The best of both worlds with retractable greenhouses

Fruit and vegetable growers typically fall into one of two “worlds,” says Richard Vollebregt, president of Cravo Equipment Ltd. “There’s the indoor world and the outdoor world,” he explains. “We’re trying to create a third alternative—the best of both worlds.”

Cravo (www.cravo.com) is a 30-year-old company that began producing retractable greenhouses in 1984. Originally, the focus was on serving the needs of flower growers, but recent studies have shown the potential benefits of retractable greenhouse technology for fruit and vegetable growers.

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

Generally, environmental problems are easier to understand but more difficult to control, observes Vollebregt. By using retractable greenhouse technology, a measured plan can be put in place to address environmental challenges. For example, with the flip of a switch, a grower can close the greenhouse roofs anytime the temperature falls below 65 degrees Fahrenheit. “We’re trying to show that if you take the best of the natural, open-field environment and the best of the closed greenhouse environment and you put them together, what you have is plants that are grown outside when the weather is perfect for them, and protected when the outdoor environment is not perfect,” he explains.

A Cravo retractable greenhouse roof opens above a field-grown crop of tomatoes, allowing the plants to take advantage of ideal growing weather and develop natural defenses to insect and disease pressure.
 
During the heat of the day, the roof system can be closed to prevent sunburn 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 using a single 2 hp motor.

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

By reducing the stressors on plants, Cravo claims its retractable greenhouses have the ability not only to increase yields, but also to improve the size, quality and taste of products grown, and to boost the 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 got cold and the plants started slowing down, then it got to 95 degrees and the plants started slowing down again,” explains Vollebregt. “So really, the plants had the capacity of producing 100 tons, but it was the negative extremes that limited the yield.”

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

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

The study in Mexico found that insects, however, didn’t like the plants that were being grown in the retractable greenhouses. “When you grow outside, your insect pressures are always highest during the extreme heat. The higher temperatures cause insects to reproduce faster, while at the same time stressing the plant and making it more susceptible to insects,” says Vollebregt. “Plants are better hosts when they’re not strong.”

Plants in the retractable greenhouse get a chance to develop their natural defense mechanisms in outdoor growing conditions. For example, building up a thick waxy layer on leaves in response to UV rays, wind, etc., which helps to protect against water loss in response to. This can happen with the roof open for even a few hours a day during ideal growing conditions, he adds, but then, during the heat of the day, the roof can be closed to prevent excessive water loss. Keeping the plant healthy prevents it from exhibiting signs of weakness, such as droopy leaves, that will attract insects seeking an easy target. At the same time, the retractable roof can keep soil temperatures from getting so hot that plant growth is hampered.

The systems are particularly valuable to growers in moderate climates, allowing them to take advantage of good growing conditions while also helping to manage—or even eliminate—the extremes.

The retractable greenhouse system can help growers in colder climates, as well. The goal for many of these growers, according to Vollebregt, isn’t to become year-round greenhouse growers, but just to stretch the growing season out in order to be in the market earlier and later than most local produce, when the prices are better. “What generally limits their time in the market is the fact that they can’t plant until a certain date after frost danger, and they’re out of the market when it becomes too cold again. Rain is sometimes a factor, but temperature is generally the biggest determinate of the harvest window. But, if you can close a roof over the field, you might be able to plant four months earlier without risk of frost damage and the plant will grow faster because warmer temperatures are being held in. And, the roof can be closed at night as fall approaches, allowing the harvest to be stretched out longer.”

Cravo’s retractable greenhouses (which can include retractable walls) are available in a variety of designs and materials, depending on the grower’s geographic location and crops. Different shades and types of polyethylene and shade cloth materials can be specified and these are protected against UV damage, with an estimated life expectancy of five to eight years for flat panel systems. Aluminum frames and stainless steel drive cables support the roof mechanism. The lower-cost solution is a flat-roof, which protects against cold and heat, but not rain. A more expensive “A-frame” option offers protection against rain. “Most growers want to cover big areas as economically as they can. You can close 2 acres of roof in seven minutes with one 2 hp motor,” says Vollebregt. This means that growers can quickly react to changing weather conditions.

The retractable roof is actually a series of panels, each covering a certain area. A 30-foot roof panel, for example, retracts to just 2 feet. The system allows enough space between rows and height (up to 20 feet) to allow for tractor access in the fields.

What is the reaction of growers when they see or hear about retractable greenhouses? “To those who come from the outdoor farming world, the concept of closing a roof when outside conditions are adverse is very logical, but what they want to understand is the data on the yield. Conceptually it makes sense, but they want to know if it’s going to help them make money,” says Vollebregt. “People who are already growing in closed greenhouses really struggle with the concept of opening a roof, because they feel like they’re losing control. And they’re fearful, because they’re growing plants that haven’t developed their natural defenses.”

Cravo has developed formulas, presented in tables on the company’s Web site, reviewing cost-benefit analyses. The cost of the retractable roof system is balanced against the ability to increase yields and lengthen the growing season, while at the same time decreasing the amount of acreage planted, thereby lowering costs for water and chemicals. “It’s not for every crop,” admits Vollebregt. “And, it’s not that every field should have a roof over it, but for certain crops, in certain conditions, it makes a lot of sense.”

Patrick White is a freelance writer and editor who is always on the lookout for interesting and unusual stories. He can be reached at pwhitevt@aol.com.