My poorest performance from Mister Butters came two years ago after a frigid winter with virtually no snow cover. So I thought the problem was that Mister Butters was a bit tender. One can't go just by zones. If there is no snow cover and almost 20 below, that's much worse than a zone 4 or 5 with the same temperature. So Mister Butters may (or may not) have a relatively narrow range for optimal performance. It was okay this year, but not stellar. But many cultivars were not okay after the funky spring and extended dryness.
