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Human assisted movement of reforestation stock poleward or upward in elevation has been proposed as a tool to address climate change in regular reforestation programs. However, moving warm adapted seed sources to colder environments could carry the risk of frost damage if seed sources are moved too far. Here, we assess genetic differentiation in growth potential, survival and cold hardiness of a wide-ranging tree species, white spruce (Picea glauca [Moench] Voss). We use data from a 34-year-old common garden experiment planted in the approximate center of the species range. Cold hardiness was negatively related to growth and positively to survival. Generalized additive models identified mean coldest month temperature and latitude (as a proxy for the day length regime) as an exceptionally good predictor for the onset of cold hardiness (R-adj(2) = 0.91). The results suggest that day length, an environmental factor that is not influenced by climate change, is an important factor controlling the timing of the onset of hardiness. Survival was only moderately well predicted, primarily by precipitation of the provenance origin (R-adj(2) = 0.36) indicating that other adaptive traits besides cold hardiness should be considered in assisted migration prescriptions. Survival of seed sources appears primarily compromised when transplanting sources from wet origins to a dry location. Nevertheless, acceptable migration distances without significant tradeoffs were up to 500 km north and 1500 km west towards a central planting location.