Good day, I am seriously questioning the USDA Plant Hardiness Zone Map.  I'm located in Charlotte, North Carolina.  My zip code says I'm in zone 8a.  For the past years except 2013, my zone was 7b.  But other zip codes in Charlotte says 7b.  Mine is the only one that's registered as 8a.  I don't understand.  I just emailed the USDA questioning this.  I just want to know what makes my area hotter than others in town.  I think my zone is really 7b.  I'll post their reply soon.

I know what you mean Dennis. Los Angeles is a zone 9 (a,b...whatever) but when I entered my zip-code it said I am in 10A zone. this must have to do the wind feed and elevation factors maybe.

I see an opportunity for you to flatter She Who Tolerates The Figs:

"See honey, you're so hot even the USDA notices!"  

Dennis, it is your fig forest that has made a micro-climate change!!  :)

Seriously, my understanding is that they are claiming there are warming changes related to urbanization - heat from buildings, radiation of sunlight from manmade surfaces, etc

Any given zip code is probably part of a larger zip code set.  Meaning that they probably don't have good low temperature information for each and every zip code that is out there.  They might have good, credible data for maybe one or two points in a county and maybe there are 4 or 5 zip codes in a county.  Or perhaps a certain zip code spans two counties, and they have to decide which to stick it in.  Or maybe they don't do it by county, maybe they do it by city or region.  At any rate, they have actual data for a certain number of physical locations, and they have to then map this data to cover all zip codes.  Certainly this process has its nuances and quirks.

That being said, assuming it's not some sort of error, there are at least two plausible reasons that you could see adjacent zip codes with different hardiness zones:

1.  If you are near the edge of the 7b/8a line.  We're talking about a 5 degree differential here, so even though your particular zip code says 8a, it's likely that you're really in between. 
2.  If there is a microclimate where your zip code is.  For example, if you are on the eastern edge of a large lake or something like this, could shave a couple degrees off low temperatures, thereby raising your hardiness zone.

California has so many micro-climates, Sunset Western Garden magazine broke down the USDA zones even further.  I am Sunset Zone 19 and USDA Zone 9a.  Two towns over, they get snow that actually sticks, so they are in a different zone. 

It's colder in the valley we overlook than it is up on the hill.  Weird, but the citrus growers below have turbines because their temps drop to freezing.  You can hear them go on at nights when the temps drop, but you can visibly see the temps change in your car as you drive up the hill from the valley below.  Weird.

Suzi

If you're in Charlotte proper, I'd wager the heat island effect is why you've moved up a little. I'm sure it wouldn't hurt to pretend it's still a zone colder for those once in a long while cold snap/polar vortex events.

Wow!  USDA responded!  They said they will double check and let me know.  Now that's what I call service!  They must still be working on my Phytosanitary Certificate I requested.  In other words, my contact information is in their system.

Here is the link to an interactive USDA map <http://planthardiness.ars.usda.gov/phzmweb/interactivemap.aspx>

You can zoom this thing in down to the level of main roads. It will tell you exactly where  you are in relation to a zone boundary.

Wow!  Neat link!  It changed my whole zone!  I wondered why Avocados do well here.  I've gone from 9a to 9b, and the difference is altitude.  Bookmarked it!

Suzi

All of the zip codes I've ever lived at (28211, 28226, 28277) in the Charlotte city limits are 8a. Downtown is still 7b according to the tool.

We are 7a here in Fair Lawn NJ on the new USDA map. We were 6B on the old one. I wonder if the granularity increased and they are identifying more micro climates?

According to this I am extrapolating from across the lake from ohio, I would be a zone 7a, which some Canadian sources say I am on the canadian hardiness maps as well, but it seems to bounce around 6b and 7a.

Garden web had me at 7 and the Arborist  site has me at 8....I think I'll settle for 7.5...

Please explain Sunset Garden Zones vs. USDA Zones.....do I/should I .....pay attention to both when selecting a variety of fig?

Will knowing my the Sunset Zone give me an indication as to whether or not a fig will RIPEN in my climate zone?  Keeping a fig tree alive is one thing, but keeping it alive and eating ripe figs seems to be the goal....

Are the "Sunset Zones" related to the heat units, or, heat-days, for a given area?  I never understood any of this, and have read very little about this other type of climate information.  In fact, catalogs almost never, if ever, list a plant with a Sunset Climate Zone recommendation....especially when related to figs.

I'm confused.  Please give me the short and quick, and smarten me up.


Frank