I live in zone 4b, & I'm forever finding plants that I fall in love with, only to then read "zone 5". I live in the extreme southern part of zone 4. I'm guessing this is where the term "marginally hardy" would come in? So for these zone 5 "wishlist plants" of mine, is it possible that many of them might do just fine in my garden? I mean, how much difference can there be across a span of 10 miles?!?
You've hit on a common concern. Gardeners are always trying to extend the limits of plant growth! Hardiness Zones ratings are crude measures at best. They use average minimum winter low temperatures as the basis for Hardiness Zone divisions. Other factors such as winter moisture, temperature fluctuations, and wind can also affect hardiness. If you'd like to try Zone 5 plants plant them in a protected location and protect them over winter. You may be able to create a warmer microclimate in which the plants will thrive. But be prepared to occasionally lose some plants during particularly severe winters.