Structures & Equipment
Inside View: March 2011
By Gary Jones
By Gary Jones
When it comes to greenhouses, there are primarily two main types:
glass-covered structures, and polyethylene or rigid plastic styles.
When it comes to greenhouses, there are primarily two main types: glass-covered structures, and polyethylene or rigid plastic styles. For glasshouses, the Venlo-style and the wide-span type are the primary choices. Location/industry history and crop type/cultivation systems largely determine which is favoured by individual growers.
For example, growers of protected cut flowers and long-season vegetables usually prefer Venlo greenhouses, particularly in Canada, while those producing young plants or potted plants frequently choose the wide-span greenhouses, especially in California and southern Europe. When it comes to plastic-covered houses, growers in Canada typically opt for double-poly covered houses, to save fuel in our cold winter climate, while growers in many European countries prefer single-poly covers. Of course, these are broad generalizations, not “must-dos.”
BIGGER PICTURE DESIGN CHANGES
■ Apart from safety, considerations, greenhouse designers are driven by two major factors: heat retention and light transmission. Over the last decade or so, a number of innovations in glasshouse design have been evolving:
- Structures have become taller – Gutter heights have grown from 4.0 metres to as much as 6 m to 6.5 m standard. (Even taller glasshouses are also available for special purposes such as accommodating an extra layer of hanging baskets and/or carousel watering system.) Poly-covered houses are even taller.
- Vents are getting larger to allow more light transmission. Some designs have now done away with the frame around the vent glass, and vent arms are connected directly to the glass itself.
- Spans are getting wider. Standard double-span Venlos, for example, have gone from 6.4 m about 15 years ago to a typical 8 m double-span now. This allows more space for growing and working, with fewer gutter posts.
- Section lengths are getting longer. Standard 4 m section lengths of 10 years ago have now made way for 4.5 m and 5 m sections.
These developments are primarily driven by the “Holy Grail” law for greenhouse design that one per cent more light leads to one per cent more crop yield. Reducing the amount of (opaque) structural components such as glazing bars, gutters and purlins is clearly beneficial, but the ability to achieve a more uniform environment within the greenhouse is also a critical factor for the growers who use these structures. Ask any grower who has small glasshouses how hard it is to control temperature fluctuations in the summer . . .
BEAUTY IS IN THE DETAIL
■ These innovations are all well and good, and bring significant benefits. But they also bring some challenges. Like everything else, rarely is there no cost for moving forward! For example, lower profile roofs, wider span widths and longer section lengths require wider and longer panes of glass. In turn, these require more specialized handling when installing and cost more to replace when they break. Prins Greenhouses has developed an emergency repair kit for the grower in the event of breakage of large panes of glass. Similarly, for maintenance purposes, Prins has also introduced a specially adapted bike for use in the ultra-narrow profile gutters that high light transmission greenhouses now incorporate. These ultra-narrow gutters came on the market about three years ago, and already there is a considerable acreage built.
Following the “less-is-more” theme, have you ever bashed your head on those large, sharp-cornered metal box aspirated screens that we use to measure greenhouse temperature and calculate humidity? Oh, it’s just me?
Well, if you have but are just too embarrassed to admit it here, you can now thank Argus Controls for bringing out its new “Omni Sensor.” These small, lightweight aspirated boxes designed for the Titan® greenhouse control system provide measurement of greenhouse temperature, humidity, light levels and optional CO2. The boxes are easily plugged in, and Argus has added a convenient onboard display, allowing the user to see the current values for all readings and to enter calibration adjustments for the CO2 sensor while in the greenhouse. (Just as important, they are lightweight and hurt much less!)
SETTING NEW STANDARDS
■ No matter how good the individual components of a greenhouse, be it glass or poly-covered, the structure is only as good as the way it is actually built. Construction standards are essential. To this end, a new greenhouse construction standard is being finalized in Europe and will be binding throughout all member states. Given the global nature of our industry, likely this will also be used here.
Gary Jones is chair of Production Horticulture at Kwantlen
University, Langley, B.C. He serves on several industry committees and
would welcome comments at Gary.Jones@Kwantlen.ca.