The new USDA plant hardiness zones map, the first in more than 20 years, reflects Minnesota's trend toward milder winters, which allow some slightly warmer-climate plants to survive. Many Minnesota locations saw change in their rating. Most of southern Minnesota, including the metro area, jumped from 4A to 4B rating; a tiny part of the metro jumped to the 5A rank, matching the hardiness zone that covers virtually all of Iowa. Hardiness zones are based on the average of the coldest temperatures from 1976 through 2005.
Source: USDA, United States National Arboretum