Plant hardiness zones are based on the average annual minimum temperature over a 30-year period, not the lowest temperature the thermometer has ever hit. They are guides to help you determine which plants will survive in your area. They are not steadfast law. Many circumstances besides hardiness zones, like soil quality, sun exposure, moisture and sudden or prolonged temperature swings, can affect how well plants will do in your yard.
There are also always microclimates that will vary within zones. (Even within yards.) Trust your own knowledge of your property and your garden.
You can download the map at the USDA Plant Hardiness Map Site
What's New in the 1990 Plant Hardiness Zone Maps (PHZM)?Appearance and Data
- The resolution is noticeably better. Mapping was improved by using a Geographic Information System (GIS). The higher resolution makes seeing the distinction between zones clearer and easier. It also makes it possible to separate small anomalies, like cities, which tend to be warmer than surrounding areas, cooler high elevation area, and areas near mitigating large bodies of water, into their proper zones.
- It's the first map designed to be viewed via the internet. Zones can be zoomed in on a finer scale.
- A more sophisticated algorithm was used to compile low-temperature values from actual weather reporting stations.
- The zones are based on 1976–2005 weather data. The 30 year period was chosen as a means to factor in year to year fluctuations and variations. They did a trial check on more recent data and it did not make a measurable difference in zones. (The previous map was based on 1974-1986 data.)
Climate Change Conclusions Climate changes are usually based on weather averages in a 50 to 100 year period. This map uses only a 30 year average and is not meant as evidence of temperature changes. While most zones are ½ zone warmer than the previous map, the change can in part be attributed to better mapping and weather tracking. The USDA points out that some mountainous regions that had been hard to observe are now in cooler zones.
There's an interactive map where you can pop in your zip code or click on the map and get your zone, actual average temperature, the temperature range for your zone and your longitude and latitude.