At the northern limit of the Eastern Massasauga Rattlesnake’s (Sistrurus catenatus (Rafinesque, 1818)) range, individuals spend up to half the year overwintering. In hummock hibernacula found in peatlands, it is likely that subsurface temperature and water table position are contributing factors dictating habitat suitability. As a step towards assessing the vulnerability of hibernacula to anthropogenic changes, we combined subsurface temperature and water table dynamics to assess the likelihood that unflooded and unfrozen conditions were present in hummock hibernacula. Our results indicate that taller hummocks are more resilient to an advancing frost line and fluctuating water table by providing a larger area and duration of unfrozen and unflooded conditions, and a critical overwintering depth that is farther from the hummock surface. In two study sites along eastern Georgian Bay, an unflooded and unfrozen zone was present for over 90% of the overwintering period for hummocks taller than 25–27 cm. Our findings highlight the vulnerability of peatland hummocks to variability of winter weather where deep freezing and (or) water table rise may nonlinearly reduce resilience. This suggests that height is not the only component affecting the suitability of hummock hibernacula and that further research should examine the structure and spatial arrangement of hummocks within a peatland.