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 theday 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 duringnight. The presence of a frost blanket reduced night-time radiative coolingof leaves and buds and allowed their temperatures to stay closer to ambientconditions 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.