Retractable roofs first came on the production greenhouse scene in 1991. In these past 16 years what have we learned about their role in cooling, ventilating and shading?
The concept of opening the greenhouse’s roof to get cooler air to plants is a logical one that most growers are catching on to, according to Richard Vollebregt, President of Cravo Equipment Ltd., Brantford, Ontario, Canada. Long growing seasons, varied needs of plants and changing weather conditions make retractable roof greenhouses and their versatility an asset.
This January, a freeze surprised growers on the West Coast, where the promise of mild temperatures lulled growers into leaving crops unprotected against the elements.
“The plants that were in the greenhouse were okay,” Vollebregt says. “What happened to all the plants that were outside? They died.”
The balance between healthy, strong plants that are grown outside and protected plants in the greenhouse that need to get outside is one growers need to deal with.
“There is a renewed interest in protecting outdoor crops, but doing it in such a way that you don’t end up with soft plants or with the problems associated with being in a closed greenhouse again,” Vollebregt says. “If you just put conventional greenhouses up to protect the plants from the freeze, you end up with soft plant material.”
Economics Of Plant Needs
Some major bedding plant operations have spent $20 per square foot for open-roof greenhouse space, according to Vollebregt.
“That is okay if you have a market for plant material 12 months of the year,” he says. “But what happens if you only need the extra space for the spring crop for a six-month period? Can you afford to build a $20 house? Not really, because you are not turning enough dollars in six months to pay for a $20 investment.”
The trick is to figure out how growers can build houses that give a good crop quality for the springtime, but aren’t so expensive that growers must grow plants 12 months a year.
“When you go to a nursery, you will see 50 to 60 percent of its total growing area has greenhouses on it, and yet in 30 to 40 percent of the area, they are just growing plants outside with no protection,” Vollebregt says. “That is a very high investment on the plants in the greenhouse. They have a big, expensive greenhouse over top and they have no protection on the plants outside. So they have a lot of money in plants sitting on the ground outside that are totally vulnerable to the freezes, rains and all those kinds of conditions.”
That’s not to say there isn’t a place for open roof glass houses, Richard clarifies.
“There absolutely is,” he says. “The lower cost retractable A-frame is a perfect complement to the higher cost A-frame house. So you have to determine your business needs. Does it need more growing space for 12 months a year, or are you looking to expand your seasonal production? Automatically, one of the houses will stand out as what you need.”
Retractable roofs have been around for about 16 years and roof coverings have evolved in that timeframe.
“There are some now in use that are lasting 10 years,” Vollebregt says. “Whereas before, some of them were in the 5- to 6-year range.”
With all the trapped heat and energy in greenhouses, retractable roof greenhouses enhance the cooling effect. A ventilation system is not big enough to exhaust heat as quickly as greenhouses take them in. Retractable roof greenhouses take trapped heat out of the equation.
“You open up the roof and you have all vents and no roof covering, so there is no trapping of heat,” Vollebregt says. Curtain systems also provide shading and cooling in the greenhouse; heat is still allowed to accumulate in the greenhouse. External shade systems stop the light and heat before they reach the greenhouse. In this setup, a ventilation system can keep up with the reduced amount of heat that is in the greenhouse.
“The external shading system is doing the exact same thing as a cloud in the sky does,” Vollebregt says. “It is stopping the sun before it reaches the greenhouse, which means it’s cooler inside. For locations in the South, the external shade system is a good way of cooling existing greenhouses that are just too hot. It doesn’t give you any heat retention, but it does give you cooling and shading, so you have light reduction and cooling out of the same curtain.”