A Sunset Zone is a climate zone for gardening determined by a system developed by Sunset Magazine in the mid-20th century. There are 24 Sunset Zones, divided by a number of factors to help gardeners identify the most appropriate plants for their needs. While initially developed for 13 western states, the Sunset Zone system has been expanded across the United States as an alternative to the widely used United States Department of Agriculture (USDA) zone system.
Several factors go into the determination of a Sunset Zone. Winter and summer highs and lows are considered to provide information about the temperature extremes in the region. In addition, weather patterns like humidity and rainfall are factored in; regions can have very similar temperature patterns but wildly different weather, making it very hard to grow the same kinds of plants in both locations. The Sunset Zones also get into details like prevailing winds.
Zones one through three cover cold, snowy regions with a short growing season and generally harsh climates. Four through six are located in rainy, wet areas like the coast of the Pacific Northwest. Zones seven through nine and 14 through 17 encompass inland valley regions like the San Fernando Valley of California, where climates tend to be very warm and ideal for agriculture.
The Southwest desert region like that found in Arizona and New Mexico is described in zones 10 through 13, while the unique climate of Southern California spans zones 18 through 24. The diversity and detail of the Sunset Zone system is designed to help gardeners identify not just which plants will survive the winter, but which plants will thrive in given growing conditions. This zone system for gardening also allows for microclimates, small pockets in otherwise uniform regions where growing conditions are markedly different.
Gardeners in the Western United States are often familiar with the Sunset Zone system, as the Sunset Western Garden Book is one of the most venerable and widely used gardening references in the area, and many nurseries indicate plant hardiness by Sunset Zone. The expansion of Sunset's system to other regions of the United States has involved careful evaluation of climate conditions and the development of appropriate maps to help gardeners find their zones.
Lists of plants suitable for different Sunset Zones are available and people can also explore native plant lists divided by Sunset Zone if they are interested in growing a native plant garden. These lists are readily available on the Internet and in gardening guides.