There seems to be a major issue on “Plant Hardiness Zones”. People feel that they are written in stone instead of a general guideline. This is far from the truth, especially in what is called a “Micro Climate”.
Heres part of an article from Wikipedia…
Microclimates exist, for example, near bodies of water which may cool the local atmosphere, or in heavy urban areas where brick, concrete, and asphalt absorb the sun’s energy, heat up, and re-radiate that heat to the ambient air: the resulting urban heat island is a kind of microclimate. Microclimates can be found in most places. Another place this can occur is when the ground is made of tar or concrete; because these are man-made objects, they do not take in much heat, but mainly reradiate it.
So there are many things that affect the climate in your area.
I remember leaving work one night and driving home. It was a spring like night with a trickle of rain. 5 Miles out, I hit a bump in the road and immediately was thrown into a snow storm. That bump was literally the line between spring like weather and snow.
Where I live, in Oklahoma, I have a hill to the west of me. I have many a night (The hill is only 1/2 mile away) watched it rain on the other side of the hill. My friend, 2 miles south of me, though we are in the same zone, he has to plant a month later!
A friend in Kentucky, who is in the same exact zone from me, waits for me to post when I am planting something, then adds 2 months!
Talking to old time Farmers I have learned that the plant hardiness zones are just a general guideline, nothing more. I learned I was starting potatoes a month and a half late!
You can have a great garden using the Planting zone map but remember to use general air and ground temperatures and weather forecasts also.
Planting zones are just a guideline!