Washingtonia filifera damaged. Will it be fine?

[Foxpalms]

On 2/21/2024 at 7:28 PM, Chester B said:

I have some experience with filifera and winter.  They absolutely hate winter humidity.  I bet the lack of airflow had a major cause in the state of the palm.  I had one in a pot for a few years, and tried different methods of keeping it looking good in winter to no avail.  My final year with it I had it on my front porch so that it did not receive any rain or precipitation of any kind.  Minimum temps it saw was 32F/0C.  Winters are very wet and high humidity, and even with it in a pot in well draining soil, never cold and never rained on, by spring it was covered in black spots and had lost most of its fronds.  In the future if you're protecting a filifera I really think you need to open it up from time to time on warm days to allow airflow in there.  Robustas can handle humidity much better and seem to be one of those palms that you can box up.  Those are my thoughts.  Good luck.

That is something I also notice. In London during the winter the humidity is high however still a bit lower than it is in the PNW or costal areas of the UK during the winter.  Here they get a bit of those fungal spots of the fronds but only a couple whilst in costal areas you will see more of those fungal spots on the fronds during the winter. Robusta here doesn't get that.

[UK_Palms]

On 2/23/2024 at 12:18 PM, Foxpalms said:

@UK_Palms This one has been growing unprotected at 55N in the UK. Im not sure about filifera but robusta dominant hybrids or pure robustas will take -5 to -6c in north west London without spear pull during their once every 10 year freeze. The ones in the urban areas just north of RAF northolt all survived last winter. I drove past a few of those palms a few weeks ago and they all looked fine with a bit of damage but not lethal amount.

 

Wow, that Washingtonia at 55N has grown loads since I last saw it posted about a year or two ago now. It has almost doubled in size since then, which is surprisingly quick given that it is up at 55N. It didn’t get effected by the 22-23 Freeze either really. which was the worst winter since 2010.

I can’t remember the town or location of it. Where is it growing again? Isn’t it in Seaton or somewhere? I also wonder how that CIDP is doing up in North Shields at 55N as well. That is about 10-15 foot high. Both the furthest north / furthest from the equator Washingtonia and CIDP in the world are in that area on northeast England.

Edited February 24, 2024 by UK_Palms

[Foxpalms]

4 hours ago, UK_Palms said:

 

Wow, that Washingtonia at 55N has grown loads since I last saw it posted about a year or two ago now. It has almost doubled in size since then, which is surprisingly quick given that it is up at 55N. It didn’t get effected by the 22-23 Freeze either really. which was the worst winter since 2010.

I can’t remember the town or location of it. Where is it growing again? Isn’t it in Seaton or somewhere? I also wonder how that CIDP is doing up in North Shields at 55N as well. That is about 10-15 foot high. Both the furthest north / furthest from the equator Washingtonia and CIDP in the world are in that area on northeast England.

I'm not sure on the exact location but it is somewhere around there. The same person also has a Sabal Palmetto, a few types of butias and brahea's.

[Arecaceus]

IMO this looks like a W. filibusta if not just robusta. Pure filifera (as pure as it gets in cultivation) is very cold hardy. The palm looks very moldy so I guess most damage comes from being wrapped for 3 months. My W. filifera has seen similar temperatures in early 2021 with a lot of burning and mold (not being wrapped) and completely recovered by the end of summer in one of the coolest and wettest years we ever had. This year my filifera (which is bigger than yours) took -5/-6° in very wet conditions and it doesn't show any damage so far and it's already pushing growth.

NOW for my W. filibusta after the same conditions it has about 30-40% burnage and is also already pushing because of the mild temps but last year I had to trunk cut it even though it has seen colder temps before then but that winter (22-23) temps were fluctuating so much from frost to +20°C all the time combined with unsual amounts of rain thus causing plants going out of hibernation. What saved it was cutting it down to a point where I could expose healthy tissue (looks bright yellow and feels hard and fresh) and treat it with Hydrogen Peroxide (3%) several times. I couldn't keep it dry easily as it was already too tall but what you're doing is helping very much! I would cut it down as early as possible because mine wasn't rotten too deep down to save it. It might have made it without this procedure but seeing the results of fellow palm growers doing this encouraged me to risk it. It immediately started to push. Good luck!

[RichardS]

After about 3 months, with the rain protection that I built, the palm started pushing new growth and it has mostly recovered. There are still some signs of the damage that it had recieved. There are two small leaves that grew from the dried spear and therefore remained partially dry, but after that, all new leaves grew fine.

This is the palm in mid april, mid september and 1st november respectively.

Now, however it is facing another issue. Last few weeks were really quite wet, damp and cold, which the palm doesn't seem to like and we even had some snow. The temperatures could have been as low as -5°C (23°F), possibly a tiny bit lower during two nights (though it's hard to say how cold it really was near the palm). These lows aren't that low as some people here also said, but apparently when they are combined with very high humidity it can cause some problems and I think that's what I'm dealing with now, but I'm not sure how bad it really is.

This is what the palm looks like now.

You can see that the old leaves from the damaged spear started to completely dry out, but that's far from the only thing I noticed.
Here you can see one of the leaves being slightly discolored. The left leaf is lighter and more desaturated compared to the new leaf on the right.

The worst looking leaf is on the other side though. It is starting to dry out and has subtle dark spots on the dried parts.

Another one of the newer leaves also has some spots that looked dried out and appear to have small holes in them.

The spear also appears to have some wavy dryish spots along it's height.

In the crown of the palm there are also things like some dark spots, some light spots on the green parts and dark discoloration on stems of some leaves.

The leaf in the 4th image that's drying out also has some wierd damage on it's stem.

The palm has been completely unprotected the whole time, but I'm not sure what I could have done to prevent this new damage. As I said, the temperatures weren't that incredibly low. I also don't know how serious this really is and how much more damaged can it get as winter is far from over. Luckily it is supposed to warm up a bit in the following days and there should't be any freezing nights, at least next week.

Hopefuly it won't get much worse and it will get through the winter fine, but unfortunately I don't have a lot of time to take care of the palm, not just because I don't have a lot of time in general, but also the palm is about 30km from where I live and travelling there is not especially convinient. Of course if you have any advice about what could help my palm I would highly appreciate it.

[jwitt]

I would put up your solar protection at the winter angle. Cover if temp drops below 15-20f. 

Just the "solar" protection will increase the diurnal temps which will squash humidity concerns. 

[ChrisA]

The wavy dry spots you’re seeing on the spear are just the indents from the spines of the petioles (stems) when they were bunched together as they were emerging from the center.  Also the weird damage you’re seeing on the petioles a few photos lower down is probably from heavy winds that caused the petiole to get raked by the adjacent petiole.  These are not issues and just cosmetic.    

That said, I do have some concerns about your Washingtonia Palm.  I think it could use some help this winter and get some protection from the cold, especially if you get moisture and frost at those times.  If I were you, I’d protect it from any temps below the low to mid 20’s. There are several topics on PT that explain excellent ways to protect your palm!  If it could start the next growing year without losing ground I think it could become much more robust. I’d would give it similar protection for the next few winters until it becomes more massive.

I think it’ll look fantastic once it gets a good growing season in next year after protection this winter!

 

Best wishes!

[ChrisA]

I would also recommend cutting the dead petioles off closer to the trunk. That will keep your live petioles from getting more damage from the wind. 
 

Here are some examples of young robusta with trimmed up trunks with no dead petioles extending far from the trunk. You’ll see that this will leave plenty of room for the live petioles to blow around, gives a cleaner look, and will make it much less hazardous to protect. The spines on the living stalks are bad enough but the dead ones are even worse, ouch!! 😆 

[RichardS]

Thank you very much. When it comes to the solar protection, I'm not sure if you meant the glass roof or the mirrors you talked about all the way back 9 months earlier, but if you meant the glass roof, I'm not sure if that will do anything now. Back then it was completely defoliated and was therefore much shorter, it was efficient at preventing rain from getting into the crown. Now it's much taller overall and the rain will get blown right into it if it rains. I also don't think it can help with humidity at all, the glass panel is just too small now. I will definitely cut the dead petioles off closer to the trunk next time I'm there. That's a great idea as there is plenty of wind in the area so hopefuly the fronds won't take too much damage in the meantime.

[jwitt]

Using the glass or mirror to shine the sun on the palm at 40f and 100%rh and the temp of the palm is now 50f, the humidity is now less than 70%.  The greater "diurnal" temperature dries things out.  

Not much temperature change is needed. 

If humidity is the concern 

[UK_Palms]

On 12/20/2024 at 11:52 PM, jwitt said:

Using the glass or mirror to shine the sun on the palm at 40f and 100%rh and the temp of the palm is now 50f, the humidity is now less than 70%.  The greater "diurnal" temperature dries things out.  

Not much temperature change is needed. 

If humidity is the concern 

Easier said than done, I am afraid. Otherwise everyone would be using mirrors to grow Washingtonia in marginal/risky climates. Plus what if there is no sun the day after a brutally cold night? Further north (above 45N) the sunlight intensity is very low in December/January and the days are so short that you would struggle to make use of a mirror for any daytime recovery or lowering humdity. You would also need to surround the palm with mirrors further north to get any kind of effect, which would still be minimal at best. So it's just not practical for most of us further north.

December 2010 was the notoriously cold advective freeze here in southern England and most of Europe. It was also the dullest December on record here (dullest month EVER in some places). The nights weren't brutal, brutal like some continental places can get (-8C to -10C here), but there was barely any sun for daytime recovery. So many days were not going above freezing. During the freeze, there was about 3 hours of sunshine in the space of 10 days for London and Paris! So 0.3 hours a day on average during the worst freeze in western Europe for 20+ years. There was also a 7 day stretch with a combined 1 hours of sunshine during the event at London Heathrow, so 0.1 hours of sunshine on average in the middle of the freeze event.

A mirror would have been absolutely pointless during that event for most of Europe as there was just no sun. It would be more practical for drier, sunnier climates further south that are desert/semi-arid, where decent amounts of moderate strength sunshine is basically guaranteed. So it might work for you there, but I am almost certain that it won't work for the OP. Just saying.

[RichardS]

Yeah, that's true. The effect won't propably be that significant and there's not a ton of sunny weather to beign with. Definitely not comparable to new mexico. It might theoretically be worth trying though, because even though it might not help a lot, it still can help even if just a little during the sunny days. The main problem would be getting a mirror that's big enough and installing it in a way that it doesn't get knocked down by the wind. Of course this would be mainly for humidity and when combined with a slanted roof for example, it could propably have some effect. For very cold nights (less than -4C/-5C (25F/23F)) I will propably get a heated cable and a frost cloth, but so far these were luckily not a very big problem. I'm mainly concerned about that one leaf that is drying out and has those black spots over it. I think it could be caused by the humidity. It's not old enough to be dying naturally and I would like to try to prevent that from happening to the other leaves, especially the spear and it would be very benefitial if the palm could start growing in the spring without getting almost completely defoliated. If there is one of these palms growing in Liverpool that has even more winter humidity and rain than my area, then there must be a way to get them established right?

[jwitt]

4 minutes ago, RichardS said:

Yeah, that's true. The effect won't propably be that significant and there's not a ton of sunny weather to beign with. Definitely not comparable to new mexico. It might theoretically be worth trying though, because even though it might not help a lot, it still can help even if just a little during the sunny days. The main problem would be getting a mirror that's big enough and installing it in a way that it doesn't get knocked down by the wind. Of course this would be mainly for humidity and when combined with a slanted roof for example, it could propably have some effect. For very cold nights (less than -4C/-5C (25F/23F)) I will propably get a heated cable and a frost cloth, but so far these were luckily not a very big problem. I'm mainly concerned about that one leaf that is drying out and has those black spots over it. I think it could be caused by the humidity. It's not old enough to be dying naturally and I would like to try to prevent that from happening to the other leaves, especially the spear and it would be very benefitial if the palm could start growing in the spring without getting almost completely defoliated. If there is one of these palms growing in Liverpool that has even more winter humidity and rain than my area, then there must be a way to get them established right?

Think of a car dashboard under glass, with mirrors increasing the light.  Even beneficial on a cloudy day.

Would a 5f bump in daytime max temp and corresponding drop in humidity be beneficial? I would think so. 

 

[RichardS]

3 minutes ago, jwitt said:

Think of a car dashboard under glass, with mirrors increasing the light.  Even beneficial on a cloudy day.

Would a 5f bump in daytime max temp and corresponding drop in humidity be beneficial? I would think so. 

 

Yeah of course it would be beneficial, but it's hard to say if it would work so well. The palm is outside and there is air constantly circulating around it. That doesn't happen if you are in a car. To replicate that effect I would have to put the palm in some sort of a greenhouse like structure and you know, at that point I could just heat the inside of the structure as well as regulate the humidity. That would not only be pretty hard to build, but It would also cost a decent amount of money. If you let's say, left the car doors fully open the whole time, then it would be a more fair comparison in my opinion. During the summer it would have the effect you are describing but here in the winter you have pretty low solar radiation and and the day is very short as well. If you combine that with overcast weather I think the effect would be minimal and 5f sounds pretty optimistic, although I have not tried this before and you could be very much right. During a sunny day I don't doubt that there would be a noticeable improvement if the mirror was big enough. If it was combined with a roof over the palm then it would also help to prevent rain from getting into the crown and on the leaves and there also wouldn't be as much moisture in the soil directly near the palm. If I could buld something like that, at worst the effect would be very subtle and at best pretty good. That's why I said it could potentially be worth trying.

[jwitt]

45 minutes ago, RichardS said:

Yeah of course it would be beneficial, but it's hard to say if it would work so well. The palm is outside and there is air constantly circulating around it. That doesn't happen if you are in a car. To replicate that effect I would have to put the palm in some sort of a greenhouse like structure and you know, at that point I could just heat the inside of the structure as well as regulate the humidity. That would not only be pretty hard to build, but It would also cost a decent amount of money. If you let's say, left the car doors fully open the whole time, then it would be a more fair comparison in my opinion. During the summer it would have the effect you are describing but here in the winter you have pretty low solar radiation and and the day is very short as well. If you combine that with overcast weather I think the effect would be minimal and 5f sounds pretty optimistic, although I have not tried this before and you could be very much right. During a sunny day I don't doubt that there would be a noticeable improvement if the mirror was big enough. If it was combined with a roof over the palm then it would also help to prevent rain from getting into the crown and on the leaves and there also wouldn't be as much moisture in the soil directly near the palm. If I could buld something like that, at worst the effect would be very subtle and at best pretty good. That's why I said it could potentially be worth trying.

Leave you car doors and windows open and the dash is still warm.  My thoughts.

[UK_Palms]

2 hours ago, RichardS said:

Yeah, that's true. The effect won't propably be that significant and there's not a ton of sunny weather to beign with. Definitely not comparable to new mexico. It might theoretically be worth trying though, because even though it might not help a lot, it still can help even if just a little during the sunny days. The main problem would be getting a mirror that's big enough and installing it in a way that it doesn't get knocked down by the wind. Of course this would be mainly for humidity and when combined with a slanted roof for example, it could propably have some effect. For very cold nights (less than -4C/-5C (25F/23F)) I will propably get a heated cable and a frost cloth, but so far these were luckily not a very big problem. I'm mainly concerned about that one leaf that is drying out and has those black spots over it. I think it could be caused by the humidity. It's not old enough to be dying naturally and I would like to try to prevent that from happening to the other leaves, especially the spear and it would be very benefitial if the palm could start growing in the spring without getting almost completely defoliated. If there is one of these palms growing in Liverpool that has even more winter humidity and rain than my area, then there must be a way to get them established right?

 

That one in Liverpool has probably never been protected but also never seen anything colder than -5C / 23F going back to 2010. So it hasn't really needed much in the way of protection there. It is also incredibly slow growing and does not look great due to the lack of summer warmth and sun up at 53N in northwest England, but it doesn't have to deal with proper cold winter temperatures that will take it out. That is the difference. Most years the lowest temp is probably around -3C or -4C. Everything is secondary to the coldest minima in terms of survival.

Looking at Bratislava, you had -12C in December 2010 and -15C in February 2018. In January 1987 you had -20C. I am quoting the city temperatures as well, not the airport on the outskirts of the city which was even colder. There's no chance of getting a Washingtonia to survive there long term with those minimums, unless you build a proper box shelter with a heating element. And then it will only last for as long as you can protect it. As soon as it is exposed to freezes below -5C or -6C you are in big trouble.

Have a look at how that Polish guy protects his CIDP and how Will Simpson protects his Washie in North Carolina, although he gets much more winter warmth between any freeze events. But the only way you will grow Washingtonia in your location is if the drainage is on point there and it gets a winter box built over it with a heating element inside to keep it above freezing. This is quite a lot of work and will cost money to construct and heat. But you will have to go to that effort. There are no two ways about it. 

[Las Palmas Norte]

2 hours ago, jwitt said:

Leave you car doors and windows open and the dash is still warm.  My thoughts.

Maybe a great place to germinate seeds while driving around town. Plant em before your vision becomes obscured.

[RichardS]

2 hours ago, UK_Palms said:

 

That one in Liverpool has probably never been protected but also never seen anything colder than -5C / 23F going back to 2010. So it hasn't really needed much in the way of protection there. It is also incredibly slow growing and does not look great due to the lack of summer warmth and sun up at 53N in northwest England, but it doesn't have to deal with proper cold winter temperatures that will take it out. That is the difference. Most years the lowest temp is probably around -3C or -4C. Everything is secondary to the coldest minima in terms of survival.

Looking at Bratislava, you had -12C in December 2010 and -15C in February 2018. In January 1987 you had -20C. I am quoting the city temperatures as well, not the airport on the outskirts of the city which was even colder. There's no chance of getting a Washingtonia to survive there long term with those minimums, unless you build a proper box shelter with a heating element. And then it will only last for as long as you can protect it. As soon as it is exposed to freezes below -5C or -6C you are in big trouble.

Have a look at how that Polish guy protects his CIDP and how Will Simpson protects his Washie in North Carolina, although he gets much more winter warmth between any freeze events. But the only way you will grow Washingtonia in your location is if the drainage is on point there and it gets a winter box built over it with a heating element inside to keep it above freezing. This is quite a lot of work and will cost money to construct and heat. But you will have to go to that effort. There are no two ways about it. 

Well I'm obviously willing to protect the palm if the temperatures get very low, especially as low as -12C, but that happens rarely just like you said and even then it's not like it's -12 every night throughout the month, it's propably like one or two nights at most, so why would it have to be inside of a heated box for the entire winter. If everyhting comes secondary to the coldest minima then it should be all right for most of the winter. I've seen people here on this having their washingtonias survive -12 with little damage and survive even much lower temps with more extensive damage. Then there must have been other deciding factors. Humidity is the main one I've heard.

Also to avoid confusion, Bratislava-Koliba doesn't exactly represent the city temperatures. The station is on a hill in a foresty suburban area. I'm not saying that the location where the palm is is warmer, but I don't think that station has much UHI influence. The palm is about 30km east of Bratislava in a small town as I said. It's a bit far which makes taking care of the palm much more difficult and the UHI of Bratislava would also help it quite a bit, but there's not much that can be done about that.

[UK_Palms]

54 minutes ago, RichardS said:

Well I'm obviously willing to protect the palm if the temperatures get very low, especially as low as -12C, but that happens rarely just like you said and even then it's not like it's -12 every night throughout the month, it's propably like one or two nights at most, so why would it have to be inside of a heated box for the entire winter. If everyhting comes secondary to the coldest minima then it should be all right for most of the winter. I've seen people here on this having their washingtonias survive -12 with little damage and survive even much lower temps with more extensive damage. Then there must have been other deciding factors. Humidity is the main one I've heard.

Also to avoid confusion, Bratislava-Koliba doesn't exactly represent the city temperatures. The station is on a hill in a foresty suburban area. I'm not saying that the location where the palm is is warmer, but I don't think that station has much UHI influence. The palm is about 30km east of Bratislava in a small town as I said. It's a bit far which makes taking care of the palm much more difficult and the UHI of Bratislava would also help it quite a bit, but there's not much that can be done about that.

 

Your Washingtonia got defoliated last winter during what was an average to slightly above average winter for you guys, which only saw -9C and it was wrapped up as well. Yet it still defoliated. Even if it regrew, it will not survive back to back defoliations like that. Throw in a bad winter like 2010, 2012, 2018 etc and you will have even bigger trouble. They will be killed stone dead in your climate without a heated box / warm house type protection. Wrapping a Washie in cloth for 2 weeks is out of the question really when winter properly bites for you guys there.

I see February 2012 was even colder for you than the 2010 and 2018 freeze events. I can see -11C daytime maxes showing on there and a -16C minimum, with 12 consecutive nights below -10C, as well as almost 3 weeks straight that are below freezing, starting in late January 2012. These are extreme temperatures and again this isn't the airport station that I am quoting, which is even colder. This is the city station, although you insist it is on the outskirts of the city still and less protected. But it's still cold there nonetheless.

These are palm obliteration temperatures at our northern latitudes! If you haven't got a heated grow box type thing your palms are 100% gone in these scenarios. I wouldn't even trust a Trachycarpus Fortunei to survive those temps shown below. A monthly average minimum of -6C as well is nuts...

 

As I have previously said, CIDP and Washingtonia are not present over here in the UK in places that saw colder than about -6C to -7C during the 2010 and 2018 events. Areas that colder than that are not viable for growing these species long term in our northern latitudes. Not without insulated structures built around them and heating elements for when the proper Siberian expresses roll into town. It is your only way to get around it. During a very mild winter, you may not need to worry about protecting it much, but about 60-70% of your winters are going to require a heated shelter/box to get a Washie through. It's 'do or die' I am afraid.

London St James Park’s coldest month in recent decades for comparison… and even this would have left some damage on CIDP and washingtonia…

[UK_Palms]

I have just seen January 2017 for your area as well. Every single night that month was below freezing and the average minimum was -7C that month. Compared to an average minimum of +0.3C for St James Park in London in December 2010.

 

What minimums do you think you had in your actual locality in December 2010, February 2012, January 2017 and February 2018?

3 of those 4 events look like they would take out Trachycarpus Fortunei, which will be completely defoliated at about -12C to -14C and 10 days/nights below freezing in a northern latitude. Fortunei has been killed dead by even less bad freezes in Scandinavia, Germany and the Baltics, where winter sunshine levels are very low. -10C and 2 weeks below freezing has killed them even.

I would say that Washingtonia is completely out of the question for your area, but if you are going to grow it you will need one of those heated box shelters most winters. Again it's 'do or die'. No two ways around it looking at the freeze history in your area. It's actually much colder than I first realised. I haven't even asked about how much snow you get there as well.

Also any talk about mirrors and glass is basically pointless looking at those temperatures and freeze durations for the months I showed. It won't save a Washingtonia without a hot box/house being constructed. @jwitt

[RichardS]

Yeah these events you pulled up were pretty harsh, I remember that one back from 2017.  I was obviously not talking about leaving the washingtonia unprotected through a siberian cold blast of this strength. Those temperatures are very low and are not common. It's hard to say what the lows were in the locality of the palm as there is no weather station nearby. It's like maybe halfway from Bratislava to Trnava, closer to Trnava though. There is no reason for the location to be warmer than the station you looked up. When it comes to snow, we used to get a decent amount, but in the recent years, not much. This year in 2024 there was just one day with snow cover that lasted just several hours. Last year there were more days with snow I think like 3-4 in december, that's the last winter when my palm got defoliated. Hard to say if it was because of the lows during the cold wave in january or because of humidity, or because it was wrapped for 3 months straight, or a combination of those factors. I wrapped the palm in late november and hadn't seen it for 3 months. Not a great thing to do and I learned the hard way. Maybe you're right and it's going to turn out as something not viable and I'll maybe try growing a trachycarpus or experiment with something else entirely like sabals. Many people here have a trachycarpus, if you look for them you will be able to find them and people don't protect them here. They were apparently even found growing without human interaction just nextdoor in Vienna. Some have allegedly also survived -20C in northern slovakia without protection, but those were grown from seed there and were well adapted, they grew a much longer and more dense fur.

[jwitt]

34 minutes ago, UK_Palms said:

I have just seen January 2017 for your area as well. Every single night that month was below freezing and the average minimum was -7C that month. Compared to an average minimum of +0.3C for St James Park in London in December 2010.

 

What minimums do you think you had in your actual locality in December 2010, February 2012, January 2017 and February 2018?

3 of those 4 events look like they would take out Trachycarpus Fortunei, which will be completely defoliated at about -12C to -14C and 10 days/nights below freezing in a northern latitude. Fortunei has been killed dead by even less bad freezes in Scandinavia, Germany and the Baltics, where winter sunshine levels are very low. -10C and 2 weeks below freezing has killed them even.

I would say that Washingtonia is completely out of the question for your area, but if you are going to grow it you will need one of those heated box shelters most winters. Again it's 'do or die'. No two ways around it looking at the freeze history in your area. It's actually much colder than I first realised. I haven't even asked about how much snow you get there as well.

Also any talk about mirrors and glass is basically pointless looking at those temperatures and freeze durations for the months I showed. It won't save a Washingtonia without a hot box/house being constructed. @jwitt

33 minutes ago, UK_Palms said:

I have just seen January 2017 for your area as well. Every single night that month was below freezing and the average minimum was -7C that month. Compared to an average minimum of +0.3C for St James Park in London in December 2010.

 

What minimums do you think you had in your actual locality in December 2010, February 2012, January 2017 and February 2018?

3 of those 4 events look like they would take out Trachycarpus Fortunei, which will be completely defoliated at about -12C to -14C and 10 days/nights below freezing in a northern latitude. Fortunei has been killed dead by even less bad freezes in Scandinavia, Germany and the Baltics, where winter sunshine levels are very low. -10C and 2 weeks below freezing has killed them even.

I would say that Washingtonia is completely out of the question for your area, but if you are going to grow it you will need one of those heated box shelters most winters. Again it's 'do or die'. No two ways around it looking at the freeze history in your area. It's actually much colder than I first realised. I haven't even asked about how much snow you get there as well.

Also any talk about mirrors and glass is basically pointless looking at those temperatures and freeze durations for the months I showed. It won't save a Washingtonia without a hot box/house being constructed. @jwitt

"It was completely covered with frost cloth for almost three months and the lowest temperature it experienced was -9°C or 15°F for two nights in a row.."

From a zone Slovakian 7b/8a.

For these conditions glass and mirrors may help. 

I don't care if "those" conditions are Edmonton. 

So I will not get into a climate debate. 

All I am saying,if I was attempting such a palm in a northerly 7b/8a, I would increase the solar light/radiation, and get what the benefits of doing that, entails.  

Higher diurnal temperatures and lower humidity.