Yep, me too. I am now in 8. I had been expecting this, because of the temps. My problem is the "swings". Very late freeze or a very low winter temp. The joke in the South is, "Don't put up your shorts, it will be 80* on Christmas!" It has of course happened before, but it now happens much more often.

Yeah, I'm still gonna play it safe when it comes to plant hardiness. The new map says I am now in zone 6b. Yet just a few years ago the temp dropped down to -13*F, which is the kind of temps zone 5b experiences.

Rodney

I've been seeing news headlines about this for the past few days. One article I read this morning stated that zones are determined using the lowest recorded temperature for each area - which can be highly variable nowadays, I think.

My zone has changed a bit too - officially. I've been unofficially using zone 10 info for years now.

As a yearly average it may be correct. But wouldn't a 10 year or 20 year assessment of low temperatures be more useful for perennials and trees? Having a damaging Winter event every 5 or 10 years should be a consideration.

Still in 7b here but I think that being squeezed in-between the Appalachian Mountains and the Chesapeake Bay keep things a bit more moderated here. Thx for posting the map site.

A long-term assessment might be a good idea, but it wouldn't be the USDA Plant Hardiness Zone Map. USDA says ...

"The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which perennial plants are most likely to thrive at a location."

Most perennial plants farmed by gardeners and growers don't grow for 5-10 years. Trees sort of grow by themselves.

Like the dates of last and first frost, this is an average metric. The zones are NOT about the lowest recorded temperature at a site. It's about being likely to thrive. Useful for planning purposes. No question that it might sometimes get colder than what the zone says it should.

If you investigate how the map was created, other than advanced GPS technology offering much greater detail, the database remains the same - it still covers a 30 year period (1991-2020 for the 2023 map). And it only reports the average minimum low experienced over that 30 year period. Temperatures could of course drop below that but on average, they do not.

The thing that I keep thinking about is how the changing climate also means new insects and diseases in areas where they were not a concern before. I feel like that could negate some of the expanded growing ranges.

The last map was published in 2012, so the new map certainly averages more recent years. The database for this map is also for a 30-year average, but not the same 30-years as for the previous map. So the database does not remain the same, though the database duration does.

But yes, as noted, you can't depend on it to tell you absolutely what the worst freeze will be. Just like for the average first and last frost dates, you can't depend on those dates to absolutely be the dates of first and last frost.

This is the article I read yesterday:

https://www.npr.org/2023/11/17/1213600629/-it-feels-like-im-not-crazy-gardeners-arent-surprised-as-usda-updates-key-map

And we haven't had temp.s here below 40, I think, for the past several winters - no freezing at all.

It will be interesting to see what El Niño brings us this winter.

The past two winters we've had temperatures of 4F. Way, way below our averages for what used to be our zone 8b. Seriously, maybe half of our winters we don't even get a freeze! I covered all my veggies with multiple layers, brought my porch plants inside, and most survived. But a couple of my sago palms planted in the ground were badly injured, even though they were lightly covered, and two pittosporum trees were killed. Averages being what they are, I have to assume that we're going to get a few heat waves in January in the next few years to make up for that!

Some research suggests that climate change is destabilizing the jet stream and polar vortex allowing it to meander further south, which can make winters suddenly colder. So although the mean is warming, brutal cold snaps are more likely.

The coldest temp this year so far was sub zero back in Jan and Feb. Our primary home in NY changed from zone 6a to 6b, our other home in PA changed from 5b to 6a. I have so many potted rose trees, during the winter months they are stored in the basement and garages. Normally rose trees can only grow in pots here due to the grafted part won’t survive the winter. This year I planted 12 grafted hardy rose trees in the garden. Will see if they can survive this winter.

" My initial comment about playing it safe is further reinforced by this info. I knew it was about average minimum temperatures, not necessarily the coldest it has ever gotten in a location, but I think that bumping almost everyone up half a zone or more knowing that it can get colder in their location (and has done so very recently) may be a bit irresponsible. And I don't know how they can use the word "thrive" the way they do (used elsewhere on their site, as Dan's comment shows)."

That is my opinion also. Those maps are supposed to be a guide for selecting stock - including fruit trees & vines - which will survive in a given zone. They're pushing climate change, when the long-term risk of a colder winter should be the focus. If I remember, the Arbor Day Foundation tried to push a similar updated map a few years ago - and they were wrong. We had a bad winter a few years ago; got down to -28 in my area... the new map claims I now have warmed to -10F??? A lot of people lost grapes and fruit trees that were borderline. I wouldn't just take that map with a grain of salt, I'd throw it in the compost.

It's all about averages. You can't argue with averages. As I've said several times, we tend to abide by the average date of last frost when we plant out, knowing full well that we might still get a frost after that date. Should we toss those frost dates in the trash because they might not be quite right? There is a nice explanation of the new map at https://www.ars.usda.gov/news-events/news/research-news/2023/usda-unveils-updated-plant-hardiness-zone-map/. 

Not at all clear that those maps are intended for trees. I guess it comes down to definitions. Maybe USDA should be more forthcoming about these maps not being appropriate for orchardists as opposed to "gardeners and growers". Let's face it, at least for a home gardener, for crops that aren't trees or large aged vines, it's all about protection. What are the chances that you'll need protection to preserve your crop? When my temperatures dipped a zone or two below my own in the last few years, it just took a tarp or two to protect. Same with when I get an unexpected frost after planting out frost-intolerant summer veggies. That's because the cold snaps were sudden, and over soil that was a lot warmer. Certainly, if you have weeks of intolerable cold, you have a real problem with survival. For those with acres of crops, yep, you just lose the crop. But there are many reasons why you might lose a crop, and abiding by zones is just playing the odds of averages. Farmers are always playing odds.

As noted in the explanation I pointed to above, the USDA Risk Management Agency refers to the map’s plant hardiness zone designations to set certain crop insurance standards. The risk here is assumed to be one year for one crop rather than many years for survival of an entire tree. But yes, the word "thrive" here applies to a yearly crop, and not to long term survival of trees.

Yes, it is the swings that are getting me. Most of the year it is much warmer than normal. However, just last Christmas we went down to 9* for three days. Pipes froze, we had rolling blackouts because of electric demands, and a lots of things that should of been fine, either died or were damaged.

i have been joking for years that my house is sometimes in zone 9 and sometimes in zone 5.