Turf loss from freezing injury results in costly re-establishment, especially with turfgrasses such as perennial ryegrass (Lolium perenne L.) having poor low-temperature hardiness. Studies are limited as to the influence of N and K on cold tolerance during dehardening periods in late winter when grasses are most susceptible to freezing injury. The objective of this study was to evaluate perennial ryegrass low temperature hardiness during deacclimation in response to N and K and associated effects on crown hydration, median killing temperature (LT50), shoot growth rate, tissue K concentration, soil exchangeable K, and low temperature disease. Treatments included five rate levels of N (49, 147, 245, 343, and 441 kg·ha-1·yr-1) in all factorial combinations with 3 rate levels of K (49, 245, and 441 kg·ha-1·yr-1). Low temperature tolerance was assessed using whole plant survival and electrolyte leakage (EL). Interactions between N and K were detected for all field measurements. The effects of N and K on survival LT50 were detected only during late winter periods in February 2004, N and K differences were lost by March. Late winter cold survival was negatively correlated with crown moisture, growth rate, and tissue K. Tissue K concentrations ranged from 28.6 to 35.9 g·kg–1 DM while soil K ranged from 121 to 261 mg·kg–1. Soil extractable K was not correlated with tissue K. Survival and EL LT50 were uncorrelated due to N and K interaction. Survival LT50 ranged from –9.0 to –13.6 °C. Maximum cold hardiness occurred when low to moderate N (49 to 147 kg·ha-1·yr-1) was applied with medium-high to high levels of K (245 to 441 kg·ha-1·yr-1), which corresponded to soil exchangeable K levels ranging from 200 to 260 mg·kg–1. Alternatively, similar K fertilization and soil K levels combined with high rates of N (343 and 441 kg·ha-1·yr-1) increased freeze stress and low temperature fungi (Typhula incarnata). At N rates routinely applied to perennial ryegrass, higher soil extractable K beyond those levels currently recommended for optimum shoot growth could provide some benefit in enhancing cold hardiness. Late fall applied N did not appear to increase the potential for winter injury.