What kills cold-hardy palms: cold air or frozen soil?

[KarenRei]

When cold hardy palms succumb to the cold, what is generally the killer - cold air, or the soil freezing too deep?

Looking through my database of edible / medicinal / etc plants and filtering for palms, sorted by hardiness, I see for example:

Zone 7-10: Jubaea chilensis Nannorrhops ritchiana Sabal etonia Trachycarpus artianus

Zone 8-11: Brahea dulcis Brahea serrulata Ceroxylon alpinum Ceroxylon quindiuense Ceroxylon utile Chamaerops humilis Corypha palmetto Sabal mexicana Sabal minor Sabal palmetto Serenoa repens Trachycarpus fortunei Phoenix dactylifera

The last one rather surprised me, but I guess it makes sense - deserts can get pretty cold in winter nights.  Of the above list, Jubaea chilensis looks the most interesting. There's also many more that don't have zones listed, but I have their habitats, and could look into said habitats' climates.

Here in Iceland we have an interesting opportunity in that geothermal hot water is cheap.  We can run hot water lines in the ground and keep it from freezing beyond more than a few centimeters.  But obviously, the only way to stop the aboveground parts from getting cold is to allocate them greenhouse space, which is a significant cost per square meter, and ideally reserved for ultratropicals.  So it matters what's the limiting factors - root freezing, or leaf / trunk freezing?

Other issues of relevance:

• Our winters aren't actually that cold.  Our typical lowest winter temperature is generally around -15°C (5°F) or so, with a typical January low being around -3°C (27°F).  Aka, mid zone-7. But:
• Our winters are long, and summer temperatures quite low (average air temperature: ~15°C (59°F)).  Aka, not exactly friendly to plants that need high summer temperatures!  But...
• We can have the soil temperature as hot as we want outside.  So if it's *root* temperature that's the limiting factor in the summer (mineral / water uptake)... hey, no problem!  In general, planting times and growth rate limits are most often cited relative to soil temperatures (e.g. we can't grow tomatoes outside here in the summer because tomato growth rates are little to none when soil temperatures are below 15°C - regardless of air temperatures)
• We're also very windy.  But we can to some extent control that with windbreaks.
• We're also dark in the winter.  But we can - as with indoors - supplement it (at a cost)

What are your thoughts?  Do you have any experience as to whether the limiting factors are more about air temperature or soil freezing, or have you seen any research on the topic?  One way to compare it might be the difference between the hardiness of palms in the ground vs. their hardiness in large pots; if those in pots die but those in the ground survive, then the potted trees are probably being killed by root freezing.

Maybe I should just experiment...pick up a hot plate at a store and use on one of my potted date palm seedlings outside (since I don't have any of the zone 7-10 species). I'd hate to have to use a dioecious species as a guinea pig, though, before I know which ones are males and females...  the "date palms" at the store sold as as houseplants wouldn't work, as they're P. robellini, which is only hardy to zone 9. Maybe I should just import some Jubaea chilensis seeds for a trial...

 

Edited February 18, 2018 by KarenRei

[kinzyjr]

For your climate, Trachycarpus will be the best performer.  All of the Sabal species will die due to lack of summer heat.  Jubaea, I'm not sure about.  I'm not sure what their heat requirements would be, but they typically do not do so well here in Florida due to our oppressive humidity.

[bgl]

Karen,

As you probably knew long before you found out about PalmTalk, you are in uncharted territory, and experimenting really is the only way to go for many of these palms. I remember quite a few years ago we had a Forum member who lived in Ålesund, Norway, and actually grew palms successfully in his garden there. Very mild climate and never freezes due to the proximity to the Gulf Stream. You have some similarities but the drawback of course being that you are even further north. I can't address the various palm species (and how they might do there) since I have no experience but - and again as you probably already know - the limiting factors will be:

Lack of heat - year round

Lack of daylight during the long winter

Lack of abundant rainfall (many of the palms will absolutely thrive, but there are of course exceptions)

Soil temperature is probably less critical for many of the cold hardy ones since you're not likely to experience permafrost. That being said, running geothermal water in pipes underground can only help. Hey, I have known people in Southern California who did that!

No matter what - we will certainly be interested to know how things progress for you! Good luck!

Bo-Göran

[Cikas]

Trachycarpus martianus is cold sensitive Trachycarpus species. It is for USDA 9. Trachycarpus Wagnerianus is the most cold hardy trachycarpus. 

[richnorm]

You might get more specific advice on the  European Palm Society forum.   You may get away with Trachycarpus fortunei but my guess would be that none of the others on your list could be maintained outside of a greenhouse with supplementary heat.  A lot of palms with extreme cold tolerance actually need a lot of heat to grow.

[Cikas]

I think in your climate even Trachycarpus Wagnerianus and fortunei would die. They can take cold, but they can not take months of below freezing temperatures. They are not that hardy. Palms species that hardy do not exist. Both soil and air temperature is important. 

Edited February 19, 2018 by Cikas

[TexasColdHardyPalms]

A few pieces of misinformation on here. Chileans dont mind humidity, they hate nematodes just like trachycarpus.  Jubaea is NOT a zone 7 palm, only a zone 8b, warmer 8a. It will not survive repeated 10-14f winters. 

Trachycarpus takil and nanus are the most cold hardy of genus. Fortunei and waggy are zone 7 palms. Nannorhorps needs a lot of heat in the summer, but are leaf hardy to 5f.

[PalmatierMeg]

Sabal etonia is not a zone 7 palm. Sabal minor is.

[KarenRei]

Lots of bad info from PFAF and Useful Tropical Plants I see    I should post a correction in the relevant articles.

Sounds like my best bet will just be to set up a small scale experimental apparatus and seedlings from a bunch of random imported seeds of differeng species to determine where the limitations (summer growth, winter survival) are leaf temperatures vs. root temperatures.  It was admittedly sort of a long shot wondering whether anyone would have any data on the subject, although peoples' experience with growing plants in marginal climates in pots vs. in the ground would have helped hint at the answer, since pots chill roots deeper, faster.

Edited February 19, 2018 by KarenRei

[Cikas]

11 hours ago, TexasColdHardyPalms said:

A few pieces of misinformation on here. Chileans dont mind humidity, they hate nematodes just like trachycarpus.  Jubaea is NOT a zone 7 palm, only a zone 8b, warmer 8a. It will not survive repeated 10-14f winters. 

Trachycarpus takil and nanus are the most cold hardy of genus. Fortunei and waggy are zone 7 palms. Nannorhorps needs a lot of heat in the summer, but are leaf hardy to 5f.

I disagree. Trachycarpus Wagnerianus and fortunei are USDA 8 palms in North European climates (cold and humid). In USDA 7b they need protection. Trachycarpus Takil is less cold hardy than fortunei and Wagnerianus (in North Europe specimens of Takil were damaged while fortunei and Wagnerianus were not). Nanus is rare palm in cultivation, soo there is no enough data right now. 

[Alicante]

13 hours ago, Cikas said:

I think in your climate even Trachycarpus Wagnerianus and fortunei would die. They can take cold, but they can not take months of below freezing temperatures. They are not that hardy. Palms species that hardy do not exist. Both soil and air temperature is important. 

I agree.

In my hometown (Rotterdam, Netherlands) even T. Fortunei doesn't perform very good and it's a 8b zone. They prefer quick warm-ups after winter.

[KarenRei]

38 minutes ago, Alicante said:

I agree.

In my hometown (Rotterdam, Netherlands) even T. Fortunei doesn't perform very good and it's a 8b zone. They prefer quick warm-ups after winter.

Just to reiterate (because it's not clear from this response): we're not talking about plants just planted in the ground and left to their own devices.  We're talking about soil heating.  Aka, the ground never freezes (the trunk has a constant water / nutrient supply from the roots), and in the summer, the soil is as warm as it is in the tropics (aka, very productive roots).  The question is how the plant as a whole performs when the aboveground portions are in a suboptimal climate but the underground portions are in an optimal climate.

I'm actually surprised that you can grow any palm at all in Rotterdam without some sort of "assistance"; that's impressive that you've pulled it off. Rotterdam is just one zone away from us, without a particularly hot summer. I wouldn't even dream of just putting a palm (even a hardy one) in the ground outside here on its own.  That would otherwise be known as "sentencing it to death"   

(Although I must admit that I've been tempted to take excess banana pups and plant them along popular hiking routes in the summer, just to confuse the heck out of hikers for a couple weeks    )

Edited February 19, 2018 by KarenRei

[KarenRei]

If I were to do a trial run with a couple species (don't need to have any edible properties), what would you recommend?  Something wind tolerant, fast growing as a seedling, and easy to get seeds for at a reasonable price (or contrarily, to find as a houseplant at a garden centre).  Maybe I should just go down to a garden centre and see what sacrificial lambs... er... "test subjects" they have 

Edited February 19, 2018 by KarenRei

[richnorm]

4 hours ago, KarenRei said:

If I were to do a trial run with a couple species (don't need to have any edible properties), what would you recommend?  Something wind tolerant, fast growing as a seedling, and easy to get seeds for at a reasonable price (or contrarily, to find as a houseplant at a garden centre).  Maybe I should just go down to a garden centre and see what sacrificial lambs... er... "test subjects" they have 

Trachycarpus fortunei.  

[cm05]

I’d say cold air kills more than cold soil, assuming the palm is in-ground. The opposite would be true for a potted palm.

Your biggest hurdle would be the conditions above ground. Your winters aren’t too cold, but they’re cold for too long and your summers aren’t particularly warm. 

I don’t think the palm would be bothered by your winter darkness, many people in cold zones wrap/cover their palms for the winter without issue.

Trachycarpus fortunei, takil, wagnerianus, and nanus are your only realistic options, and they'd really benefit from being in a greenhouse for at least part of the year. My Trachycarpus fortunei doesn’t begin growing at a normal pace until the mean temperature is greater than 50°F / 10°C, such conditions don’t persist long enough in Iceland (only 2-3 months) for the palm to sustain itself, and especially not if it recieves any sort of damage from cold and/or moisture. The Trachycarpus fortunei’s growing in Sitka, Alaska all died out over time for that reason, and Sitka is 5°F / 2.8°C warmer than Reykjavík.

I’m in New York (also zone 7) and I’ve seen Trachycarpus die out after 2-3 consecutive cold winters despite the rest of the year being tropical in comparison to Iceland and Alaska.

[GottmitAlex]

For this particular question there is never a cut and dry answer. What I have come to conclude is it all depends on the particular circumstance. In my case, In these past 3 years (two with planted coconuts) we haven't seen anything under 5c. But I have lost a coconut due to root rot. It was the actual soil that coloquially speaking, "done her in".  The soil was clay. Heavy clay actually and as many know wet clay soil will stay wet for a very very long time. In my circunstances, time enough to rot one of my green Malayan dwarf roots. Under the same playing field sand the clay soil, all my other cocos have sailed through quite nicely. Yes, their plots are 3x3x3ft. (More or less) of pure sand.  Water drains and evaporates in 2-3 days. 

[Bigfish]

16 hours ago, PalmatierMeg said:

Sabal etonia is not a zone 7 palm. Sabal minor is.

Meg, it might surprise you to know that Sabal etonia does very well in zone 7b in Tennessee!  There are a couple of specimens in Chattanooga that are doing quite well there, even flowering and fruiting every year.

 

 I think the duration of freeze is way too much in Iceland for any palm, unfortunately.  Trachycarpus was tried in Sitka, Alaska (coastal), and lived for several years, but the prolonged cold was eventually too much for it.

[Jim in Los Altos]

On 2/18/2018, 4:34:53, Cikas said:

Trachycarpus martianus is cold sensitive Trachycarpus species. It is for USDA 9. Trachycarpus Wagnerianus is the most cold hardy trachycarpus. 

That's the first thing I noticed on that list. It's the least cold hardy of the Trachycarpus genus but the most attractive IMO. I doubt it can handle less than low 20s Farenheit. 

[Alicante]

14 hours ago, KarenRei said:

Just to reiterate (because it's not clear from this response): we're not talking about plants just planted in the ground and left to their own devices.  We're talking about soil heating.  Aka, the ground never freezes (the trunk has a constant water / nutrient supply from the roots), and in the summer, the soil is as warm as it is in the tropics (aka, very productive roots).  The question is how the plant as a whole performs when the aboveground portions are in a suboptimal climate but the underground portions are in an optimal climate.

I'm actually surprised that you can grow any palm at all in Rotterdam without some sort of "assistance"; that's impressive that you've pulled it off. Rotterdam is just one zone away from us, without a particularly hot summer. I wouldn't even dream of just putting a palm (even a hardy one) in the ground outside here on its own.  That would otherwise be known as "sentencing it to death"   

(Although I must admit that I've been tempted to take excess banana pups and plant them along popular hiking routes in the summer, just to confuse the heck out of hikers for a couple weeks    )

Rotterdam had 5 Trachycarpus in a zone in the open air planted by a local nursery in early 2009. They survived the 11th coldest winter Rotterdam experienced (2010) and they were recovered by that same summer, but they didn't manage to do it in the 7th coldest winter Rotterdam has experienced (2013) unless one of them which looks very good nowadays.

Rotterdam has a pretty strong mid-Spring warm up, the airport high average is 17.5ºC in May, in the city itself can be from 0.5 to 1ºC warmer,

^^ This is a pic from Autumn, 2009. They were planted in the Spring of 2009. This pic from below shows the only palm remaining in June, 2017:

There are few others across the city and there are some in private gardens as well.

This one is from eastern-central Belgium, a nice looking Jubaea Chilensis on full ground! It gets winter protection, tho. 

 

 

[KarenRei]

You all amaze me with your zone pushing.  Seriously, putting a palm in the ground in Sitka, Alaska with no additional heat at all?  And I thought I was pushing boundaries!

I'll keep you updated on how the experiment goes.  I have pretty much all of the components that I'll need except for some insulation, light bulbs, and the test subject.  There's not much of a palm selection at the stores here; the hardiest thing I could find when I looked yesterday was Phoenix roebelenii  :Þ   I think I'll pick one up, as well as ordering some Trachycarpus fortunei seeds to plant indoors under lights; that way, I can start the experiment with the former, monitor how it deals with the tail end of winter and measure any summer growth or dieback, and then when it (presumably) dies next winter I'd have a T. fortunei seedling to replace it. On the other hand, if it somehow doesn't die next winter, then that would open up quite a broad range of possibilities.  I presume it will die, though.  Our January low/high temps this year were, for example:

Celsius: -8 -2 -8 2 -3 1 -3 0 -4 1 -2 1 -1 7 1 4 2 7 0 4 1 8 2 6 1 3 -4 -2 -6 -1 -6 -1 -6 -2 -7 -3 -9 -2 -7 0 -5 2 0 3 0 4 0 1 -6 -1 -6 4 1 3 -1 3 -1 1 -1 1 -6 1

Fahrenheit: 17 26 17 35 26 30 25 33 24 34 27 35 29 44 35 39 36 45 32 39 33 46 35 43 33 37 24 28 21 29 20 30 21 28 19 26 15 28 19 31 21 35 27 37 33 39 32 34 21 28 20 39 33 38 30 37 30 35 29 33 21 34

I'll probably target a soil temperature of ~12°C/50°F at depth in the winter and ~34°C/93°F at depth in summer; I imagine the heat from a few light bulbs in an insulated box and a little pumice on the soil surface should do the trick (I have a spare fixture I was going to throw away, and an empty shipping crate onhand that would be perfect with some minor changes). I'm thinking about having a glass or clear plastic window on the top as well to add a small amount of winter supplemental lighting, if I feel up to caulking some scrap into place for that.  Plus one large plastic pot, some supports to keep it in place, a bowl with a hole to drain excess water, and some ropes to anchor the whole system in place... I think that's all it should take to test this out.

Edited February 20, 2018 by KarenRei

[KarenRei]

Well, I've been rigging up the experimental apparatus, and it's been a mixed bag.  The size of the crate and the bulk of the insulation quickly made it clear that there was going to be no "window" to let light out - so I fetched pieces of glass from my land for nothing     I used a long old extension cord with a tattered end, trimmed it clean and soldered it to the internals of an old light fixture, to get a 4-bulb "heater".  I installed 42W halogen bulbs, but within 10-15 seconds it had gotten so hot inside (even without any insulation over them) that I quickly realized this was way too much power.  I installed 7W LED bulbs, which seemed to be fine. I finished the assembly. Supporting the pot over the insulation and managing draining took several abortive attempts; I eventually ended up with hanging the pot on cables from the crate, and draining the bottom through a plastic bag to outside. 

After running it for several minutes indoors, I took it outside and set it in a snowbank (it was also lightly snowing).  Went back inside.  Several minutes later, the breaker threw.  This surprised me; I'd really gone overboard on waterproofing (going as far as to caulk the sockets themselves).  Seeing no signs of moisture inside, I checked the cable, and found no obvious reason for a short.  So I started pulling the rock wool to get at the light bulbs.  They were burning hot.  Geez, even 7W LEDs?  I must have melted some wire insulation inside the bulbs to cause the short.

My thermometer inside the pot never read above 27°C, but it surely would have gotten hotter with time; it just didn't have long enough.

New plan to try tomorrow. Light bulbs are just too much heat in too concentrated locations.  I'm going to try LED christmas lights.  I'll have to cut off the lightbulb sockets from the extension cord, solder on a normal extension cord end (picked up one today, just in case), and then try to spread out the lights evenly along the pot edge.  I can start off by setting them to a blink mode where each LED only spends a small amount of time on - and then if I need more power I can ramp up by choosing a different blink setting (up to constantly-on).  Hopefully that won't burn anything out...

My last option would be to buy a heating pad with a thermostat. But I know they'd charge way too much here, and it would take weeks to get one imported. Maybe I should buy one regardless...

I picked up my test subject as well.  I discovered that the pygmy date palm was priced at a ridiculous level (they wanted the equivalent of $200, for a small houseplant).  Not going to spend that for something that will likely be killed.  I went to a cheaper store; the only true palm they had was too large (and correspondingly expensive), which left me to choose between an elephant's foot tree or a cycad.  As I have a moral objection to cycads (toxic palm mimics    ), I went with the elephant's foot tree. Similar (if not better) cold hardiness to Phoenix roebelenii, although perhaps not as good wind tolerance.  Meanwhile, I've ordered Trachycarpus seeds to cultivate indoors until they're big enough to trial outside.  Also ordered some Butia seeds, in case the test subject does better than expected and I want to try something less hardy than the Trachycarpus.  Last, I also ordered some Salacca wallichiana seeds, but those are just to be raised permanently indoors, they'll never live outside for obvious reasons   

Edited February 21, 2018 by KarenRei

[richnorm]

You can buy very cheap thermostats online.  Best get one which has a temperature range input rather than single point so the heat source isn't constantly being turned on and off.  Fancier versions used for amphibian tanks require no soldering.

[Bennz]

KarenRei,

 

 

I've got no experience with trying this with palms but I did do the experiment with bananas, my results may have some relevance to you (or not!).

My climate is oceanic temperate with a mean annual air temperature of about 14.5C, but a solid frost free zone 10b bordering on 11a in places. Some hardy bananas ripen quite well over the summer, but winter-ripened fruit is not much good because it's just too cold. So I tried lifting the soil temperature with piped heated water to try to get more consistent fruit.

5C above ambient soil temp had great results, the plants grew vigorously and there were no problems. So I lifted the soil temp to 10C higher and the positive results continued, but the plants started to show some signs of 'choking', ie the leaves started to bunch up a bit instead of maintaining their usual dimensions. 

My heating was very low tech, and I had to adjust the thermostat relative to the soil temperature. So we got in towards winter and I decided to have fun and lifted the soil temperature to 25C, when the natural soil temperatures were down to 12C. Everything was fine until one night when the air temperature dropped to 5C and the bananas collapsed dramatically, they went black overnight as if someone had hit it with a flamethrower. Interesting contrast was that the same banana cultivars alongside (Cavendish and orinoco) with natural soil temperature were both totally untouched by the 5C night air.

I had to pay for my soil heating, and in my case the conclusion was  heating the soil  would be more expensive and less effective than building a greenhouse. Combining the two would be the perfect solution.

If palms act anything like bananas you might create some major unexpected problems by lifting soil temperatures to tropical temperatures without a corresponding increase in air temperature.

[KarenRei]

Very interesting data - thanks so much for sharing!

It sounds like summer heating (to some degree) may be a big boost, but not to excess, and winter heating might be bad.  My suspicion was that winter heating would always be fine for evergreen plants which can tolerate air temperatures below zero, but would have to be carefully timed with deciduous plants, since they use soil temperatures to decide when to go into leaf. But maybe some timing / degree of heating issues apply to non-deciduous trees as well.  The question of course is how much it depends on species; on frost tolerant vs. not frost tolerant; on woody vs. herbaceious; etc.. 

Heating is very cheap here (geothermal water), while construction is expensive, hence the interest in soil heating wherever it can be useful. Thanks again for the data!

Edited February 22, 2018 by KarenRei

[Axel Amsterdam]

Established trachies are hardy in the western part of Holland, from Rotterdam to Amsterdam. This one has been there for decades 

[Axel Amsterdam]

And another established trashy around 20 km from Rotterdam.

There are hundreds of smaller sized trachies in the west, typical winter lows are around -6C. As long as temps don't exceed -14, they don't experience much leaf damage and they will take the frozen soil too. I must add that we don't have continuous winterweather for months, we have outbreaks of frosts whereby the day temps can stay below zero (32f) for days, but then move a little upwards again, never warm though.  

Most semi hardy palms I lost (CIDP, Washingtonia, butia) always had healthy roots even after the meristeem was killed (surgery). So for me the risk of rootdamage by frost is low, the meristeem goes first in my climate.     

[KarenRei]

How deep does the soil freeze there, do you know?  If you're getting several entire days below freezing at time, it's probably not insignificant.  Around here, if I'm not mistaken, it's usually something like 70-100cm in a bad winter.

Some updates.  First, the original (scrapped) heating approach:

It's been swapped out for the LED christmas lights, but I've also ordered a thermostat-controlled heating pad, which should get here in several weeks.

I've been breaking in the temporary pretends-to-be-a-palm:

Probably won't even get the Trachycarpus seeds for a few weeks, let alone get them to a size worth trying outside.  Anyway, the beaucarnea tolerated the breakin period like a champ, so I transplanted it into the box this evening (hopefully not damaging the root; it really didn't want to come out of the pot, and its root connection has always been rather flimsy  :Þ  ) 

 As you can see, it's mounted to the top of my trailer frame, anchored with two bungees and several lashings.  Maybe a bit flimsier than I'd prefer, but it already withstood (without the tree) our most powerful storm of the year - and now that the tree is in, I bound it in place as well.  Anyway, if it does fall, I'd rather it be with the wannabe-palm than an actual tree that I care about     I've got it there so that A) I can move it around, B ) it'll get sunlight over the very large obstruction to the south, and C) there's an unrealistic amount of shelter from the wind here, I wanted to get it more exposed for it to be a realistic experiment.

Anyway, the first real test should be starting unexpectedly soon.  While the past couple days it's been above freezing, starting tomorrow it's supposed to get down to -7°C, and up to at least Tuesday it's highs from -2° to 0°C and lows from -7°-5°C.  In theory, that should kill a B. recurvata, if I'm not mistaken.  Small plants are only supposed to be hardy to -5°C.

I guess we'll find out if I need to look for a better test subject soon enough 

One observation so far: soil temperature declines a lot more when it's raining or there's snowmelt going on.  Water flowing through the soil mix warms up, drains out the bottom, and takes the heat with it.  Good lesson learned for the larger scale - drain water on the surface away from plants and off the lots as effectively as possible.

Edited February 28, 2018 by KarenRei