Plant Zones: Sunset, Heat or Hardiness?

Plant zones are wonderful tools to inform gardeners about the landscape with which they live. However, with multiple systems of evaluation, it can be confusing to determine which zoning system should be chosen for the area. Due to the sheer volume of information, most zoning systems have a certain focus in order to be accurate and useful. By understanding the focus of each system, the usefulness of each zoning system becomes apparent and can be utilized by the gardener for their specific needs.

Sunset Zones

The 24 Sunset zones were designed for region-specific information pertaining to the 13 most-western states (including Alaska and Hawaii). The system takes into account a number of different climatic conditions such as humidity, mountain ranges, valley systems, rainfall, summer highs and winter low conditions. Most western-specific nurseries, seed catalogs, and gardening websites use this system for zoning and plant classification due to how specific and accurate it is. Sunset has attempted to group the nation into 40 climate zones, but lost some of the specificity and thus the system never caught on. If gardening in the West, or looking for western information, Sunset is the system to use.

Heat Zones

The 12 Heat zones have been created by the American Horticultural Society to indicate the levels of heat which plants can undertake before dying. The zones recognize the number of days a plant can withstand temperatures above 86 degrees before damage begins to be inflicted on flowers, leaves, buds, and root systems. Zone 1 (less than one day of heat) occurs at higher latitudes and altitudes, whereas Zone 12 (over 210 days of heat) are at lower latitudes and concentrated in areas where drought often occurs. The heat zones are wonderful resources for southern states where colder temperatures are of minimal worry and "over wintering" is often not much concern when compared to lack of rainfall or drought. Southern gardeners should take note of their heat zone and plant varieties which are heat/wilt resistant or can withstand extreme drought.

Hardiness Zones

Developed by the United States Department of Agriculture in the 1960s, Hardiness zones are the standard zoning for the United States and now the world. Most all generalized gardening publications and Internet sites use the Hardiness zones as their plant classification system. Hardiness zones focus on the low temperatures and average frost dates to determine which of 11 zones plants will be able to withstand and overwinter. Northern latitudes or places with exceptionally harsh winters will want to focus on the plant hardiness zones as their means to figure out which species will be able to survive the harsh conditions of the landscape. However, knowledge of the area's hardiness zone even in the western and southern states is important as a reference when looking up gardening information. Most encyclopedias, seed catalogs, and international forums of communication will often make statements like "Hardy to Zone 7" or merely list "Zone 9" under the specific requirements of a species without further climatic information. By knowing the Hardiness zone of the garden significantly more information is available to the gardener.