Global warming is likely to have large effects on plant development. In wheat, there is a consensus that leaf appearance rates increase with temperature in a roughly linear fashion up to an optimal temperature, which typically is assumed to be 20 °C (for daily mean air temperature). Above this value, however, there is disagreement as to whether the rate continues as a maximal value or declines with supra-optimal conditions.  This paper examines effects of temperature on leaf appearance and phenology in spring wheat (Triticum aestivum L. cv. Yecora Rojo) using infrared heaters (T-FACE system) and planting dates to expose crops to an exceptionally wide range of temperature at Maricopa, AZ. Heated treatments with controls were grown in January, March, and September of 2007 and 2008. Additional plantings occurred approximately every seven weeks. The primary objective was to characterize leaf appearance rates at temperatures above 20 °C. Leaf appearance rates were estimated using a range of cardinal temperatures (CTs) and assumed forms of response curves. Initial tests indicated that a base temperature (T_base) of 3 °C and optimum (T_opt) of 23 °C, referenced to a conventional daily mean temperature, provided the lowest RMSE for leaf number (0.39 leaves). Partitioning the dataset into cool- and warm-season plantings, resulted in estimates of T_base of 1 °C for the cool-season plantings vs. 4 °C for the warm-season plantings and in values of T_opt of 18 °C and 21 °C for cool- vs. warm-season plantings.  A simple daily mean temperature approach worked better than more complex temperature functions. These results demonstrate the sensitivity of estimates of CTs to temperature conditions during the experiments.