Fruit and vegetable growers typically fall into one of two“worlds,” says Richard Vollebregt, president of CravoEquipment Ltd. “There’s the indoor world and the out-door world,” he explains. “We’re trying to create a thirdalternative—the best of both worlds.”

Cravo (www.cravo.com) is a 30-year-old company that began pro-ducing retractable greenhouses in 1984. Originally, the focus was onserving the needs of flower growers, but recent studies have shown thepotential benefits of retractable greenhouse technology for fruit andvegetable growers.

“When you grow outside you have problems with excessive coldand heat and wind and rain—all these environmental problems,”explains Vollebregt. “But, when you go into a closed greenhouse, youhave problems with fungus or mold, plant abortion, nutritionaldeficiencies—physiological problems. So you don’t get rid of yourproblems in a closed greenhouse, you just change them.”

Generally, environmental problems are easier to understand butmore difficult to control, observes Vollebregt. By using retractablegreenhouse technology, a measured plan can be put in place to addressenvironmental challenges. For example, with the flip of a switch, agrower can close the greenhouse roofs anytime the temperature fallsbelow 65 degrees Fahrenheit. “We’re trying to show that if you take thebest of the natural, open-field environment and the best of the closedgreenhouse environment and you put them together, what you have is

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Continued on page A10

Uncovering New PossibilitiesThe best of both worlds with retractable greenhouses

by Patrick White

Flat-panel, retractable roof systems offer growers the most economi-cal protection against sun, heat, cold and wind, but A-frame systemsthat add rain control are also available.

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plants that are grown outside when the weather is perfect for them, andprotected when the outdoor environment is not perfect,” he explains.

A similar approach has been taken at some modern sports stadiums,where the natural turf—and the spectators—can be shielded fromrain, wind, snow and severe sun heat when the weather is at its mostextreme, but still enjoy the sun and breeze on more pleasant days.

By reducing the stressors on plants, Cravo claims its retractablegreenhouses have the ability not only to increase yields, but also toimprove the size, quality and taste of products grown, and to boostthe insect/disease resistance of the plants.

“Let’s say you’re growing tomatoes and you get 30 tons per acre.And you start thinking, ‘Why didn’t I get 100 tons?’ Well, first it gotcold and the plants started slowing down, then it got to 95 degreesand the plants started slowing down again,” explains Vollebregt. “Soreally, the plants had the capacity of producing 100 tons, but it wasthe negative extremes that limited the yield.”

Four years ago, a study was conducted at the University of Arizonathat demonstrated the benefits of growing tomatoes using theretractable greenhouses.“The yields were far better than anyone expect-ed, and they were comparing them to high-tech closed greenhouses,”says Vollebregt. “Then we did a project in Mexico that compared theretractable greenhouses to open field production of tomatoes. Theresults showed not only increased yield and larger size, but also on aver-age a 90 percent reduction in losses due to fungus and insects. “

The reduced disease and insect pressures were particularly sur-prising to researchers. “When you go into a closed greenhouse andyou do get an insect in there, the reproduction rates are very rapidbecause you’ve got ideal conditions for the insect: a good foodsource, no wind, high humidity, high temperatures. And, the plantsin the greenhouse are softer and more succulent, and being lazy,those are the plants that insects will go to first,” says Vollebregt. “Sothe fear that growers have with retractable greenhouses is that if theyopen the roofs, all the insects will come in, and that’s a logicalthought because that’s what happens with closed greenhouses—when you open the roof vents or sidewalls, you can have majorproblems with insects and things like powdery mildew.”

The study in Mexico found that insects, however, didn’t like theplants that were being grown in the retractable greenhouses. “Whenyou grow outside, your insect pressures are always highest duringthe extreme heat. The higher temperatures cause insects to repro-duce faster, while at the same time stressing the plant and making itmore susceptible to insects,” says Vollebregt. “Plants are better hostswhen they’re not strong.”

Plants in the retractable greenhouse get a chance to develop theirnatural defense mechanisms in outdoor growing conditions. Forexample, building up a thick waxy layer on leaves in response to UVrays, wind, etc., which helps to protect against water loss in responseto. This can happen with the roof open for even a few hours a dayduring ideal growing conditions, he adds, but then, during the heatof the day, the roof can be closed to prevent excessive water loss.Keeping the plant healthy prevents it from exhibiting signs of weak-ness, such as droopy leaves, that will attract insects seeking an easytarget. At the same time, the retractable roof can keep soil tempera-tures from getting so hot that plant growth is hampered.

The systems are particularly valuable to growers in moderate cli-mates, allowing them to take advantage of good growing conditions

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A Cravo retractable greenhouse roof opens above a field-grown cropof tomatoes, allowing the plants to take advantage of ideal growingweather and develop natural defenses to insect and disease pressure.

During the heat of the day, the roof system can be closed to preventsunburn damage and excess water loss, which will weaken the plant,decreasing yield and inviting pest pressure.

Two acres of roof can be closed or opened in just seven minutes usinga single 2 hp motor.

Uncovering New PossibilitiesContinued from page A8

Grow 208 A1-A16 1/18/08 10:55 AM Page A10

while also helping to manage—or eveneliminate—the extremes.

The retractable greenhouse system canhelp growers in colder climates, as well. Thegoal for many of these growers, accordingto Vollebregt, isn’t to become year-roundgreenhouse growers, but just to stretch thegrowing season out in order to be in themarket earlier and later than most localproduce, when the prices are better. “Whatgenerally limits their time in the market isthe fact that they can’t plant until a certaindate after frost danger, and they’re out ofthe market when it becomes too cold again.Rain is sometimes a factor, but temperatureis generally the biggest determinate of theharvest window. But, if you can close a roofover the field, you might be able to plantfour months earlier without risk of frostdamage and the plant will grow fasterbecause warmer temperatures are beingheld in. And, the roof can be closed at nightas fall approaches, allowing the harvest tobe stretched out longer.”

Cravo’s retractable greenhouses (whichcan include retractable walls) are availablein a variety of designs and materials,depending on the grower’s geographic

location and crops. Different shades andtypes of polyethylene and shade clothmaterials can be specified and these areprotected against UV damage, with an esti-mated life expectancy of five to eight yearsfor flat panel systems. Aluminum framesand stainless steel drive cables support theroof mechanism. The lower-cost solution isa flat-roof, which protects against cold andheat, but not rain. A more expensive “A-frame” option offers protection againstrain. “Most growers want to cover big areasas economically as they can. You can close 2acres of roof in seven minutes with one 2hp motor,” says Vollebregt. This means thatgrowers can quickly react to changingweather conditions.

The retractable roof is actually a series ofpanels, each covering a certain area. A 30-foot roof panel, for example, retracts to just2 feet. The system allows enough spacebetween rows and height (up to 20 feet) toallow for tractor access in the fields.

What is the reaction of growers when theysee or hear about retractable greenhouses?“To those who come from the outdoor farm-ing world, the concept of closing a roof whenoutside conditions are adverse is very logical,

but what they want to understand is the dataon the yield. Conceptually it makes sense,but they want to know if it’s going to helpthem make money,” says Vollebregt. “Peoplewho are already growing in closed green-houses really struggle with the concept ofopening a roof, because they feel like they’relosing control. And they’re fearful, becausethey’re growing plants that haven’t devel-oped their natural defenses.”

Cravo has developed formulas, presentedin tables on the company’s Web site, review-ing cost-benefit analyses. The cost of theretractable roof system is balanced againstthe ability to increase yields and lengthenthe growing season, while at the same timedecreasing the amount of acreage planted,thereby lowering costs for water and chem-icals. “It’s not for every crop,” admitsVollebregt. “And, it’s not that every fieldshould have a roof over it, but for certaincrops, in certain conditions, it makes a lotof sense.”

Patrick White is a freelance writer and editorwho is always on the lookout for interesting andunusual stories. He can be reached atpwhitevt@aol.com.

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