Analysis of Thermal Environments in Polyethylene Overwintering Structures1

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Abstract
Equal average container temperatures during prolonged periods of low outside air temperatures were achieved by unheated structures covered with either a double-layer of clear polyethylene or a double-layer with the inner layer translucent and the exterior clear polyethylene. Single-layer structures were less effective, with clear polyethylene affording more protection than translucent. Excessively high mid-day temperatures in both the single- and double-layer clear houses increased the probability of desiccation injury and affected the quality of the nursery stock. Fluctuations of air temperatures below 0°C were most rapid in the clear houses, and appear to depend greatly on the relative humidity, which affects the thermal conductivity of the air. Container temperature fluctuations were similar whether the container soil water was frozen or unfrozen. The best covering tested thus far appears to be a double-layer with the interior translucent and the exterior clear polyethylene. This covering moderates low container temperatures, high mid-day temperatures, desiccation problems, rapid temperature fluctuations and concomitant management problems.

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