Protected production systems are used to modify the crop’s microenvironment and extend the growing period early in the spring or late in the fall (Arancibia, 2018; Lamont, 2005). In addition, protected systems enhance vegetative growth and increase productivity, which may improve the sustainability of vegetable production operations. A wide variety of structures such as hotbeds, glass cloches, coldframe, low and high tunnels, and various types of greenhouses have been used as protected systems to extend the growing season (Lamont, 2005). Although farmers use protected cultivation systems for warm season vegetables, LTs can also benefit cool season vegetable crops by increasing early vegetative growth, reducing ET and possibly irrigation.
Low tunnels effectively extend the growing season in vegetable production (Arancibia, 2018; Lamont, 2005). Among the different types of covers available to use with LTs, spunbonded rowcovers of various thicknesses are most popular. They are semitransparent porous fabrics that allow airflow and ventilation, hence helping avoid condensation that may damage the foliage in contact with water (Arancibia, 2018). Low tunnels covered with spunbonded fabric increase vegetative growth and yield by increasing soil and air temperature (Arancibia, 2018; Arancibia and Motsenbocker, 2008; Gerber et al., 1988; Ibarra et al., 2001; Jolliffe and Gaye, 1995; Nair and Ngouajio, 2010). In addition, LTs are movable, allowing for crop rotation with cover crops in sustainable production systems.
Many vegetable species are shallow rooted and are sensitive to mild water stress (Feigin et al., 1982; Sammis, 1980). Therefore, irrigation is important in vegetable crops to maintain adequate soil moisture for continuous growth and development. However, more than 90% of the water used by plants is lost through transpiration (Morison et al., 2008). In most agricultural systems, poor WUE occurs when soil evaporation is high as compared with plant transpiration in the same field (Gallardo et al., 1996). Water lost through evaporation and transpiration is known as ET, which depends on environmental conditions and plant stage (size). Increased ET results in increased crop water needs. Factors influencing ET are SR, crop growth stage, daylength, air temperature, relative humidity (RH), and wind speed (Allen et al., 1998; Jensen and Allen, 2016; Zotarelli et al., 2010). Therefore, fully grown plants demand larger amounts of water, especially in warm, sunny, and windy days (Abdrabbo et al., 2010). Under LT, however, rowcover reduces direct sunlight and blocks wind, which reduces ET even at higher temperatures (Arancibia, 2009, 2012). Therefore, reducing ET in crops grown under LTs may reduce irrigation requirements and improve WUE.
The use of LT can be beneficial to extend the harvest season of brussels sprouts (Brassica oleracea L. Group Gemmifera). Brussels sprout is a cool season, frost-tolerant vegetable crop from the family Brassicaceae. It is an important source of dietary fiber, vitamins (A, C, and K), calcium (Ca), iron (Fe), manganese (Mn), and antioxidants (U.S. Department of Agriculture, 2018). In 2017, the United States imported fresh and frozen brussels sprouts valued at $56 million but exported only $16 million of similar sprout products (U.S. Department of Agriculture, 2017). Therefore, the United States is under-producing brussels sprouts. The main brussels sprouts production season is fall, but spring production is also possible, and extending the harvest season by growing under LTs may help increase local production for direct sale markets.
The hypothesis for this study was that LTs create a more favorable environment in both spring and late summer-fall that would reduce ET and irrigation while increasing vegetative growth and yield. Therefore, the objectives were a) to determine the differences in microenvironmental conditions between LT and open field, and their association with irrigation requirement and b) to determine differences in vegetative growth, production, and WUE in brussels sprouts grown under LT and open field.
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