Influence of Container Size, Insulation, Moisture Content, and Medium Type On Low Temperatures In Containers

Despite the ubiquitous usage of containers for the production and marketing of woody plant material, little research has been conducted to determine the influence of container size, medium type, medium saturation, and location within the container on low root-zone temperature.  In northern regions, growers must go to great expense to mitigate low-temperature induced root zone injury. Overwintering of containerized nursery stock is costly due to overwintering structure direct costs - the costs associated with a reduction in space use efficiency, increased labor, and mechanical damage resulting from increased plant movement.  These costs make it difficult for nurseries in northern regions to compete with southern growers.
Using controlled ultra-low temperature coolers, we studied how container size, insulation, medium type, medium saturation and location within the container influence root-zone temperatures. We found that peat and pine bark mixes significantly delayed freezing within containers compared to Stalite or sandy loam media.  While the peat-based and pine bark mixes have substantially higher water holding capacities and lower thermal conductivities, following freezing they perform similarly to the other medium types. Following homogeneous solidification within the media due to freezing, all medium types cooled rapidly.  During the phase change, the thermal diffusivity of the medium abruptly decreased as medium temperature reached slightly below 0.0°C.  This abrupt thermal diffusivity decrease remained for media with high water contents and low thermal conductivities - such as pine bark mixes and peat mixes - for days compared to a few hours for sandy loam or Stalite.