High tunnel use throughout the United States is on the increase as a result of earlier crop production and the opportunity to exploit local markets with regionally grown produce (Carey et al., 2009; Knewtson et al., 2010). In regions with short growing seasons, extending the production period can have significant economic benefit (Rader and Karlsson, 2006; Rowley et al., 2010; Waterer, 2003). Tomatoes (Solanum lycopersicum L.) are the most commonly grown high-tunnel crop because early, local, high-quality tomatoes have a high value in the market (Knewtson et al., 2010; Ward et al., 2011). Tomato is a common high-tunnel vegetable crop in Utah, and growers regularly ask if tunnels provide enough early protection when day–night temperatures vary significantly and, if so, when to plant. Producing tomatoes before outdoor field production begins extends revenue into a normally unproductive period and allows growers to benefit from out-of-season price premiums (Ward et al., 2011).
For climates near zone 5 on the USDA hardiness scale, growing in a high tunnel makes it possible to plant and produce tomatoes more than 1 month earlier than outdoors (Wells and Loy, 1993). By protecting plants from rainfall, high tunnels facilitate uniform watering, which is important for preventing disorders such as fruit cracking, blossom end rot, and many foliar diseases. High tunnels also provide more optimal temperatures for growth and improve quality by preventing disorders associated with poor pollination such as cat-facing and puffiness (Dorais et al., 2001). Krizek et al. (2006) showed that high tunnel-grown tomatoes had a higher sensory score for sweetness, flavor, texture, taste, and overall eating quality when compared with field-grown fruit.
The benefits of high-tunnel production have been explored in other states including Missouri, Connecticut, and New Jersey (Chism, 2002; Gent, 1992; Reiss et al., 2004), but research in arid high-elevation regions is lacking. In arid high-elevation climates like Utah, daytime temperatures spring can be quite warm (10 to 20 °C) even when nighttime temperatures are below freezing (Moller and Gillies, 2008). These extreme diurnal temperature variations can limit the length of the growing season for outdoor-grown vegetables. However, warm temperatures during the day and sunny skies could be used advantageously when high tunnels are incorporated into a farm’s production system. Utah gets more solar radiation on average than other parts of the United States that conduct high-tunnel tomato research. On average, the western United States receives 18–21.6 MJ·m−2·d−1 of solar radiation in April and 21.6–25.2 MJ·m−2·d−1 in May, whereas during the same period, regions of the central and eastern United States receive 14.4–18 and 18–21.6 MJ·m−2·d−1, respectively (Wilcox and Marion, 1994). Increased solar radiation should result in more accumulated heat units in a high tunnel compared with climates with more cloudy conditions. Arid climates also have the advantage of low relative humidity that should limit the development of problematic diseases common to more humid regions (Lamont, 2005; Wells and Loy, 1993).
In most of the Great Basin region (USDA hardiness zones 5a to 6b), tomatoes are typically transplanted outdoors in mid- to late May and begin to ripen in early August, whereas high-tunnel tomatoes can be planted 5 to 7 weeks earlier. Choosing an appropriate planting date is an important decision for growers in these climates. A 2-week delay in planting was shown to delay ripening by 2 weeks for early high-tunnel tomatoes transplanted in Connecticut (Gent, 1992). However, early-season cold weather makes planting risky during late March and early April even when grown in high tunnels. Row covers and low tunnels have been shown to provide some frost protection for early field-planted tomatoes (Ankara, 2001; Emmert, 1956; Waggoner, 1958) and could be used in combination with high tunnels.
Typical outdoor early spring temperatures (March and April) in northern Utah range from 10 to 20 °C during the day and –5 to 5 °C at night (Moller and Gillies, 2008). Day temperatures inside a high tunnel normally reach 20 to 30 °C, which is considered to be optimal for tomato (Kinet and Peet, 1997). However, nighttime temperatures in a high tunnel are typically only 1 to 4 °C warmer than the outside temperature, and consequently the high-tunnel environment is still susceptible to chilling or freezing conditions at night (Wien, 2009). Tomato plants are susceptible to chilling injury when exposed to temperatures below 10 °C (Kinet and Peet, 1997) and grow best when temperatures are 25 to 30 °C during the day and 16 to 20 °C at night (Csizinszky, 2005). Tomatoes are known to compensate growth when day or night temperatures are below optimum provided temperatures are optimal during the opposite period (Calvert, 1964).
The addition of rowcovers can increase temperatures 2 to 6 °C, and polyethylene rowcovers alone were shown to protect tomato plants from freezing when outside temperatures were –3.8 °C (Emmert, 1956; Waggoner, 1958). Low tunnels have also been shown to promote earlier tomato harvest and increase total yield within an unheated glasshouse, presumably by improving temperature conditions (Ankara, 2001). Waterer (2003) showed that heat unit accumulation, as measured by growing degree-days, was similar for low and high tunnels and both provided adequate plant protection when crops were planted in late May. However, this was after the risk of significant frost was passed. In New Jersey, polyester energy curtains placed inside a high tunnel were found to increase average night air temperatures 2.3 °C compared with the open field. In tunnels without curtains, temperatures were only 0.9 °C warmer than in the field (Both et al., 2007). In the arid, high-elevation regions of Utah, temperatures below –7 °C often occur from mid-March to late April when growers would be transplanting tomatoes into high tunnels (Moller and Gillies, 2008). These low temperatures may seriously limit early high-tunnel tomato production even when additional rowcovers are used. Therefore, targeted supplemental heat may aid in additional cold temperature protection during the early season.
Many high-tunnel production manuals advocate using protective structures like rowcovers and low tunnels inside high tunnels (Blomgren and Frisch, 2007; Byczynski, 2003; Lamont et al., 2005). However, few reports are available on the integration of supplemental heat additions and low tunnels within high tunnels. High tunnels typically do not use electricity; however, backup heating could provide the protection necessary to keep valuable plants alive in the early spring (Blomgren and Frisch, 2007). Supplemental heating is expensive, but costs may be reduced when used under low tunnels inside high tunnels, where heat would be concentrated around the plants. This approach has been used successfully for greenhouse tomatoes, where the amount of fuel needed to run the heating system was significantly reduced by targeting the heat at plant level using convection tubes (Hanna and Henderson, 2008). Soil warming has been shown to increase biomass accumulation and nutrient uptake for many crop greenhouse environments (Gosselin and Trudel, 1985; Hurewitz et al., 1984; Shedlosky and White, 1987). Although root zone heating can partly offset the adverse effect of low night air temperatures on greenhouse plants (Gosselin and Trudel, 1985; Janes and McAvoy, 1983), no published studies are known that assessed the effectiveness of root zone heating when high-tunnel air temperatures are near or below freezing.
The objectives of this project were to evaluate appropriate planting dates that provide early fruit production, to determine if low-cost supplemental heat sources can provide low-temperature protection for early-planted tomato, and to determine whether high-tunnel tomato production using these systems is economically viable.
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