Analysis of Thermal Environments in Polyethylene Overwintering Structures1

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.

The Three-dimensional Redistribution of Water and Solutes in a Frozen Container Medium1

“…If this hypothesis is true, then the present results may correspond to that which occurs under field conditions, that is, wherein the container top becomes coldest and thus freezes first. In addition, large temperature gradients between different regions in containers do not exist under field conditions (15). There fore, the physical configuration of the experimental system probably did not grossly alter the redistribution of H20 and solutes as it occurs under field conditions, although the re distribution of H20 may have occurred during thawing in these experiments rather than during freezing, which is likely under field conditions.…”

“…Because of the paucity of information regarding the mecha nism^) by which low root zone temperatures and/or a phase change could result in desiccation injury, the problem remains a serious one. The seriousness of the problem is indicated by the observation that the microclimate conditions conducive to desiccation arise relatively frequently even in double-layer polyethylene houses (15).…”

Root Hardiness of Container-grown Ornamentals1

“…Havis (2) concluded from his observations that the killing temp was generally that of the mature roots and that the small portion of young roots at the outer edge of the container could be lost without affecting top growth. However, the temp difference between the edge and the center of an 8.8 liter container in a polyethylene overwintering structure is not significant (10). Therefore, if the young roots at the edge are killed, those throughout the container are killed.…”

“…However all segments of the industry in cold northern en vironments share the common problem spatial temp control and greater response capabilities than most modified-compressor type freezer systems. 10-hr photoperiod (20 klx intensity). After 4 to 5 weeks, they were transferred to an 8-hr photoperiod (6 klx) at a constant temp of 1.5° for 6 to 7 weeks.…”

“…Lateral shoots have been induced to growth after cytokinin treatment to apples (11), macadamia (1), tea crabapple (3), roses (4), poinsettia (10), and holly (16). Wider branch angles, which may be an advantage, often result from buds induced to grow by cytokinin treatment (14).…”