For hardiness itself, if a plant can survive a zone 4 winter then it better be able to survive a zone 7. My initial question was the second number and as I understand Daisy's explanation it has to do with needs of the plant for blooming and/or other things which have nothing to do with just surviving winter.
I agree with all of you that planting is about location, location, location. One spot may be shaded nearly all day while another is sun all day. Exposure to/protection from wind will affect them positively and negatively depending on temperatures & humidity - the same wind that requires protecting a rose in winter is wonderful for helping dry it and preventing fungi problems in summer. No spot is necessarily bulletproof year round, we have to be diligent and aware every season.
Gardening is rewarding; it is also challenging. If all that was involved was sticking a plant in the ground and walking away it might be easier, but how much fun would it be? "Having" to tend the garden gives us time to stop and spend a moment with each one we planted, to appreciate its uniqueness, maybe to reflect on the idea that sometimes it is growing in spite of us. It can be humbling to realize we might have planted for "me", but an entire community of bugs, birds, and critters are living out their lives from and around it. The challenge of placing it where IT needs to grow rather than where it's best for US helps remind us the world does not revolve around us. I appreciate being able to walk out on my porch to look at orchids rather than having to tromp through a rain forest for days to see it in its "real" world, same with any plant in the garden. The privilege of having the plants of our choice close at hand is worth the little bit of effort to determine which ones to plant and exactly where.
