Impact of Container Material on Substrate Heat Buildup in an Outdoor Nursery

The current study was conducted at the University of Kentucky Horticulture Research Farm in Lexington in conjunction with locations at Mississippi, Michigan, Texas, and West Virginia under the USDA–SCRI program. Buxus x Green Velvet 'Boxwood’ were planted in four types of #1 (~3.8 L) containers (treatments): 1) polyethylene PF400-SM (control) (Nursery Supplies Inc., PA); and alternative containers 2) Western Pulp 7X7RD (Western Pulp Products Co.,TX); 3) Keratin (Horticultural Research Institute, Washington, D.C.); and 4) Root Pouch^TM (Root Pouch Inc., OR). This one-factor completely randomized design experiment was conducted in Kentucky, Texas, Mississippi, Michigan, and West Virginia from June to October 2012.  All plants were irrigated at 7 am and 7 pm to replace 100% daily water use. Two thermocouples (Type T copper-constantan thermocouple wires; Omega Engineering, CT) in each plot measuring substrate temperature were placed in the container of central growing beds at one inch below the substrate surface at one inch away from container wall facing south and at the center of container. Data were recorded using a datalogger (CR1000; Campbell Scientific) programmed to scan every 30 s and determined maxima, minima, and averages hourly. Average substrate temperature showed around 6°C to 9°C increase in black plastic containers as compared to alternative containers at one inch away from container wall and an increase of about 2°C to 4°C at the center of container during August in Kentucky. Substrate temperature was exposed to critical temperature (>37.8°C) for more than 3 hours on 15 different days in black plastic containers and about 9 days in keratin containers and none was observed for wood pulp and fabric containers during the study in Kentucky. Substrate temperature was increased by about 16°C (plastic), 14°C (keratin), 10°C (wood pulp), and 7°C (root pouch) from sun rise to midafternoon and substrate started cooling down from late afternoon with root pouch and plastic cooling the fastest, followed by keratin and wood pulp containers. Other locations observed similar trend in thermodynamics among the containers. Plastic containers exposed plant roots to rapid changes in substrate temperature than alternative containers types causing decreased plant root dry weight at harvest compared to plants grown in wood pulp. Highest substrate temperature observed in plastic was attributed to its black, non-porous and thin container walls. Porous walls of wood pulp and root pouch containers improved heat exchange and also allowed increased evaporative cooling resulting in reduced heat buildup.