My 8a destination last weekend was Dallas, Texas - half a continent and an entire universe away, at least when it comes to plants. There were no flying monkeys, but one quick look around told me I wasn't in Cary anymore. Yes, there are some common denominator plants - crape myrtle, butterfly weed, switch grass, and the ubiquitous Knockout rose - but the similarity ends there. Its topography and its hot, dry climate are worlds away from the Southeast. So much for the zone hardiness map.
Back before I knew what I was doing, I did what every other novice gardener does: saw something pretty, checked out its cold hardiness, and stuck it in the ground. Maybe it was my Northeastern, cold-centric mindset, but I assumed that not freezing to death was the only thing a plant had to worry about. Seven years and countless dead plants later (my kill rate is well over 30%), I have learned that winter temperatures are just the tip of the iceberg. Heat, humidity, soil type, and annual rainfall matter just as much.
Poppies, peonies, and tulips (hardy from zones 3-8) need a period of winter chill in order to flower. Hostas (also zones 3-8) always seem to look better in the north than the south. They hate heat, as do fellow shade plants brunnera and pulmonaria, so don't be fooled by that zone 8 hardiness listing. The agastaches I insist upon growing (zones 4 - 9) may do fine in Dallas, but they have trouble with clay and wet winters (hello, Cary). For me, they will not survive more than a year or two, no matter how much Permatill and shredded pine bark I add to the soil. Ditto those cold-hardy agaves and the biggest heartbreaker of them all, Mexican bush sage.