The United States National Arboretum has updated its USDA Plant Hardiness Zone Map for 2012. This is the first official update since 1990, and more than 80 million potential gardeners have moved a half-zone warmer.
If you're not familiar, here's the low down:
The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
Although the map doesn't let you zoom very far in when you click on the state, and the color variations are a bit hard on my eyes, it appears that Wake County (where I live, home to the state capitol of Raleigh) is still in Zone 7b, but most of eastern north carolina has moved into Zone 8a - including the county just east of Wake County (Johnston County), and the one just south of Wake County (Harnett County). I live maybe 15 miles from Harnett County.
So, your zone may have changed.
Don't let this fool you into thinking you can plant any earlier. If you looked up your average last frost date last year, it's not gonna be any different. Do NOT use USDA Zones to determine last frost date. I know, some of you are saying "Who would do that?" Well, I give you about.com.
The USDA Hardiness Zone Maps only predict low temperatures. Some of us think "Wow you're in Zone 9, that must be hot!". Nope. It's not hot in North Bend, Oregon (Zone 9a). Well, I mean it's certainly nice there, maybe a little rainy.
My point is, that the Hardiness Zone Maps don't predict weather, climate, or heat. They ONLY predict cold so you know what to plant and what not to plant.
Zone 7b has a minimum temperature of 5-10 degrees. The web site is a bit misleading as it refers to the "average annual minimum winter temperature". Don't confuse this with the "average low", which for January in Raleigh is around 30 degrees. I don't actually know how they come up with 5-10. I've lived here since 1998 and I haven't seen it get below 10 even once. It's only been below 15 a couple of times. But the State Climatology Office informed me yesterday that the all time record low recorded at RDU International Airport was -9 degrees back in 1985.
So here's the deal. If something is hardy to Zone 7 (not specifically Zone 7b), it should be able to handle 0 degrees but it might not be able to handle -9 degrees, and the plant could thereby die and NOT come back - or be heavily damaged by the freeze.
The zone maps are not a guarantee. You can plant Zone 8 plants in Zone 7 if you want and they might survive a "normal" cold winter, but they probably won't survive an extreme cold winter like that. You are just taking a bit of a risk.
On a side note, some plants can't survive the low temps, but some plants NEED them too. The Blue Spruce, commonly referred to as the christmas tree in my home state of Michigan, is hardy only to Zone 8b. Frosts are rare in zone 9 I guess and these trees need a little freeze in the winter (they're also not very tolerant of it being 100 degrees).
So whatever you do, plant accordingly. Check your zone - it's possibly you've moved into a zone where you can more easily plant Palm Trees or perhaps a zone where Rosemary is a perrenial shrub (this is you, if you've moved from 6b to 7a!)