How to make an accurate cold-hardiness calculation

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People on PalmTalk always discuss specific minimum temperatures for a palm's survival -- e.g. Bismarckia 22 degrees, Sabal Palmetto 12 degrees, Kentia Oliviformis 24 degrees, etc.  It's as though a magic minimum temperature is all a gardener needs to know.  Surely that can't be accurate.  Doesn't the number of hours below freezing have anything to do with it?  Doesn't the number of consecutive freezing nights have something to do with it too?  Take for example a Bismarckia that sees 22 degrees for ten minutes and is then basking in 65-70 degrees two or three hours later.  Surely that Bismarckia has a better chance of survival than the one that has three consecutive nights of freezing (including 22 degrees one night), and only sees a high of 45 degrees for four days straight.  Am I wrong? If not, is there a magic number of hours below freezing that a Bismarckia can take? 

I am only using Bismarckia as an example, but you see what I mean.  Please share your knowledge on this thread. I am fretting about this freak cold snap in Florida right now -- way more time below freezing than I am used to, at least since 2010.           

    

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There are a lot of factors that go into this.  Was the plant actively growing before the cold spell?, precipitation, wind, duration of cold, etc.  I can tell you a lot of the data from California on this site are pure fantasy numbers for Texas, which I think is the toughest place to grow palms and other tropical.  Texas winters are usualy warm/hot before a cold front comes through and zone for zone day time highs are far lower than anywhere in Florida or California.  I know of no 10A Florida or California climate that has two days in a row that never break 40F and certainly no Florida zone 8 that spends 72+ hours below freezing or 48 hours below 28F which we have done three times in the last 7 years. 

What really matters is what was the actual temperature of the plant tissue which is easy to obtain on the fronds themselves but very tough when gauging live or die meristem temps.  The important thing is for all of us to report our findings on the forum for each species along with as much information/data as possible so that we can make a very informed decision in the future.  I can almost guarantee that anything that lives in 9A Austin will survive anywhere in 8B Florida or California.  Seeing pictures of Royals, pygmy dates, majesty palms, Queen Sagos etc growing in Tallahassee which is an 8B (18.3F) on the USDA interactive map is insane as you can't grow those anywhere in a Texas 9A.  Tallahassee is listed as the same hardiness as Bryan/College station, which cannot even grow Queen palms.

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This is great information, TexasColdHardyPalms.  Thank you.

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I have learned to attempt to grow your garden with palms and plants that are appropriate for your established zone and when cold fronts like this become reality you prevent yourself from heartache. With that said, my dypsis decipiens, allagoptera arenaria, bismarckia, parajubaea and a few others look really pissed off right now, but they should all live to see another summer.

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Some coconuts (even seedlings) can externally survive several 6-8 days of cool weather 42F-50.  But one problem is root rot if the soil is constantly damp and moist during the cool spell.

I have lost a coconut due to the latter condition. Four coco seedlings planted in sand survived the former.

 

So each factor is essential to the calculation.

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@GottmitAlex, bring up a good point as well.  Wet soil is a better conductor of temperature than porous soil with air pockets.  Palm roots are very sensitive to temperature which is why potted plants are more susceptible to death at a higher temperature than ones planted directly in the ground.  Some palms (Sabals) can have their roots freeze without much issue while others such as coconuts begin to die and then rot at temperatures much higher than freezing.  The wetter the soil the colder the roots will be in the winter which goes back to the actual tissue temperature. 

This is even more important with agaves and cycads than with palms.

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What do yall think about utisol clays? Mine seems to drain SLOW, but also absorbs slow and we get a good mess of runoff. Its a rocky red clay and i feel the slow. Absorbtion might help keep the feet less wet, but once its wet its wet for a while.

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Are those "magic numbers" based off of when a palm defoliates? I have heard of many Sabal palmetto surviving Southeastern Virginia in single didgets and sometimes they were not even defoliated. I have two Butias that pretty much get defoliated every winter but never die and regrow during the spring, and the tempatures goes well below what their "magic number" hardiness is, I never use anymore protection on them than just a (thin) layer of burlap on the extreme cold nights. I have not put any protection on them yet this year and they are half defoliated right now after the tempatures hitting 7 degrees one night. I don't think my thin layer of burlap even did much to them, other than the first winter in the ground, which burned off their green house fronds pretty fast. 

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2 hours ago, PalmTreeDude said:

Are those "magic numbers" based off of when a palm defoliates? I have heard of many Sabal palmetto surviving Southeastern Virginia in single didgets and sometimes they were not even defoliated. I have two Butias that pretty much get defoliated every winter but never die and regrow during the spring, and the tempatures goes well below what their "magic number" hardiness is, I never use anymore protection on them than just a (thin) layer of burlap on the extreme cold nights. I have not put any protection on them yet this year and they are half defoliated right now after the tempatures hitting 7 degrees one night. I don't think my thin layer of burlap even did much to them, other than the first winter in the ground, which burned off their green house fronds pretty fast. 

Again we need to report not only defoliation conditions but death conditions, which many of us have in that freeze damage section.  2011 killed 100% of the palmetto in Carlsbad New Mexico that had been there for 20 years that i can remember. Palmetto are a 7b palm at very best and a few year in a row of defoliation will cause decline and kill them. 

@mdsonofthesouth soil wetness isn't going to be anywhere near as important as ambient temp for the species you grow as long as there isn't standing water.  The more tropical the species the more cool sensitive the roots seem to be. 

 

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Posted (edited)

Thats correct, Amsterdam has loads of rain in autumn and winter but my syagrus was never killed by that. I just kept the trunk warm with a heating cable and a plastic wrap. Equally my Washy and Brahea armata can easily cope with the winter rains. I think its 90 per cent about keeping the growing point and meristeem from going under certain temps for too long, too many days in a row. 

 

More tropicals species are different and more sensitive to cool soil temps and wetness

Edited by Axel Amsterdam
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Posted (edited)

On 1/4/2018, 5:18:04, Sandy Loam said:

Sabal Palmetto 12 degrees

   

And even those minimums don't mean anything.  The temps at my house the last three nights: 10, 9, 11.  All the sabals, butia's and trachy's in the neighborhood are just fine.  Sabals are native here, but I'm willing to bet most of the species in people's yards are of the Florida variety.  They're still looking just as good as last week when it was in the 50's.

 

Some of the Washy's around town show some burn, but most should recover.

 

We haven't gotten above 34 in the past 3 or 4 days, either, so not only are we seeing record low temps, but we're seeing incredible, unprecedented time below freezing.  This spring will be the real test as to how tough these palms REALLY are.

Edited by Anthony_B
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