I still remember the day it "snowed" in Monterrey, my city because the winds that came from the north (USA) were too cold. I lost two Cocos nucifera and one Phoenix roebelenii that day, the snow only lasted like 3 hours during the day because the sun started rising fast and the ice was melting. A lot of tropical plants were lost that day all around the city. Here are some local Roystonea regia that survived the freeze, yes that house has Roystonea regia outside and in the garden and all of them survived except the one next to the biggest one which was replaced by another Royal, it's impressive how cold hardy these palms are, they are actually starting to be more common around here. A couple of mature coconuts, foxtails and adonidias survived that day as well.

14/02/2021

Recovery / 2021 - 2022

Replacement

I think they are a lot more hardy than people think. Winter 2020 we reached -5.6 celcius here for a brief time in the night and we had perhaps 5 nights it went down to -3 as well as countless nights with -1. 

The Royals did well, the worst damage I saw was roughly 50% burnt foliage. That palm recovered very fast in the next few months.

Odd that the Robilinis died for you. They had no damage here. 

Royals are one TUFF palm they take hellish heat and some cold and just keep on trucking!!!

Remarkable!

After a certain size, (let us say, when petiole obtains the girth of about 1 ½ hand finger, plant becomes substantially more cold tolerant, as long as roots are not exposed to air temperature (ie not potted but planted in the ground specimen).

When I lived in arizona, a royal at treeland nursery was defoliated by a short radiational freeze to 18F.  6 hrs later from that low it was 50F so it was a fast radiational event.  Slower events may kill them outright at 25=26F.  I have 5 juveniles killed in 2010 at 28, 29F on consecutive nights but the large adult royals just lost the lower 1/3 of leaves in the crown.  Some cold events have a large temperature gradient between the ground and say 15' above ground and its the temp of the bud of the palm that determines death.  Zones dont tell the whole story we have discussed this in the past that its the plant tissue temperature not air temperature the determines death.   Plant tissue temperature drops more slowly than air temperature that is why air temp is unreliable in determining hardiness in cases where the temperature moves quickly upwards after the minimum.  For 9B florida you need to get them through that size window to have the best chance.  And yes up against a warm building that plant tissue temperature will be notably warmer that out away from it.

What size were the juveniles? I have 3 new this year, early plantings, that im worried about. 2-3ft clear wood so not babies but not established

I’m not too surprised. I’ve seen some really massive royals and thought “how can this thing freeze to death when it has a 3’ wide trunk?” I think their size can provide some serious insulation.

3 hours ago, Bkue said:

What size were the juveniles? I have 3 new this year, early plantings, that im worried about. 2-3ft clear wood so not babies but not established

4-5 meters tall

10 hours ago, Bkue said:

What size were the juveniles? I have 3 new this year, early plantings, that im worried about. 2-3ft clear wood so not babies but not established

15 gallon size, not trunking was the size of those killed.  I would wrap them at this size for a couple years to protect the bud.  As they gain height they will become more hardy.  2-3' of clear trunk sounds like they are well on their way.  On my second try I planted some bigger ones with 3-4' clear trunk.  They have burned 2x in  29-30F but came right back.  They also recovered from (2)cat one hurricanes in about a year to 15 months, growing ~12 new leaves the following year. 

They are cold hardy if it gets to -2º, but the next day is 20º. They would probably survive that, but won't survive prolonged cold, if is 10º constant for 3 months they just die.  And hate moisture when it's cold.

Pretty amazing when you see something like this, survived 22F in February 2021 in McAllen, TX. These aren't particularly big nor old. No concrete, no wind block, no canopy, no water moderation, etc just the power of low latitude and sunshine.  While big old royals had the highest survival rate overall, plenty of young ones survived too. 

2015

 

2 months after 22F

 

20 months after 22F

1 hour ago, Brandon39.5 said:

They are cold hardy if it gets to -2º, but the next day is 20º. They would probably survive that, but won't survive prolonged cold, if is 10º constant for 3 months they just die.  And hate moisture when it's cold.

my nights are like 4 degrees right now and its pumping out its last frond of the season it looks like.

34 minutes ago, Xenon said:

Pretty amazing when you see something like this, survived 22F in February 2021 in McAllen, TX. These aren't particularly big nor old. No concrete, no wind block, no canopy, no water moderation, etc just the power of low latitude and sunshine.  While big old royals had the highest survival rate overall, plenty of young ones survived too. 

2015

 

2 months after 22F

 

20 months after 22F

Definitly more hardy than given credit for. Was the 22f just a brief dip at night? 24/-5.6 here for a brief moment and the Royals around here had anywhere from mild burn to 50% all recovered quickly.