Yes, so sad, i was hoping a bit about that liquid, but i had not to many illusions.
Here the climate is unluckily very cold on some winters and then i proceed as follow for my palms:
When i get a dry climate's palm i first prepare a hole about one cubic meter, the i mix the content with volcano rocks and river sand, i make a little hillock.
I plant my palm, then for the winter i surround the palm with a sewed bag made by frost blanket closed by tighting string.
i cover the foot of the plant with dry grass. above it i roll a silicon christmas light string for outdoor use type as Duralight about 5m long (16W/meter).
I prepare a foldable roof made by a pair of concrete's iron gate, soldered together by a pair of recycled door's hinges to make it easy to tidy on spring they are covered by polycarbonat 2 layers pannels. On each corners are soldered a nut to screw the corner's posts inside for easy installation and removal.
i install my 4 posts around the plant each one of them is surmounted by a soldered screw and its nut. then i screw the roof above them.
i surround the installation by a reed carpet to insure a good air flow inside to prevent moisture and condensation. Once the material is ready , need about 15 to 20 minutes for installation each winters per plant , then i think it is a good deal for just a few money
don't forget to close all opened sides by a plastic pasted with a good quality outdoor use tape (such DuckTape)
Using it i could preserv on -5°F(-21°C) Jubaea Chilensis (perfect), Butia Capitata (alive but in poor state) , Chamaerops humilis (50% defoliate).
For Basjoo bananas i just cut the leaves surround the trung by dry grass, surround by a reed carpet and cover by a plastic hat on top, no need any heating systems, that would even be dammagable.
good luck everyone