Studies were conducted to better understand the effect of temperature, photoperiod, cultural practices, and growth regulators on the hardening and dehardening of peach tree wood. Greenhouses were constructed to control temperatures to which peach trees were exposed. It was noted that hardening and dehardening usually followed the temperature fluctuation fairly closely. However, trees exposed to temperatures above 60 F (15. 6 C) became hardy to 0 F (-17. 8 C). A photoperiodic response in regard to an increase or decrease in hardiness was not definitely observed in peach wood. Extended photoperiod did not produce any significant effect on hardiness. Defoliation, bloom date, and fruit set were all the same as the controls. The cultural practices of fall pruning and heavy late summer fertilization with nitrogen did not result in a change in the hardiness level of the trees when compared with the controls. Likewise gibberellic acid spray had no effect. Defoliation produced serious physiological damage when done in mid-August, while defoliation in mid-September was not nearly so damaging, Defoliated trees were less hardy than the controls until mid winter.