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Cees VandenEnden
HortiSource Consulting Inc / Green Q
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Roof Coverings

The time that you would order plastic at the lowest price or, when building a glass greenhouse, order “just glass” is behind us. New developments are coming out so rapidly that ordering your roof cover is becoming a scientific task; considerations include what crop you are growing, which production period you are aiming for and so on.

All these questions will influence your present and future choices. In the past we developed plastic greenhouses as an economic affordable alternative to the higher initial price of glass covered greenhouses. We accepted the slightly lower light levels of the plastic. But even building a glass covered greenhouse was a rather simple decision; you just ordered glass.

The last few years started a transition; we looked at the specific types of plastic or asked for the manufacturer of the glass as this information had some impact on the light levels each material could produce. So far the statement 1% light equals 1% production was still accepted as a kind of law. However, it was found that in plastic greenhouses, with lower light transmission, production levels were being achieved that could compete very well with glasshouses with higher transmission rates. This added a new dimension in the search for higher light transmissions; clear glass in the past had light transmissions of 84-88%, by producing low Iron level glass this went up to 91%. This was excellent for winter circumstances but we still were facing plant stress in the summer because of direct radiation on the plants. Here it is that plastic coverings are doing a better job; because light through plastic is always a bit more diffused, plant stress will be delayed. This knowledge brought some people to the idea to look into diffused glass and some results were really eye-opening; production increases of 5-9 % were realised although the total light level decreased by 4-6 % and this was a factor in producing under low light levels in the winter.

Newly developed production techniques are introducing new and very interesting options. The company that produces the 91% glass, GlasCompany BV in Holland, took the development of the CrystalClear glass a step further and treated that glass on both sides with an anti-reflection material that made the average light transmission closer to 95%. The anti-reflection coating will ensure better light transmission under all angles of light entrance; past measurements were always expressed under a straight angle, a situation very seldom realised. With the growing interest in diffused glass they ran some tests with CrystalClear, putting a treatment on the outside that diffused the glass to the chosen level of diffusion. This nano-technology only roughs up the glass very slightly and has very little or no effect on the glass getting dirty. Initial test show a light transmission of 88-90 %. The expectation is that production will increase significantly despite the lower light transmission levels; diffusing the light lowers stress levels, delays the need for screening and may eliminate the need for white washing for a big part of the summer season. During the winter period, because of the treatments, it can increase the penetration of the light to the plants and increase the light efficiency by the plants.

It is difficult to make statements about the level of production increase without having tested this material, but expectations are high, varying from 10% as a moderate level to more than 20% from some outspoken people. In any case, this development can turn the industry upside down. The extra cost seems to be significant but if this becomes mainstream, can come down significantly. Payback times between 3 and 5 years are reasonable to work with and could make this a worthwhile investment.

In plastics we see similar developments and the choice will be driven by the need of your crop.
When building a new greenhouse, all these developments will make your decision on what to build and with what specification a difficult one. Developments are rapid at this time and many new products are entering the market, proven or not. With the economics of this time you cannot afford to make any costly mistakes. Take the time to develop your plans. Research the right combinations for you, as covering the greenhouse is only one facet of the new developments; energy efficiency and savings with closed and semi-closed greenhouses are also a big factor. Have outside advice to help you consider at your options. The relatively small cost of advice can improve your chances of succeeding.

Cees VandenEnden

HortiSource Consulting Inc.

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