When discussing plant hardiness zones, which index is commonly referenced?

The USDA Plant Hardiness Zones are the standard reference for understanding plant hardiness in relation to climate. This system divides North America into various zones based on the average annual minimum winter temperature, providing valuable guidance for gardeners and landscapers on which plants are likely to thrive in specific regions. Each zone is assigned a number, with warmer zones accommodating plants that may not survive in colder areas. This standardization allows users to make informed decisions about plant selection based on the growth potential of various species in specific climates.

The other indices mentioned may pertain to specific needs or scenarios in horticulture or shipping but do not serve the broad purpose of assessing plant hardiness against climatic temperatures like the USDA Plant Hardiness Zones do. Thus, the USDA system remains the most widely referenced index for determining plant viability in different locations.