Saturday Short Subjects: Zone Change

On Wednesday, January 25, the United States Department of Agriculture (USDA) released a new version of the Plant Hardiness Zone Map, the agency’s first update to the map since 1990. This useful tool for gardeners, farmers, foresters, nurseries, and data lovers, was jointly developed by USDA's Agricultural Research Service (ARS) and Oregon State University's (OSU) PRISM Climate Group. 

The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees F) and 13 (60-70 degrees F). Each zone is a 10-degree Fahrenheit band, further divided into A and B 5-degree Fahrenheit zones. According to the official USDA press release: “Plant hardiness zones do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.”

It should be no surprise that the map indicates that the US is a warmer place than it was two decades ago. The USDA press release also states, “The new map uses data measured at weather stations during the 30-year period 1976-2005 and is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States.”

Many in the horticultural world feel that this map is long overdue. In 2003, according to an article in Mother Jones, the American Horticultural Society released a draft version of a new map based on data from 1986 to 2002, which showed dramatic movement of the zones northward. The USDA pulled this map from circulation, promising to release a new map in 2005.

The Arbor Day Foundation, a nonprofit organization with over one million members dedicated to planting trees, grew tired of waiting. The group released its own map in 2006, “in response to requests for up-to-date information.” You can view that map here. Be sure to check out the animation showing you the shift in zones from 1990 to 2006. FYI, in the 1990 map, I was in Zone 6.

Back to the new USDA map. It may have been a long time coming, but the map is extraordinarily rich in data and easy to use.

The new map is more detailed geographically and takes into account such features as hills and valleys. It has an interactive feature. You can enter your zip code to find the information for your particular microclimate. Those in the East Bay Area might even find that your town is in a cool zone nestled between two slightly warmer ones. I now live in Hardiness Zone 7a, with average annual extreme minimum temperature of 0 to 5°F. 

I’ve told you mine. Do you know your zone? Check it out here. Your new plants will thank you.

Why Saturday Short Subjects? Some readers may recall  being dropped at the movie theater for the Saturday matinee — two action-packed feature films with a series of short subjects (cartoons or short movies, sometimes a serial cliffhanger) sandwiched in between. Often the short subjects were the most memorable, and enjoyable, part of the morning. That explains the name. The reason behind these particular posts is that we are all short on time. My Short Subject posts should not take me as long to write or you as long to read (or try).