   Plant Hardiness Zone Update - May 23, 2007 Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County Despite arguments about the causes, scientifically valid data indicates that our climate has experienced some changes and many places have become warmer. The National Arbor Day Foundation has developed an updated Plant Hardiness Zone Map based on the most recent 15 years' data available from the National Oceanic and Atmospheric Administration's 5,000 National Climatic Data Center cooperative stations across the United States. It should be noted that this new map is not an official government document and its release has generated some controversy. Plant Hardiness Zones are based on average annual low temperatures using 10 degree increments. For example, the average low temperature in zone 3 is -40 to -30 degrees Fahrenheit, while the average low temperature in zone 10 is +30 to +40 degrees Fahrenheit. The USDA Plant Hardiness Zone map was developed using minimum temperature data and was most recently revised in 1990. It represents the lowest average temperatures that can be expected in that broad area based on a 12-year average (1974-1986). There are 10 different zones rated for plant hardiness and an additional zone that is essentially frost-free. The 1990 map also divided zones 2-10 in half to denote 5-degree differences in average minimum temperature rather than the old 10-degree differences. The USDA Plant Hardiness Zone Map of 1990 can be viewed on-line at: www.usna.usda.gov/Hardzone/. The National Arbor Day Foundation’s (NADF) 2006 Plant Hardiness Zone Map shows some significant deviation from the official USDA map described above. Significant portions of many states have shifted at least one full hardiness zone. Much of Illinois, Indiana, and Ohio, for example, have shifted from Zone 5 to a warmer Zone 6. Some areas around the country have even warmed two full zones. Conversely, a few areas also shifted to a cooler climate zone. Interestingly enough, these areas were in the west, and some changes appear to be in Yavapai County (it is difficult to tell from the on-line maps). The NADF web site has the 2006 revised map along with animations showing the shifting of hardiness zones at: www.arborday.org/media/zones.cfm. The NADF website also has a tool that allows users to input their zip code and returns their plant hardiness zone. I tested this tool and found that Prescott was firmly in Zone 7 (minimum temperature 0 to 10 degrees F), Cottonwood spanned Zones 7 and 8 (minimum temperature 0 to 20 degrees F), Camp Verde spanned Zones 8 and 9 (minimum temperature 10 to 30 degrees F), and Sedona spanned Zones 6 and 8 (minimum temperature -10 to 20 degrees F). This leaves a lot to interpretation and seems to make it more difficult rather than easier to select appropriate plants if you don’t have local experience to help you interpret these data. The American Horticultural Society created a Heat Zone Map which addresses the ability of a plant to tolerate extreme heat and drought. It has divided the country into zones based the number of “heat days” (days with temperatures exceeding 86 degrees F). The zones range from 1 (less than one heat day) to 12 (more than 210 heat days). This map is available on-line at: www.ahs.org/publications/heat_zone_map.htm. Heat Zone data are steadily being incorporated into the plant literature and nursery tags. If you feel lost in the world of Plant Hardiness and Heat Zones, then don’t despair. Remember that low lying areas and canyon bottoms are going to be colder in the winter. South and west exposures are going to be warmer and microclimates created by natural or constructed features will also influence suitability of a site for a given plant species. In closing, one of the best tools for determining whether or not to try a given plant in your garden is the Sunset Western Garden Book. This fantastic reference uses its own climate zones which integrate temperature extremes, latitude, elevation, average precipitation, and humidity. It is regularly updated and there is a brand new 2007 edition available. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |