The United States Department of Agriculture recently released a new Plant Hardiness Zone Map (PHZM) to reflect data collected from 1976 to 2005 and includes two new zones: 12 (50-60 degrees F) and 13 (60-70 degrees F). The map, which can be searched by ZIP code on the USDA’s website, is based on the average annual minimum winter temperature, divided into 10-degree Fahrenheit zones.
The last version of the map was released in 1990 with data collected from 1974 to 1986. A comparison of the two shows that zone boundaries have changed and each zone is warmer. According to the USDA, if your hardiness zone has changed, it does not mean you should change what you are growing in your garden. What is growing successfully now will most likely continue to be successful. The USDA also stated that since the map represents 30-year averages of extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of global warming.
Hardiness zones are based on the average, not the lowest temperature that has ever occurred or might occur. Gardeners should keep that in mind when selecting plants, especially if they choose to “push” their hardiness zone by growing plants not rated for their zone. And while this edition of the USDA PHZM is the most detailed to date, there might still be micro-climates that are too small to be detected.
The new map has a higher level of resolution and can show smaller areas of zone delineations. For example, cities hold more heat because they have large amounts of concrete and blacktop, so a city or town may be assigned to a zone warmer than the surrounding countryside. Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain may be an area of cooler zones. A location near a large body of unfrozen water may provide milder winter weather and be in a warmer zone.
While these numbers are certainly interesting, the USDA also encourages gardeners to rely on their own experience before making any sweeping changes to their practices and plant choices.