In 2010, U.S. blueberry (Vaccinium sp.) production occurred on nearly 68,400 acres (Joshua, 2010). In Florida, ≈3500 acres were planted with a crop value of near $50 million in 2010 (Florida Department of Agriculture and Consumer Services, 2011). Southern highbush blueberry farms are disseminated throughout Florida from the northern border with Georgia (lat. 30°33′N) to the southwest region (lat. 26°38′N), which comprises most of the cool temperature areas in late fall, winter, and early spring. Blueberries are transplanted in open fields either into plastic-covered or bare soil hills, which are irrigated with sprinklers, mini-sprinklers, or drip lines. The soils in the majority of the southern highbush blueberry growing areas are sandy with rapid drainage and slightly alkaline pH. Planting hills are formed with pine bark or with a mix of pine bark rototilled into the sandy soil. These practices are aimed to lower soil pH because of the pine bark acidity (pH < 6) and to promote rapid drainage during the rainy summer months. Planting densities vary depending on the cultivars but the most common are between 1600 and 2800 plants/acre (Lyrene and Williamson, 1997). Flowering of southern highbush blueberry occurs from January to March and most of the planted cultivars in Florida have chilling requirements between 200 and 400 h per winter with temperatures ≤45 °F. Those required chilling hours may be obtained depending on the location of the fields in the state. Fruit are harvested from early April to mid-May and the highest market prices occur before 15 May, dropping from ≈$7.00/lb in March to around $3.00/lb in May (Lyrene and Williamson, 2000; Williamson et al., 2012). Therefore, practices that improve earliness may allow growers to access higher prices in the market.
Overhead irrigation (4 to 5 gal/min per sprinkler head) is used as the preferred freeze protection method. To obtain adequate freeze protection with sprinklers, the correct amount of water needs to be applied (Locascio et al., 1967). The principle associated with this practice is called “heat of fusion” and it is described as the heat released by water during the freezing process, where 1 g of water releases 80 calories of heat as it forms ice (Perry, 2001; Snyder, 2001). Because of the “evaporative cooling,” another property of water, as 1 g of water evaporates, 540 calories of heat are absorbed from the surrounding environment, thus when compared with the 80 calories released by freezing, nearly seven times more water must freeze than evaporate to provide a net heating effect (Perry, 2001). A main disadvantage of this freeze protection method is that during prolonged freezing periods, it could quickly deplete underground water sources (e.g., aquifers), which might be shared with urban settlements around the southern highbush blueberry operations. A single night of freeze protection (8 to 12 h of irrigation) with high volume sprinklers might use between 2 to 3 acre-inch/acre of water, which might translate in very large water volumes to protect the crop (Tyson et al., 2011). Consequently, technologies aimed to save water would be desirable to reduce pressure on underground water sources, nutrient leaching potential, and pump fuel and electricity costs for growers.
The use of protective structures, such as high tunnels, has been proposed as an alternative for southern highbush blueberry freeze protection and to increase fruit earliness. High tunnels are generally unheated, plastic-covered structures with passive ventilation (Lamont et al., 2002). Potential benefits of high tunnels for vegetable and small fruit production include freeze protection, high early yields, protection against some diseases and rain, and efficient fertilizer and water use (Lamont, 2005; Ogden and van Iersel, 2009; Strik, 2012). High tunnels may increase air and soil temperatures in areas of cold weather (Kadir et al., 2006; Reiss et al., 2004). Previous research indicated that potential benefits of high tunnels for production of fruit crops, such as raspberry (Rubus idaeus), strawberry (Fragaria ×ananassa), and cherry (Prunus avium). In raspberry, high tunnel production has enabled growers to extend the season during cold months and to access off-season markets during high price windows (Demchak, 2009; Gaskell, 2004). Lang (2009) examined the performance of cherry under high tunnels and found that larger sugar concentrations and fruit size were obtained when covering trees for a few weeks during fruit setting and harvesting in comparison with trees growing in the open field. Jett (2007) suggested that plants under high tunnels increased marketable fruit by 95% in contrast with only 60% in the open fields, which is mainly due to protection against rain and wind (Demchak, 2009). Pattison and Wolf (2011) demonstrated that strawberry earliness could be advanced up to 4 weeks and duplicated yields under protected culture. Kadir et al. (2006) and Salame-Donoso et al. (2010) proved that strawberry leaf area and number, shoot biomass, and soluble solid content improved under high tunnels. In southern highbush blueberry, previous studies conducted in Italy, Portugal, and Japan indicated that production could be from 1 week to 1 month earlier under high tunnels compared with open-field production (Baptista et al., 2006; Ciordia et al., 2002; Ozeki and Tamada, 2006). However, there is no information on the effect of this type of structure on southern highbush blueberry fruit earliness under the subtropical Florida conditions. The objective of this study was to compare the early fruit weight of southern highbush blueberry cultivars in high tunnels and in open fields in Florida.
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