Abstract
The warming climate has lengthened the growing season across mid‐latitude regions, driven largely by an earlier onset of the frost‐free period in spring. Earlier spring warming can advance plant phenological development but may also increase exposure to subsequent frost or freeze events that damage vulnerable new vegetative growth, commonly referred to as a ‘false spring’. We examine long‐term trends in growing degree day (GDD) accumulation prior to the last frost (0°C) and last hard freeze (−2.2°C) in spring across the continental United States. Using daily temperature data from 186 stations over the period 1952–2021, we calculate GDD totals and rates on the date of last frost and freeze for each year and assess long‐term trends using least‐squares linear regression. We find an earlier spring onset at 54 stations based on the freeze threshold and at 56 using our frost criterion. Increasing GDD trends are evident at 22 locations for frost and 18 for freeze, mostly in the northeastern and north‐central United States. At most of these locations, the date of the last frost or freeze has not advanced significantly. However, after accounting for spatial autocorrelation using a false discovery rate adjustment, most GDD and GDD‐rate trends are no longer statistically significant at the continental scale. These results indicate that, despite widespread earlier spring onset, there is limited evidence for a coherent, large‐scale increase in false spring risk across the United States over the past seven decades. Localised increases in early‐season heat accumulation may nonetheless have important ecological and agricultural implications, underscoring the need to consider both phenological responses and atmospheric circulation variability when assessing frost and freeze risk in a warming climate.