Question of the Week
Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on using high tunnels for vegetable production in warm season climates.
High tunnels are used in southern climates to protect crops from pests, extreme solar radiation, and inclement weather. These high tunnels, which are commonly used throughout the Middle East, South America, and the southern U.S., differ in their primary use for season extension. This is opposed to the use of high tunnels in temperate regions where high tunnels provide warmer growing environments so that the growing season can be extended earlier in the spring, later in the fall, and possibly throughout the winter.
Due to the difference in use from temperate climates, high tunnels in more tropical weather conditions require somewhat different designs and construction. As with any high tunnel, the underlying factor in the design is ventilation. For a passively ventilated roof, it is suggested that the area of the roof vent be 20% of the floor area and located on the leeward side of the high tunnel. In addition, ventilation can be achieved through roll-up or roll-down sides. In fact, in many tropical areas, insect netting is used for the sides of the structure which allows for better ventilation.
One option to consider for reducing the amount of light and heat in a high tunnel is to use shade cloth rather than a plastic film. Shade cloth is a strong polyethylene fabric that can be purchased in different densities, usually between 30% to 80%. Shade cloth provides ventilation, improves light diffusion, and reflects heat, all of which help keep the high tunnel cool. A 30% density shade cloth can reduce the air temperature by 4 degrees Fahrenheit. The reduction in light intensity is beneficial to crops such as lettuce and greens. A further reduction in temperature can be achieved with the shade cloth by using sprinkler irrigation or mist.
Several growers have been able to reduce temperatures even more than with a shade cloth by using an aluminum reflective shade. These fabrics contain an anti-oxidation coating that cools the high tunnel. Some growers claim they can achieve a temperature reduction of 15 degrees Fahrenheit by using an aluminum-based shade cloth.
The web site high tunnels.org (www.hightunnels.org) is a great resource for high tunnel growers. It provides information on growing in a high tunnel, resources and suppliers, as well as a list serve. You may be able to find information on using a high tunnel in your region and can also post any questions you may have to the group. In addition, the resource section contains several suppliers of shade cloth and aluminum fabrics, such as FarmTek (http://www.farmtek.com). One supplier you may want to investigate is Haygrove (http://www.haygrove.co.uk/). These unique three season high tunnels are becoming very popular amongst fruit and vegetable growers and may be of interest to you. Although based out of the UK, Haygrove does have a dealer and warehouse located in Pennsylvania (http://www.tunnelbuzz.com). Finally, I’d like to pass along the link to greenhouse consulting company that specializes in tropical vegetable production: http://cuestaroble.com/tropicalgreenhouse.aspx. This site contains information on constructing and growing in high tunnels located in tropical areas.
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