New Climate Zone Map
The US Department of Agriculture has released an updated climate zone map for the United States. This revised map was developed with the assistance of Oregon State University. For the first time, the new map offers a Geographic Information System (GIS) based interactive format which makes it internet friendly. It has a “find your zone by ZIP code” function. The new map can be accessed at planthardiness.ars.usda.gov.
Plant hardiness zones are based upon the average annual extreme minimum temperature at a specific location. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.
The map includes 13 zones. Each zone is a 10 degree Fahrenheit band, further divided in 5 degree zones a and b. Zones in western Washington and Oregon range from 7a (0 to 5 degrees F.) to 9b (25 to 30 degrees F.). Compared to the 1990 map, zone boundaries in the new map have shifted in many areas. The new map is generally one 5 degree half zone warmer than the previous map through most of the United States. This is the result of using temperature data from a longer and more recent time period. The new map uses data measured at weather stations during the 30 year period1976-2005. This map seems to reflect a gradual warming in our climate.
Many books and plant tags list the USDA plant hardiness zone rating for specific plants. However, the Sunset Western Garden book has its own set of hardiness zones which are completely different from the USDA zones. If you use the Sunset book in selecting plants, be sure to use their climate zone maps. For example, Vancouver and Portland are in USDA zone 8b (15 to 20 degrees F.). The Sunset zone for Vancouver and Portland is zone 6. Zone 8 in the Sunset Western Garden Book is for the Central Valley of California.