I think the standard deviation of a climate is an important consideration when analyzing its' characteristics. I've never seen anyone directly mention standard deviation as a factor but I think we all know the potential implications, not all zones of the same type/number are created equal. 

Consider Brownsville Texas, a zone 10a, similar to much of central coastal Florida right? But the all time record low in Brownsville is a whopping 12* F! That's 8a, two full zones lower than average, if you were to actually plot out all the lows I image in you would get a wide deviation in general. On the opposite end of the spectrum, consider Key West Florida, a zone 11b. The all time record low is 41*F, only 1/2 zone lower than the average yearly low. 

I would love to see some work done on this, along with the USDA number, also see some kind of quantification of the standard deviation, it might make it easier to estimate just how much chance your favorite Palm might have. Not all zones of the same number are created equal, this would be a good metric to differentiate them a little more. 

I did some work on this and came to the conclusion standard deviation is only partially useful. In an ideal world temps would be normally distributed then you could calculate with some confidence the probability of the temp dropping below XXf is x% and then you could calculate the odds of survival for any given palm.  The problem is low temps aren't normally distributed and outlier events occur more frequently than the data suggests they should. 

I think if you live in a 10a climate that might see a 8a cold event then you're options are seriously limited, unless you are happy with growing palms as pseudo-annuals! For some people that might be ok, for others (including myself) it will be a case of not going there.

The USDA zones are only as useful as the information input.  Obviously the average coldest temperature of the year is an essential starting point, but the extreme lowest temperature expected in the lifetime of the desired plant is much more important. If you live in a 10a climate and expect a freak 8a low once a century, then maybe worth the risk. If you expect it every decade, then probably not, IMO. OTOH in your hot summers you get great growth so maybe the short term gain is worth the pain.

Outside the US it is a different story. My case as an example, the nearest official climate station to me has recorded the extreme temperatures over the last year as 5C (41F) low in July, and 30C (86F) high in January. The previous year the extreme low was 2C (35F), and the extreme high 27C (81F). At my house I recorded 0C (32F) once back about 20 years ago. So in a climate like mine the USDA zone is actually the same as the standard deviation, the extreme all time record low is only a few degrees below the average annual low. But having a farm mostly in USDA zone 10b, with some 10a (and possibly a bit of 9b and 11a) doesn't mean the same thing for growing tropicals as it would in US mainland due to the lack of heat.

When determining what will likely grow somewhere, average temps are the first consideration. Rainfall is less important as irrigation can enhance natural rainfall (unless the climate is too wet and the plant requires drier conditions). Then absolute minimums and extreme maximums come into consideration for long term survivability of a plant. After that it gets more complicated as you're probably considering the microclimate the plant will be subjected too. Then the climate can vary year on year giving you cool years and warm years, which can either tip your microclimate into more favourable conditions or tip it into unfavourable conditions. A highly fluctuating climate is very hard to predict, and a harder place to grow marginal plants long term.

1 hour ago, Bennz said:

I think if you live in a 10a climate that might see a 8a cold event then you're options are seriously limited, unless you are happy with growing palms as pseudo-annuals! For some people that might be ok, for others (including myself) it will be a case of not going there.

The USDA zones are only as useful as the information input.  Obviously the average coldest temperature of the year is an essential starting point, but the extreme lowest temperature expected in the lifetime of the desired plant is much more important. If you live in a 10a climate and expect a freak 8a low once a century, then maybe worth the risk. If you expect it every decade, then probably not, IMO. OTOH in your hot summers you get great growth so maybe the short term gain is worth the pain.

Outside the US it is a different story. My case as an example, the nearest official climate station to me has recorded the extreme temperatures over the last year as 5C (41F) low in July, and 30C (86F) high in January. The previous year the extreme low was 2C (35F), and the extreme high 27C (81F). At my house I recorded 0C (32F) once back about 20 years ago. So in a climate like mine the USDA zone is actually the same as the standard deviation, the extreme all time record low is only a few degrees below the average annual low. But having a farm mostly in USDA zone 10b, with some 10a (and possibly a bit of 9b and 11a) doesn't mean the same thing for growing tropicals as it would in US mainland due to the lack of heat.

That particular 8a record cold occurred in 1899, some have speculated that it was a 500 year event.  Currently there are quite a few large royal palms in that area that have survived for a long time, but everyone knows that any winter might be the one that wipes them out. I guess it all depends on how long is "long enough" to have enjoyed something that you have planted, and what you are willing to lose. 

As for me, if I lived down there, and I thought I could plant a royal, enjoy it for 15 years, then it dies from cold. I would probably do it anyway, then plant another ones and enjoy it as long as I could.