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Siting High Tunnels
Kristin Pool, Oregon State University
Alex Stone, Oregon State University
A high tunnel is not, by definition, a permanent structure. Nonetheless, site selection will play an important role in its performance. Considerations in siting a high tunnel include orientation, airflow, shading, windbreaks, drainage, soil quality, weeds and other pests, and purpose.
The purpose of the high tunnel is one of the first considerations when siting a tunnel. High tunnels require daily maintenance and therefore should be located in a location that is accessible by vehicle during all production seasons. Movable tunnels are typically sited in a production field, so a high tunnel's impact on the growth and management of the crops around the tunnel must be considered. There must be sufficient room around a mobile high tunnel for the equipment and/or people that will be necessary to move it. Make sure to consider how irrigation and power will be delivered to the tunnel, including during winter months if winter production is intended.
The orientation of a high tunnel affects the amount of sunlight the tunnel receives. Orientation (east-west or north-south) depends on your location. As a general rule, manufacturers recommend orienting the high tunnel to capture the most light in winter. For locations north of 40° latitude, the ridge should run east to west. For locations south of 40° latitude, the ridge should run north to south. At any latitude, gutter-connected or closely spaced multiple greenhouses will get more light if they are aligned north-south because they avoid the shadow cast by structures to the south.The purpose of the tunnel and its geographic location will determine the best tunnel orientation for light capture. For more information on sun angles, see the resources under Additional Resources below.
Shade can greatly reduce the effectiveness of a high tunnel since it limits light. Locating high tunnels north of any substantial obstacles is undesirable. High tunnels should be built at a distance at least twice the height of an obstacle away from the obstacle. For instance, if there is a 25 foot tall tree on your property, the high tunnel needs to be located at least 50 feet from that tree. If building multiple high tunnels oriented east-west, the spacing between the tunnels needs to be twice the height of the tunnels. If they are oriented north-south, the spacing can be 4 feet between tunnels.
Drainage is a very important consideration in siting a high tunnel. High tunnels displace rainfall. If this water is not redirected, it will enter the high tunnel. In order to avoid this, more permanent high tunnels should be built at a higher elevation than the surrounding land, on well drained soils, and with drainage aisles. The site should also be level to ensure even irrigation and to facilitate tillage. Movable high tunnels located in the field must be situated and managed to minimize the impact of rainfall on crops and soils both inside and outside the tunnel.
Soil Quality and Pest Management
Soil quality must be considered before building a high tunnel. Well drained soils heat up more quickly and prevent flooding, both important benefits for high tunnels. Lighter textured soils like sandy loams or loamy sands are most desirable because they will warm up more quickly in the spring, are easily worked, and promote good root development.
High tunnel production is intensive. Therefore, the time, work, and expense required to fully prepare soil for production before constructing the tunnel is worth the effort. Cover cropping and organic amendment management can be difficult in high tunnels, as standard equipment might not fit into the high tunnel, and it is difficult to incorporate cover crops and amendments in the areas along the sides of the tunnel. Therefore, it is a good idea to improve the soil before constructing a high tunnel. In addition, when possible, design the tunnel to accommodate soil management practices such as organic amendment, cover cropping, and tillage.
Know the cropping or vegetative history of the site when siting a tunnel. Avoid locating a tunnel where there is soilborne disease inoculum or significant annual or perennial weed problems, or correct those problems before constructing the tunnel.
Airflow is used to regulate temperatures in high tunnels; sites with naturally good airflow will improve high tunnel performance and reduce the need for daily tunnel airflow management. Single bay high tunnels should be oriented perpendicular to the prevailing winds, while multi-bay high tunnels should be oriented parallel to prevailing winds.
Windbreaks can be an important protection strategy for high tunnels. The plastic on high tunnels can rip or tear and the entire structure can be destroyed in high winds. Depending on the conditions, a properly located and designed windbreak could reduce the risk of wind damage and the need to remove the plastic before predicted wind events. Protection from wind will help maintain higher tunnel temperatures in colder periods. However, wind helps ventilate high tunnels and maintain cooler tunnel temperatures during hot periods.
The effectiveness of a windbreak is determined by its height, density, orientation and length. When constructing a windbreak growers should consider which prevailing winds are most problematic, such as strong winter winds accompanied by snow or hail. Take into consideration other potential impacts of a windbreak, for example, the creation of a barrier to crop pollinators.
The types of plants used to construct windbreaks will vary by farm and region. For example, since deciduous trees lose their leaves in the fall they generate less shade in the fall and winter when light is scarce. However, they also offer less wind protection in the fall and winter. Growers can increase the value of a windbreak by using plants that bear harvestable fruit or flowers, or timber. See Additional Resources below for windbreak resource materials.
References and Citations
Bachmann, J. 2005. Season extension for market gardeners: horticultural techniques [Online]. ATTRA Publication #IP035. Available at: http://attra.ncat.org/attra-pub/seasonext.html (verified 4 March 2010).Spaw, M. and K. Williams. Undated. Site Selection [Online]. www.hightunnels.org. Available at: http://www.hightunnels.org/ForEducators/Planning/SiteSelection.htm (verified 4 March 2010).Wildung, D. and P. Johnson. 2004. Minnesota high tunnel production manual for commercial growers [Online]. University of Minnesota Publication M1218. Available at: http://www.extension.umn.edu/distribution/horticulture/components/M1218-... (verified 4 March 2010).
The benefits of windbreaks for Florida growers [Online]. M.G. Andreu, B. Tamang, M. H. Friedman, and D. Rockwood. 2008. University of Florida Extension. Publication #FOR192. Available at: http://edis.ifas.ufl.edu/FR253 (verified 4 March 2010).How windbreaks work [Online]. J.R. Brandle, H. Zhao, and L. Hodges. Undated. University of Nebraska Extension Publication EC1763. Available at: http://www.ianrpubs.unl.edu/epublic/live/ec1763/build/ec1763.pdf (verified 4 March 2010).Basic windbreak design criteria for farm and ranch headquarters areas and large residential lots [Online]. Q. Clifton, B. Wight, and D. Hintz. 1986. USDA Soil Conservation Service. Forestry Bulletin No. 18. 190-LI-6. Available at: ftp://ftp-fc.sc.egov.usda.gov/OR/Technical_Notes/Forestry/Forestry19.pdf (verified 4 March 2010).Hedgerows for California agriculture [Online]. S. Earnshaw. 2004. Community Alliance with Family Farmers (CAFF). Available at: http://www.caff.org/programs/farmscaping/Hedgerow.pdf (verified 4 March 2010).SunPath. A sun chart calculator (copyrighted freeware) [Online]. D. Hennings. Undated. Available at: http://www.eclim.de/index5.htm (verified 4 March 2010). A small program calculating and displaying the sun's path for every month and for any latitude on earth. NRCS windbreaks resources [Online]. Undated. Available at:http://plant-materials.nrcs.usda.gov/technical/publications/windbreak-pubs.html (verified 4 March 2010).Earth/sun tool [Online]. Pearson Higher Education. Undated. Available at: http://cwx.prenhall.com/bookbind/pubbooks/lutgens3/medialib/earthsun/ear... (verified 4 March 2010). Use the arrows at the top to move the Earth around the Sun — incrementally or in an animation. Note that the colored lines on the globe correspond to the latitudes of the two sun angle pictures below. You can use the arrows to change the latitudes that you are viewing. Earth-sun relationships and insolation [Online]. M. Pidwirny. 2006. Fundamentals of physical geography. 2nd Ed. Available at: http://www.physicalgeography.net/fundamentals/6i.html (verified 4 March 2010).Farmstead windbreaks: planning [Online]. P. Wray, L. Sternweis, and J. Lenahan. 1997. Iowa State University Extension PM-1716. Available at: http://www.extension.iastate.edu/Publications/PM1716.pdf (verified 4 March 2010).
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