A new, 2012 interactive USDA Plant Hardiness Zone Map (PHZM) of the United States was released this week.
For the first time, the map is available for use on computers as an interactive GIS (Geographic Information System), where users type in a ZIP Code and find the hardiness zone for that area. The interactive map is powered by ESRI, creators of ArcGIS Online. The USDA PHZM was produced with the latest version of PRISM, a highly sophisticated climate mapping technology developed at Oregon State University. One of the features I like is that the interactive map allows users to see a satellite image as the base map and to change the transparency of the digital zone overlay (to 50%) to understand the correspondence between topography and the zone designations.
Compared with the 1990 version, the new PHZM is “generally one half-zone warmer than the previous PHZM throughout much of the United States.” I interpret this to mean temperatures across the United States are generally getting warmer; however, the USDA cautions that “changes in zones are not reliable evidence of whether there has been global warming.”
Hardiness zones maps are primarily about cold rather than heat. They are useful in selecting trees and plants that will survive our winter cold. The PHZM designations are based on “the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.”
All PHZMs are just guides. Other factors including light, soil moisture, temperature, duration of exposure to cold and humidity can account for plant survival.
As shown in the graphic above, the western and northern part of Redding are given the 9B designation (25-30 degrees F); eastern and southern parts of the community are classified 9A (20-25 degrees F). Mountains to the west are classed as 8B (15-20 degrees F).
Some local gardeners still prefer to use the Sunset Magazine designations for plant suitability, which attempt to address heat and micro-climates issues. This blog has many useful Master Gardener links for the Redding area. If you have a gardening question, shoot me an email and I will try and find an answer for you.
There are an estimated 18+ microclimates in the Redding area so the hardiness zones are still generalizations – some hilly neighborhoods on the west side have good air drainage, so even some sub-tropical plants such as citrus can be grown because the locations do not have extended freezes. Some low neighborhoods on the east side have cold sinks where the frost-tender plants will not survive. Conversely, an inversion layer in the foothills near Round Mountain allows some less hardy trees to survive.
Enhanced (high-resolution) official USDA Plant Hardiness GIS data are available in shapefile and raster grid formats from Climate Source, Inc. A license to download and use these datasets can be obtained by visiting the Climate Source website.