Old hardiness zones (33966 bytes)

USDA Hardiness Zone Changes

By Carl Wilson, Colorado State University Cooperative Extension Horticulturist, Denver

The USDA plant hardiness zones, last updated in 1990, are about to be revised. Gone will be the "a and b" zones. Four more numbered zones will be added for the semitropical portion of the country for a total of 15 zones.

For us in Colorado, we will go from a mountain zone 3, foothills zone 4 and plains zone 5 state to a predominantly zone 4,5,6 state. Only the very highest mountain areas (example Leadville at 10,000 ft) will be zone 3. The mountains and foothills will be a mix of 4 and 5,  the populated Front Range zone 5 with zone 6 infiltrating the southern areas plus "heat island" cities of Denver and Ft Collins, and the plains a mix of 5 north and 6 in the south (Ark Valley).

The reason for the revision is the markedly higher temperatures seen in the climate data collected from 1987 to 2001. A look at the national map reveals zone creep northward confirming what you have heard discussed in Denver for the last 5 years, "this is a zone 6 plant but don't be afraid to try it." Now we will officially be zone 6.

The other nice feature about the new map is that it has county lines on it. The USDA ARS (Ag Research Service) contracted with the AHS (American Hort Society) to crunch the data and create the map. The revision is due to be announced later this year.

A color draft version of the map is available on-line as a PDF file (Adobe Acrobat) on the AHS website,  http://www.ahs.org/publications.   The map is well worth a look. Be forewarned it is a large file to download if you have a modem connection and better suited for those with DSL service. Even if you don't download the map, the article about hardiness zones is very interesting. Note that the AHS site also soon will include a searchable by zip code feature to find hardiness zones similar to the heat zone search feature now on the site.

To view the new Colorado hardiness zones, see Colorado Counties.

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Date last revised: 01/05/2010