Plant Hardiness Zone vs. Sunset Zone: Which to Use?
The plant hardiness zones of a particular area are a great tool to inform the gardener of climate conditions in order to better predict which plants will thrive in their area. However, with different systems of evaluation, the issue becomes much more obscure. It is important to understand the various systems used in order to identify which one works best for the specific landscape and situation.
Plant Hardiness Zone
Plant Hardiness Zones are created by finding the average low temperature within the area and grouping similar low temperatures into one of 11 specific "Zones." Mainly the zones fluctuate depending on latitude and altitude across the world with specific maps for each continent and country. The Plant Hardiness Zones are the default, go-to guide for the United States and often the world. This level of general acceptance provides many advantages. Now with Internet accessibility a global phenomenon, discussions regarding gardening occur between people from Sweden, Australia, South Africa and Arkansas all talking about the same problems where common ground is necessary for accurate communication. Hardiness zones provide a basis for communication for people from vastly different places to network gardening information. Likewise, plants and gardening books can now be ordered internationally and are often listed with a hardiness zone for broad range clarification to a global market. The problems come about because the average low temperatures in many places are similar, but do not provide an accurate picture of the climate. Consider Portland, Oregon and Austin, Texas both being within Zone 8 and yet having completely different climate requirements which dramatically alter the success of some plants. Hardiness Zones are a great place to begin and a great fall back position for gardeners, but often do not provide the complete picture.
The Sunset Zone system is a much more detailed and specific climatic tool which takes into account many factors to create 24 different zones throughout the 13 western states (Hawaii, Alaska, Washington, Oregon, California, Idaho, Nevada, Arizona, Montana, Utah, Wyoming, Colorado, New Mexico). The Sunset Zones are considered precise zoning for these states because they take into account not only the low temperature but also summer high temperatures, length of the growing season, humidity, mountain and valley fluctuations, average rainfall, and other important climatic phenomenon. Many nurseries, garden centers, and websites within the western states focus on and use these zones due to their ability to provide a more accurate picture of the region. By using the Sunset website and CD Rom, plants are listed by zone providing a picture, specific light and water requirements, as well as soil preference, and description of seasonal changes. Sunset zones are an incredible resource for localized knowledge and location specific resources, however they are little used or known in other parts of the country.
The National Sunset Garden Book attempted to break the nation into 40 different zones, but is now out of print. Due to volume limitations, the national book was not as specific and thus lost much usefulness. Used copies are often widely circulated around the Internet if a gardener would like a copy.