The snowpack in high altitude and latitude regions provides thermal insulation during the cold season. Recent climate change has resulted in a decrease in both the duration and the reliability of this snowpack and may thus affect overwinter survival of biota. Here we use a manipulation approach to ask how snow depth affects ground surface temperatures and how this in turn affects survival of the overwintering rosette stage in the monocarpic plant Lobelia inflata. A shallow but consistent insulation layer (R-value of 3) was sufficient to reduce temperature fluctuations and the accumulation of sub-zero degree-days substantially. For all treatments >R3 these measures were negligible. Survival results are consistent with a crucial role of thermal insulation to successful overwintering of Lobelia inflata rosettes: without a consistent snowpack survival was low (11%); with an insulation effect of R3 or greater survival increased dramatically (81%). The winter prior to the manipulation study was characterized by an anomalous absence of snow at the onset of cold temperatures. This resulted in substantially greater accumulation of sub-zero degree-days at the soil surface and almost 100% rosette mortality in the field. This study shows that inconsistent and reduced snowpack – a prediction of climate change – will have critical effects on plant survival because of increased temperature fluctuations and extreme temperatures experienced at the soil surface.