Revisiting plant hardiness zones to include multiple climatic stress dimensions

A tradition exists for delineating "hardiness zones" for important plants in horticulture and agriculture. However, these zones are typically based on surviving cold winter conditions, disregarding other stressors. Factors such as the effects of summer heat, aridity, or excessive humidity have been overlooked, limiting our understanding of challenges faced by plants associated with human activities, particularly in a time of rapid global-scale climate change. Annual plants not exposed to winter cold and heat-sensitive plants encountering early summer heat waves may experience significant difficulties. Here, we establish hardiness zone criteria for four climatic dimensions: heat, cold, dryness, and moisture. We explore how this expanded concept of hardiness zones could be implemented in the context of 872 tree species in the United States, as a step toward understanding stressors that plants experience in different climates. The aim is to provide insights that may be informative for horticultural and agricultural practices.