Pushing The Boarders USA Style

[LostInJapan]

The newest cold hardiness zone map has been released from the U.S. Department of Agriculture (2023). By the looks of it, many places have "jumped" a half zone since the last issuance back in 2012. For example, two places I've lived, Westchester County, New York and Gainesville, Florida, have gone from zone 6b to 7a, and 8b to 9a, respectively. These signify large movements for gardeners if they hold. Consider that:

Orlando is now 10a
Miami is considered to be zone 11a, and likely higher in protected pockets
New Orleans is zone 10a
Hilton Head Island, Georgia is zone 9a
The islands off of Brownsville, Texas are zone 10b
San Francisco is zone 10b
Coastal Oregon in many places is now zone 9a
New York City is zone 7b

And so on. Essentially any place rated 10b or higher is capable of sustaining truly tropical vegetation year round with only occasional cold snaps to worry about. Zone 10a is also good enough for many tropicals (as my recent trip to Orlando revealed). Zone 9a is getting more marginal for tropical plants, but low growing herbaceous stuff planted under cover will survive, and "cold hardy tropicals" like queen palms and Norfolk Island pine are not out of the question.

And so on. Of course seasonal average rainfall and precipitation, as well as geographic factors, also factor in, but I wouldn't be surprised to see more folks "pushing the borders" even more in coming years. Here's the map:

https://planthardiness.ars.usda.gov/

How about you all in Europe? Seen any major changes in the past decade?

[LostInJapan]

Really? No interest? I envision Trachycarpus lining 5th Avenue in NYC, a new tropical garden like Fairchild in New Orleans...

[charlie]

Not sure about mainland europe but here in the UK we dont find the US climate zone system works to well, but you are right that things have warmed considerably in a single life time, trouble for the UK is it only takes one cold night to destroy the benefit of an otherwise mild winter and at 50N we are always going to get them!

I am more concerned about changes to the rainfall pattern, this winter has been terrible if its not actually raining its been grey and drizzly, everything is like a swamp!!

[Paul S]

Very much this. We just aren't keyed in to zones here. If anything we are more focussed on how hardy a plant is, rather that what areas a plant will survuve in - looking at it from the other end of the lens.

[Chad]

I think people avoid this subject area to post on in order to avoid 'politics'.

In Cornwall we are in a colder winter phase, since 2010 most of the inland Phoenix canariensis have died, even quite well established ones.

W. Arnold-Forster's "Shrubs for the Milder Counties" first published in 1948 was a reflection of the milder winters between the world wars: we haven't had such a long run of mild winters since.

The problem is taking short runs of 'weather' and thinking they mean 'climate'.

30 years averages are usually taken to be 'climate', and 'scientifically' it is surprising how few weather recording series have an unbroken 60 year data set. Even those that do often have problems with changing surroundings such as urban heat islands or airports with extensive tarmac and 'plane exhausts.

With the Atlantic Multi-decadal Oscillation having a periodicity of 60 -80 years, trying to disentangle cyclical from 'real' changes probably needs a much longer data set to be confident of observed 'change' in Western Europe.

And in maritime Cornwall we are as much limited by lack of summer heat as we are by winter chills. Otherwise we could could, for example, flower Jacaranda mimosifolia. We can't!

Chad

[MICHAEL]

I agree with Chad. The US zone system has always interested me, but it also always leaves me wondering if absolute annual minimum temperature could be the deciding factor for all plants in all contexts, Here, of course, really it's heat/sun/drought that are more likely to impose limits on what will grow - or at least what will grow in the ground out in the open. I'm about 1km from the coast and in zone 11a...compared to anywhere in southern Florida, we have a cooler winter overall, but never get the (ant0arctic incursions that obliterate their syagrus etc,..so I get why minimum temps matter in much of the US, but it is only part of the story here - if at all - many of the palms they struggle to keep going for years on end are no problem here. Yet, as we've had very little rain for months and months there are dead eucalypts scattered about all over the place - a dead plant due to the weather is a dead plant.

[MartinGilmour]

I do agree that zones can be a little meaningless to plant survival in a given area with so many factors at play. And even less useful for predicting how climate change may alter what we grow in our gardens. This in no way casts doubt on the reality of climate change (for example, I believe that even the chap who coined the phrase Atlantic Multidecadal Oscillation' now concludes that it doesn't exist).

But I seriously doubt that climate change will mean that my children will be able to grow an array of different plants in SE UK to those I can grow. A freezing night that would kill, for example, an Echium candicans, will still occur (even if less frequently) in the coming decades I am sure

[Chad]

This in no way casts doubt on the reality of climate change (for example, I believe that even the chap who coined the phrase Atlantic Multidecadal Oscillation' now concludes that it doesn't exist).

I agree, and I hope my scepticism of the details doesn't get mistaken for scepticism of the larger picture.

Michael Mann, to whom you refer here, has replaced the AMO with responses to volcanism which still leaves a 'periodic' influence. His work is not universally lauded.

Chad

[LostInJapan]

That's the spirit! Keep those comments coming please.

Even within the US, these cold hardiness zones need to be taken with a grain of salt, and always within the context of local conditions. If you plant a queen palm in coastal Mendocino County, California (zone 9b) and one in Leesburg in Lakeland County, Florida (also zone 9b), you definitely will end up with different results. So any shifts are more interesting regionally rather than across drastically different climates. That said, as Chad noted 30 year averages don't necessarily represent long term shifts in climate per se, but could be the result of other cyclic weather phenomena.

As an example, oranges were grown as far north as southern Georgia and South Carolina into the 1880s, but that all came to a sudden halt with the great freezes of the 1890s. So the industry moved south into peninsular Florida, more freezes came again, thus driving growing even further south. By the end of the 1980s, any groves north of Orlando were mostly wiped out. And of course good growing conditions aren't only about temperatures, or even rainfall. The citrus industry in Florida is tanking in a serious way now due to another problem, citrus greening, which is about to essentially eliminate commercial growing within the very near future. And all of this is happening when the climate seems to be getting warmer... go figure. It is difficult to imagine Florida without orange groves, or any other citrus for that matter.

Still, it would be cool to see palms growing outdoors in Manhattan :mrgreen:

[Chad]

How widely are the USA Sunset Climate Zones used?

Chad