Retractable roofs for increased production and profitability

Greenhouse crop farmers are increasingly making use of retractable roof technology to create optimal growing conditions, and to reduce exposure to extreme climates.

Retractable roofs help growers create otpimal growing conditions inside greenhouses by either reducing, or increasing, the amount of heat to which crops are exposed.
Photo: Courtesy of Cravo Equipment

Retractable roofs protect crops against excessive heat, cold, frost, hail, wind, diseases and pests. The roof creates an environment with a constant temperature and humidity level, making it possible for a farmer to extend the growing season, and increase profitability/ha.

According to Richard Vollebregt, president of Cravo Equipment, a Canadian company that manufacturers retractable greenhouse roofs and production systems, retractable roof technology has been adopted in the US, Mexico, Chile, Nigeria and South Africa.

“Annually, South Africa experiences 5 658 hours (65%) of ideal outdoor growing conditions,” Vollebregt said. This means that farmers lose out on 3 012 (35%) potential growing hours each year, which could be used to increase production.

Benefits of the retractable roof system
Cravo conducted numerous trials on blueberries in Culiacán, Mexico, to test the performance of retractable roofs in mild-to-hot climates. As Culiacán is located in the same climatic zone as Polokwane, the results of this study are useful to South Africa.

  • Reduces heat

In an open field, the leaf and soil temperatures of the blueberries measured 45°C and 60°C respectively, stressing the crop and reducing yield, Vollebregt said. However, the same variety, planted on the same day, in a greenhouse, under a retractable roof, registered leaf and soil temperatures of 30°C and 35°C respectively. Active new growth and flowering, as well as a significant increase in developed fruit, were observed. The diffused sunlight and reduced transpiration also lowered crop stress, thereby increasing yield.

  • Offers direct sunlight

As a retractable roof system’s sides are closed, more light enters the structure from the top, thereby enhancing photosynthesis, and increasing leaf temperature, plant density and fruit growth. Vollebregt said that direct sunlight means that relative humidity is less of a problem, as leaves dry off, reducing the plant disease burden. Thus, spraying programmes are less frequent, resulting in lowered costs and reduced chemical residues.

Trials conducted on tomatoes cultivated under a retractable roof structure showed a lower incidence of viruses, as these tomatoes developed a thicker leaf cuticle, which acted as a natural defence
against pests and diseases.

New way of farming
David Windsberg, from the US, installed his first retractable roof 15 years ago, becoming the first farmer in the world to buy a retractable roof for food crop cultivation. Before this, his crops were planted on open lands and in conventional greenhouses.

He experienced the many benefits associated with retractable roof technology, including an extended growing season, and the ability to expose his crops to more heat when needed. It was also easier to control salt build-up.

“With the roof retracted, I can get 101mm of rain inside, which removes excessive salt build-up in the soil,” he explained. He added that soil-borne problems and foliar diseases are manageable, and that insects are not a significant cause of crop losses, even if the roof retracts. He says the only challenge he experienced with adopting this technology was working out “a new way of farming.”

For more information visit www.cravo.com