Hardiness zones are primarily determined by which climatic factor?

Hardiness zones are primarily determined by minimum temperature ranges because these zones categorize regions based on the lowest temperatures they experience during the winter months. This classification helps gardeners and horticulturists know which plant species are likely to survive and thrive in their specific climate. Each zone indicates a range of temperatures that can be critically significant for the growth and development of various plants, since many plants have specific temperature thresholds below which they may not survive.

In contrast, while annual rainfall, plant disease prevalence, and humidity levels do play roles in overall plant health and growth, they do not directly define hardiness zones. Annual rainfall and humidity can influence the types of plants that might flourish in a region but are not sufficient on their own to determine the hardiness of plants. Similarly, plant diseases may affect species survival but are broader ecological factors rather than climate-based ones that define hardiness. Thus, minimum temperature ranges are the fundamental aspect that influences how hardiness zones are established.