Root Zone Temperatures and Growth Responses of River Birch (Betula nigra) in Five Above-ground Pot Production Systems

River Birch (Betula nigra) were grown for 29 months from bare root stock in five above-ground pot production systems in USDA hardiness zone 5b. Six randomized replicates of clumped birches were grown in each pot type: standard black plastic (SO), black fabric with porous fabric bottom (SP), white fabric with nonporous fabric bottom (RT), black plastic mesh with porous fabric bottom (RBK), black plastic mesh with solid plastic bottom (RBS). Pots were left in the field over winter with no additional protection. Root zone temperatures were logged every 30 min in two pots of each type; sensors were placed 4” deep and 1” inside the pot wall in the southwest direction. SO experienced the most extreme high root zone temperatures, accumulating 140 hrs above 40.6C while SP logged no hrs above the same threshold. RBK and RBS logged the most cold root zone temperatures, accumulating an average of 76 hrs below -12.2C  while SP logged only 16 hrs below -12.2C. Average daily temperature fluctuations were most extreme in black plastic pots (SO, RBK, RBS) with a maximum daily fluctuation of 46.0C and average daily fluctuation of 14.4C, compared to  black and white fabric pots (SP, RT) with a maximum daily fluctuation of 29.3C and an average daily fluctuation of 9.2C.  Increases in tree height were significantly different with SP and RBK having greater increases than RBS and RT. Total stem calipers at termination were significantly different, with SP and RBK larger than RBS. At termination, top fresh and dry weights were significantly greater for SP and RBK than RT and RBS. SO trees were intermediate for all growth responses. SP had few visible root defects when removed from the container but air-spading exposed sub-surface defects and there was no difference between SP, RT and RBS in final root defect ratings.  SO and RBK were rated as unacceptable for root defects.