In spite of cold conditions this winter, the trend to warmer minimum temperatures since 1970 has been for higher annual extreme minimum temperatures across the Southeast (and the US). Climate Central has published a set of graphs showing how the lowest temperature each station reported each year starting in 1970 has trended upward over time. Two things to keep in mind: 1970 was the coldest period in the last 100 years or so, which would tend to heighten the upward trend; and most of the records show are for cities with airports, so some increase in temperature due to the growth of cities is possible, although many airports are located far from city centers. The trends are also seen in rural areas, however, so this argument has limited value. You can pick out your nearest airport city and see the graphs here. Since the USDA plant hardiness map is tied to the minimum annual temperature, it is no surprise that plant hardiness zones have also been changing over time.