The USDA plant hardiness zones paint with a very broad brush. Your ACTUAL conditions are:
A. greatly affected by your local terraine, right down to your own garden site, and
B. changing pretty fast.

Our summers are becoming hotter, out here. We might moan about living in a fogbelt, but we're losing that, and we miss it.

Out here in the West, Sunset divides the USDA zones down MUCH more minutely. USDA thinks we live in Z9, I think -- but we are in Sunset Z24, and our hillside is on the mild end of that, because cold flows away from us.

Jeri

Your average annual low is not 28. It is remotely possible that was your lowest temperature last year, but it certainly isn't an average over anything like a reasonable time span.

Also depends on how built-up your immediate area is. Back 50 years ago when my area was still mostly citrus groves, frost and morning mist was a regular occurrence according to those who grew up here. Now that everything is well covered with asphalt, concrete, and roof shingles, those two weather events are occurring once or twice every 5 years.

Here is a link that might be useful: heat islands

These are hardiness zones, so they refer to the lowest temperature that should be expected over a winter, not to any average. I think Nashville is 7a on the new map, meaning that temperatures are not expected to fall below zero.

What I have read a number of times is that the cold hardiness zone is the average of the lowest temperature in a given area over a ten year period. The period the new zones are based on is 1996-2005. Many zones had warmer winters during that period, including our part of Idaho, and we were bumped up from zone 6 to zone 7. This is strictly a cold hardiness zone, and doesn't have anything to do with how hot a location gets. Diane

I got a kick out of my place being in a different zone than my moms when the USDA did their update. It is because my zip code says everyone here lives 15 miles up the coast at a state beach (Gaviota)

For some reason my moms place is in a higher zone, even though she is more likely to get frost and heat than I am. No idea how they came up with that!

I like the Sunset zone much better because they do take in to account the actual weather patterns.

But the most important thing to know is all of the micro climates in your own yard. Just like the west side of your house is going to be hotter in the full sun than the east side. Or that cold air drains and follows water. Hills and valleys are different than flat lands. I can be sitting in the warm sun at my house and run up to a park on the other side of the freeway (a mile or so as the crow flys)and need a jacket in the fog. Zones cant cover everything, so they generalize.

The zone map doesn't go by the average low temperature. It goes by the lowest temperatures you can expect to get. The average is just that, all the highs and lows of your daily low temperatures added together and then divided by the number of days in the count to reach an "average" low temperature. If you can get temps down to zero, even if it's only for a day or so, you are in zone 6.

That's not to say you can't grow zone 7 plants though. Every yard has micro climates in it and if you know your yard and how the weather works in it there will be warm spots where you'll be able to push the envelope a little bit. I know that the back wall of my house is a hot spot year round. It faces south and gets full sun and has a huge expanse of concrete patio in front of it that absorbs and holds heat, even in the winter. Look for the warm spots in your yard and that's where you can try some of those zone 7 plants out.

Ok, here's a direct quote from the USDA on what the zone hardiness map means: "The 2012 USDA Plant Hardiness Zone Map...is based on the average annual minimum winter temperature, divided into 10-degree F zones." I need to make a correction to my earlier post. The average annual minimum temperatures were based on the years 1976-2005 (thirty years) rather that ten years. Diane

I live 2 miles from the interior of the city. I have an 8 ft fence surrounding my tiny patch of earth. My garden is primarily southern exposure.
I'd say I'm at least a 7. So if I'm growing grafteds hardy to 6 does this mean I don't need to winter protect?
Thanks y'all.
Susan

Two miles from Nashville city center is pretty definitely a zone 7. That doesn't mean that it can't get colder, it just means that it hasn't recently.

Most years, were you to winter cover with mulch, you'd have warm wet temperatures in January that would cause cankers that would do more damage than doing nothing. BUT get used to your roses trying to break what passes for dormancy in early January and that to be followed by colder February. That Feb may kill off new growth; your job will be to remove by finger pruning the new growth so it doesn't act as a loci for fungal activity (cankers).

Have you checked the weather underground almanac for the history of your area.....total historic highs and lows....it isn't that long since the 80s when our part of the world went down to -20s F for three days.

In 7a, grafts should be protected by setting them just slightly below grade. There is no need to protect the canes. If they are killed back in a rare subzero episode, the plant will recover. In a normal winter, attempts at protection will do more harm than good by encouraging stem canker, as Ann said.

I totally agree, michaelg. I've never winter protected my roses, whether our area was rated zone 6 or 7, in all the years I've grown them, and I've never lost a single one. They are all grafted and I set the graft union a little below grade as michaelg suggested. I'm a lot more afraid of canker in a cold wet spring than winter kill. Diane

Ok but just to complicate things they are in 5 gallon plastic nursery pots with graft several inches above grade. I cannot plant due to construction. Should I repot 30 or so roses? Thanks everyone.
Susan

Don't repot.
Do marshall your collection in one location, preferably in shade (winter shade) because repeated daytime heat/nightime cold would really confuse their growing cycles.
Mulch over the five gallon pots. You REALLY don't want the pots to get warm. Around the perimeter, you could just use bagged mulch that you can use elsewhere next spring. Then spread mulch upto the graft on the roses, It would be good if you could use pine needles right around the plant stems (Pine needles are fast draining, and when they break down, even the least bit, they are acidic and any mess up in pH tends to supress fungal problems).

It's too early to do any of this. What you need to do now is check the water retention of those pots. Are any of them prone to drying out faster than the others? If so, you need to add clay to the pots. Winter drought can kill roses, not so much those in the ground in our part of the world, but in pots they are more vulnerable to drying and not rehydrating.

Great Ann. Thanks--this is the info I've been looking for. Should I put them on the northside of the house? Deep cold shade? That would be the coldest darkest spot likely. I water about every 3 days. Wind seems to dry them out most quickly.
I'm assuming if I marshall them as you suggested that I will still need to water them?
As for marshalling them should I wait until freezing temps?
Feel free to email me personally
dicta@comcastDOT NET
Susan