Leaf and bud temperatures of southern highbush blueberries (Vaccinium corymbosum) inside high tunnels

A high tunnel is a passively heated and cooled plastic-covered greenhouse used for season extension and crop protection for a variety of horticultural crops, including southern highbush blueberry (Vaccinium corymbosum L.). Frost protection remains a major challenge for early-season blueberry production, due to poor heat retention by high tunnels. Understanding the energy balance of high tunnels is essential for predicting night-time minimum temperatures and implementing frost protection strategies. We determined the effects of high tunnels on air, leaf, and bud temperatures of southern highbush blueberry. Impacts of frost blankets (one frost protection strategy) on temperatures were also explored. High tunnels quickly warmed during the day but cooled to ambient or below ambient air temperature at night, particularly during clear days and nights. Leaf and bud temperatures in high tunnels were higher than those of outdoor plants during the day but lower during night. The presence of a frost blanket reduced night-time radiative cooling of leaves and buds and allowed their temperatures to stay closer to ambient conditions inside of high tunnels. Lack of night-time heat retention in high tunnels was likely due to the properties of the polyethylene greenhouse cover, lack of ventilation, and large surface area to volume ratio of the high tunnels. INTRODUCTION A high tunnel is an unheated plastic greenhouse that is passively ventilated and can be erected over large areas (Lamont, 2003). They lend themselves well to organic horticultural practices and are becoming indispensable components of many organic farms. Blueberries are an economically significant crop for Georgia and production of out-of-season organic blueberries in high tunnels could be a financially lucrative endeavor for Georgia’s growers. The research reported here was part of a larger project to examine the feasibility of production of early organic southern highbush blueberries inside of high tunnels in Georgia (USA). The main goal of the overall research project was to discover the ideal date of closing high tunnels to achieve both early and high blueberry yield. In the first year of this project, we found that increased daily maximum air temperatures inside high tunnels facilitated early flowering, fruiting, and onset of vegetative growth (Ogden et al., 2007). The major limitation of the high tunnels was their inability to retain heat during cold nights. Night-time minimum air temperatures inside the high tunnels were equal to or lower than outside air temperatures (Ogden and van Iersel, 2008). Likewise, Montero et al. (2005) reported that the air in unheated greenhouses was about 2.5°C cooler than the outside air. The lack of frost protection provided by the high tunnels necessitated the use of propane heaters on nights with minimum temperature below 0°C to protect the developing, freeze sensitive flowers and buds. The research reported here was conducted during the second year of this project. In an effort to reduce fuel costs, we also experimented with frost blankets on nights with predicted lows of 0 to -1°C during the