You can also watch your winter thermometer like a hawk on long winter nights, or go to a NOAA weather station as close as possible to your home and look for the lowest temp reached each winter, say for the last 20 years.
That lowest temp determines you Zone.
"USDA Hardiness Zone 7a" means that 50% of your winters get down as low as 0-5 degrees F at least once, but half of your winters don't. And that is the lowest temperature REACHED, not a "daily average" . Just the nigh-time low.
A different system for predicting what grows in your area is the Sunset Zone system. They take more into account than the lowest low. They tried to break the USA up into similar GROWING regions. They consider year-round temperature patterns including summers, rainfall amount, WHEN the rain falls, and I-don't-know-what-all.
You might be Sunset Zone 32, but read the description to see if that's correct:
Mid-Atlantic:
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ZONE 34. Lowlands and Coast from Gettysburg to North of Boston
Growing season: late April to late Oct.
Ample rainfall and humid summers are the norm.
Winters are variable ― typically fairly mild (around 20 degrees F/-7 degrees C), but with lows down to -3 degrees to -22 degrees F/-19 degrees to -30 degrees C if arctic air swoops in.
http://www.sunset.com/garden/c...
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Their description is so good that you can usually recognize which region you are in from the description, not just a map.
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If you want to get very academic, and to search the entire planet for climates similar to your own as regards plants that will grow there, try the Koppen Climate Classification System.
But you'll never see a packet of seeds labelled with the Koppen Zone!
http://garden.org/blogs/view/R...
scroll almost to the end
Many Weather and Climate Related Web Sites
Koppen Climate Classification System
~ Koppen Zones by County
http://koeppen-geiger.vu-wien....
New Jersey Ocean 34029 Koppen Zone4 "Cfa"
GROUP C: Temperate climates
The second letter indicates the precipitation pattern: "f"
" f means significant precipitation in all seasons"
The third letter indicates the degree of summer heat: "a"
"a" is the warmest summer category for temperate climate in the Koppen system:
"a indicates warmest month average temperature above 22 °C (72 °F)
with at least four months averaging above 10 °C,"
If you scroll down the Wikipedia article to "Warm Temperate climates (Cfa, Cwa)", you'll see a list of places int he world that have climates roughly similar to yours, for gardening purposes:
http://en.wikipedia.org/wiki/K...
Wow, those are pretty hot places. Maybe you're really more like a Maritime climate?
