The effects of high medium temperatures on the growth and mineral nutrition of selected woody plants and on the composition of the medium solution were studied.

Medium temperature profiles were established for 3.8-liter black polyethylene containers exposed to solar radiation under Virginia nursery conditions. On clear days in mid-summer, maximum recorded medium temperatures on the southwest side of containers were as high as 45°C and could exceed 40°C for 4 to 5 hours. The high medium temperatures in exposed containers reduced the shoot relative growth rate and the specific rate of nitrogen uptake for Ilex crenata ‘Convexa’ in comparison to that of plants grown in containers insulated from solar radiation. Shoot dry weights of J. crenata ‘Helleri’ , Juniperus chinensis, Buxus microphylla, and Nandina domestica were at least 20% lower in exposed containers than in insulated containers.

In a pine bark medium, growth response of J. crenata, J. horizontalis, or N. domestica to increased N application rate was similar when root-zones were at 40°C for 6 hrs/day or more optimal growth temperature. In unlimed pine bark, root-zone temperature of 40°C for 6 hrs/day resulted in higher medium solution pH and NH₄- N:NO₃-N ratio than at lower temperature. Limestone addition to the medium negated these effects and alleviated growth reductions due to high root-zone temperature for N. domestica and J. horizontalis. The higher medium solution pH associated with heated root-zones resulted in lower medium solution and shoot tissue Mn concentrations for I. crenata.

A limed pine bark medium periodically fertilized with ammonium N was heated to temperatures of 28°, 34°, 40°, 46°, or 52°C for daily exposure duration of 1, 2, 4, 6, or 24 hours for 20 days. Treatment temperature of at least 40°C with a daily exposure duration of 24 hours resulted in an increase in medium solution NH₄-N concentration. Similar increase in NH₄-N was found for 2 hr/day exposure to 46°C, with further increases in NH₄-N at longer exposure times. The maximum level of NH₄- N occurred after 1 hr/day exposure to 52°C or 24 hr/day exposure to 46°C. Decreases in medium solution NO₃-N concentration generally coincided with the increases in NH₄- N. Results indicate that high container temperature may increase the ratio of NH₄- N:NO₃-N in the medium solution of plants fertilized with predominately ammoniacal N.