Washington (CNN) -- If you're one of the 80 million gardeners in the United States, you're more than likely familiar with seed packets that display maps to help growers determine which plants are most likely to thrive in a certain region.
Now, the plant hardiness zone map has been updated by the U.S. Department of Agriculture, the first revamp in more than 20 years.
The plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. It covers all of the continental United States, plus Alaska, Hawaii and Puerto Rico.
The USDA released the map Wednesday. The department says it's more accurate and detailed, and for the first time offers an interactive format designed to be Internet-friendly. Users can type in a ZIP code and find the hardiness zone for their area.
The new map features 26 minimum extreme temperature zones, ranging from a minimum extreme low of 60 degrees below zero in northern Alaska to 45 degrees in southern Florida and California. The average annual minimum extreme temperature is 70 degrees in Puerto Rico.
"This is the most sophisticated Plant Hardiness Zone Map yet for the United States," said Catherine Woteki, USDA undersecretary for Research, Education and Economics. "The increases in accuracy and detail that this map represents will be extremely useful for gardeners and researchers."
Compared with the 1990 version, zone boundaries have now shifted in many areas. "The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States," the USDA announced in a news release. Researchers used data measured at weather stations over a 30-year period (1976-2005) in contrast to the 1990 map, which relied on temperature data from only a 13-year period (1974-86).
The USDA is touting the advances as more accurate and detailed, "especially in mountainous regions of the western United States." The department says in some cases, the updates reflect changes to cooler, rather than warmer, zones.
The updated zone map will not just be a help to horticulturists. Some crop insurance standards are based on the hardiness designations. In addition, scientists use the zones in research models to keep track of the spread of exotic weeds and insects.
The new map -- jointly developed by the USDA's Agricultural Research Service and Oregon State University's PRISM Climate Group -- is available at www.planthardiness.ars.usda.gov.