A new edition of the U.S. Department of Agriculture's plant hardiness map has been released, adding two new hardiness zones.

The map, for the first time, is specifically designed for the Internet, allowing people to examine zones in sharper detail, even going so far as to plug in Zip codes to see zones and expected minimum temperatures.

The zone for Hays, however, sharply underestimates the extreme low temperatures that have been recorded but does estimate average lows.

Hays, coincidentally, is in zone 6 -- a fairly wide swath that sweeps through the central part of the state. As expected, the southern third of Kansas is slightly warmer while the northern tier of counties is slightly cooler.

Zones in the 2012 edition of the map are based on weather data from 1976 through 2005.

"Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976-2005," the hardiness zone site explains. "This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period."

The previous edition of the map, published in 1990, was drawn from weather data for 1974-1986.

The 30-year data selection period was made based on the best balance of smoothing year-to-year weather variation.

Two new zones, for areas with average lows above 50 degrees, also can be used to help gardeners decide when to bring tropical plants indoors.

Compared to the 1990 version, zone boundaries in this edition of the map have shifted, generally a half-zone warmer than the previous map throughout much of the United States. The change is a result of the more recent averaging period, although some changes are due to better mapping abilities and a greater number of station observations.

The map can be found online at

A companion U.S. National Arboretum website includes a list of cold hardiness ratings for selected woody plants. That site can be found at