15-20. If the sun is low in the sky (angle less than 15-20) its rays would reach directly only the sides of the greenhouse, whilst the roof would reflect almost all the light that hits it.
The framework of a greenhouse is generally made of wood or metal (steel + aluminium). Steel is nowadays the common material used, because of quality and durability. The size and thickness of the steel frame is based on the calculated required strength. The structure needs to hold up its own weight, stand up against wind forces, snow and seismic activity.
The covering material (or cladding) is one of the most important aspects of the greenhouse or greenhouse-tunnel. This is because the surface of the covering facilitates the energy exchange between the external and internal environments. The heat and light entering the greenhouse are influenced by the quality of the greenhouse cover (physical-mechanical and light conducting properties).
Two different types are commercially available being clear (=direct light) and diffused (=indirect light). The latter is more commonly used in environments where the sunlight is excessive for a large part of the year (Southern Europe). When the glass slopes more than 15 to 20%, condense water does not fall but slides down towards the gutters and is collected there. All types of glass are manufactured to have a very low transmission of heat.
• Tempered glass: used in areas often struck by hail-storms as they are harder and stronger.
• White glass: manufactured with a small iron content which allows UV-B rays to pass.
• Float glass: completely reflects all UV rays including UV-B.
• Anti-reflection glass: contrary to other types of glass, this glass will diffuse 6-8% more of the photosynthetic active radiation (PAR) in the greenhouse (95-96%).
• Double glazing: which reduces the consumption of energy (heating), but is costly.
• Thermal glass: this glass is covered on one side with metal oxides to reduce the transmission of heat (infra-red light) from the inside of the greenhouse to the outside.