The USDA plant hardiness zone map splits the country into 11 zones, with a and b half zones, based on average winter minimum temperatures over the past 30 years. Although not the last word on plant hardiness and survival, the map has been a touchstone for gardeners to compare plants and determine which are best for their area.
The USDA Hardiness zone map was last updated in 2012 and now they have a new 2023 version. As you might expect with global warming, the hardiness zones have shifted. In 2012 there was a significant area of Northwest Connecticut in zone 5 (winter minimum temperatures between minus 10 and 20). That area is gone. Most of Connecticut is firmly established in hardiness zone 6 (winter minimums of zero to minus 10) with a large area along the Long Island Sound in hardiness zone 7. In general across the country, there has been a one half zone shift towards warmer winters.
While the winter minimum temperatures might be warming, that doesn't mean we won't get winter cold snaps that will plunge the temperatures well below those minimums. The zones are based on averages, not isolated events. Plants can be killed if the temperatures in winter dip too low for a period of time, regardless of what the map says.
So, the recommendation is to grow plants well established in our hardiness zone and, if you're testing marginally hardy plants based on the map, plant them in micro climate areas such as near the house, garage, or the shelter of other trees and shrubs where they are protected from the cold.
