Great question, Bob. My interpretation is that the USDA zones are exactly what they say they are "hardiness" zones. So they really only tell you - on average! - what will survive dependably through the winters where you live. This info is much more useful in colder zones than for me, for example, living in Florida.
So when a plant at a nursery is labeled "zones 4 to 9" well sure it will probably survive winters in those zones, but that doesn't mean it will grow as well in zone 9 as it will in zone 4. The operative word is "survival" and you don't want your ornamental plants OR your fruits and vegetables to just survive. You want them to thrive.
I live in zone 9a or 9b (depending which map you look at) now but lived (and still garden at my daughter's) in Salt Lake City, Utah, which was then zones 4 or 5 but is now designated zone 6 depending what part of town you live in. Altitude makes a huge difference to the success of plants, as well as humidity - or lack thereof - and summer high temperatures. A windy exposure can also make the sink or swim difference to a plant especially in a dry climate.
So I guess what I'm saying is I'm with Sally, as she says above "try it and see". If I eliminate a plant by the trial and error method, I don't keep on buying it just because the label says it's hardy in my zone. I might try it in a couple of different areas of my garden, though. For example daylilies need a little bit of winter chill at least, and so I've now found that I can grow them on the north side of my house where they are in the shade all winter, so the soil cools off, plants stay cooler in the daytime and get colder at night than they did when I planted them in a sunnier spot.
On the other hand, I can't make lavender (or sage) survive our summers dependably even though it is supposedly "hardy". It just doesn't do well with the high humidity we suffer for at least 4 months. If I can place it where it gets very little rain, such as under an overhang, and strictly control how much water it gets, it might limp through a summer (once or twice in 15 years) but never grows well the next year anyway. So
it's treated as an annual in my garden. I buy it in the fall and just let it peter out come June or so.
