I'm often asked what zone are we: referring to the USDA winter hardiness zone map. The hardiness zone map links areas together that have, on average, similar winter low temperatures. They are broken down into 10F degree intervals; 5F degrees into sub zones a & b. Most plants are given a hardiness zone rating to correspond with this mapped area. On existing maps we are in hardiness zone 5b at Powell Gardens so on average our winter low should be between -10F and -15F. Since we had -11F at our official weather station this past winter that looks right on. The only thing is that if you average our winter lows out for the past 15 years we would average around -5F here. In the past 15 years we've been as cold as -12F and as mild as +17F; with a reading of -27F in 1989! I was just in Wichita where it allegedly got to minus 17F last winter -- but at Botanica, the Wichita Gardens, I saw no or little damage to tender zone 6 plants. Clearly there is more to it than the minimum low.
I had to take a picture of our 'Taylor' Windmill Palm (Trachycarpus fortunei). We planted this many years ago for trial as a below zero hardy palm. It has always died back but funny thing is this one always sends up a new basal shoot every summer, only to be killed back the next winter. Sure it can survive a rare below zero event in North Carolina but not the predictable and sustained below zero weather here. You could go to a lot of trouble and put winter protection around a plant like this and it might survive better.
Our Bracken's Brown Beauty Southern Magnolias (Magnolia grandiflora) are living up to their name: brown. At least that's the way they look from their southern side where the leaves were winter burned (consistently the most burned of our hardy southern magnolias each year). Evergreen shrub China Girl Hollies are fine at its base.