USDA Hardiness Grow Zones vs Sunset Climate Zones
Origins of these Zones
USDA Hardiness Grow Zones
1927 - The first US map with 8 hardiness zones was compiled at the Arnold Arboretum by Dr. Alfred Rehder, based on a survey of plants and their survivability in different regions of the country.
1938 - Dr. Donald Wyman of the Arnold Arboretum published a new map in his book, Hedges, Screens, and Windbreaks, that used 40 years of data (1895-1935) from the US Weather Bureau. The Arnold Arboretum map was updated in 1951, 1967, and 1971.
1960 - The US Department of Agriculture got into the act, publishing its first map, based on the data from 450 weather stations around the country. Unfortunately, they used different criteria from the Arnold Arboretum for establishing their zones, resulting in two conflicting maps.
Up to 1990 - The Arnold Arboretum map remained the standard over the 1960 USDA map.
1990 - The US Department of Agriculture, in cooperation with the US National Arboretum, updated the USDA hardiness map, using data from between 4,800 to 14,500 weather stations. Various publications by Dr. Marc Cathey, who oversaw the map project, have given a wide variety of radically varying weather station numbers. Although Cathey also claims to have used a 15-year dataset, the 1990 map actually used a 13-year dataset from 1974-1986 (US) and 1971-1984 (Mexico). This revision shifted most areas into zones that were one half to one zone cooler than the previous Arnold Arboretum map.
Sunset Climate Zones
The Sunset Climate Zone map for gardening was devised in the mid-20th century for thirteen western states. It has been expanded to include areas across the U.S., providing a more useful alternative to the USDA zone system.
Details of USDA Hardiness Grow Zones
Each of the current USDA 20 climatic zones (Zone 1, 2-10a,b, Zone 11) is based on the average minimum winter temperatures. Keep in mind that there is often a major difference between what will grow in the "a" and "b" regions of each zone.
Something to note is that this map does not take into account the amount of time at minimum/max temperature and humidity. Some plants can keep hardy through weeks at that temperature/humidity while others only days before they cannot recover.
Details of Sunset Climate Zones
The 24 Sunset Zones are determined by a number of factors to help gardeners identify the most appropriate plants for their needs. Winter and summer highs and lows are used to provide information about the temperature extremes in the region. Weather patterns like humidity, rainfall and heat are considered. The Sunset Zones also take into account specific environmental conditions like prevailing winds, day length and soil type.
The Sunset zone maps are more precise than the USDA's, and so are considered the standard references for gardeners in the West.
Good breakdown of zones: https://www.sunset.com/garden/climate-zones/sunsets-garden-climate-zones
Notes
The numbering of the maps do not match, so make sure you are referencing the correct type of map for the information you are provided.
The majority of labels will list the USDA zone number, so double check if you are wanting to go by the climate zones system.
Find your Zone Tools
USDA Grow Zones by Zip Code:
Sunset Climate Map:
Sources:
USDA Grow Zones Map Image: https://www.myhumblekitchen.com/2010/02/planting-zones-frost-date-and-planting/
Sunset Climate Zone Map: https://www.tomatofest.com/Tomato_Growing_Zone_Maps_s/164.htm
Origins of USDA map: https://www.plantdelights.com/blogs/articles/plant-hardiness-zone-maps
Comparison of both: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=21293
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