Mine showed no damage at 16F, but a young palm that isnt established will probably receive damage just a little below that. Once older and established a windmill palm will probably begin to show signs of damage below 10F according to duration and moisture.

-Alex

I'm afraid it's a bit more complicated than giving an absolute temperature palmguy. Normally they are listed as SURVIVAL hardy to zone 7 (min./average temperature 0.0 F). Note, this does NOT mean that the temp reaches 0 F every winter, only that it may occur. Note also that some leaf damage will occur at much higher temps. But this depends on a lot of other factors--(sun & wind exposure, soil conditions, nearby buildings, general health, size and length of time in the ground, soil moisture (drier is better), icing, the genetics of your particular plant, and also the DURATION OF COLD (do frigid periods last for months/weeks/days/hours??)--It matters. In my particular site, we had a minimum last winter of 14.0 F, had a VERY WET, windy, and snowy winter, but saw virtually no damage in the spring. I saw more damage two winters ago when the plant was a year less established and the minimum got to 4.0 F., but it did grow out of the damage fairly quickly--growth actually starts as early as March in their current location.--Hope this was helpful.--I would suggest that you protect annually in a 6a zone though--ESPECIALLY IN THE FIRST FEW YEARS.

Thanks for the replies guys. The trees have been in the ground for about 6 months and they are 3 year olds (they came from Florida so this is their first real winter). I know that if they haven't been in the ground long to be extra careful. They are getting lots of Christmas lights, fibreglass insulation and poly. They have seen about 27F maybe so far with no changes. I was just wondering at what temperature they would be injured at.

I had my Styrofoam box blow off of my Windmill Palm several years ago when it was smaller, in the middle of December, before I had secured the box very well. I was out of town that day. When I came home it was 10 degrees. I put the box back on, and in the Spring when I took the box back off, there was absolutely no damage to the leaves.

As long as the plant is established and the site fairly sheltered by winds, a larger plant should not show damage even down to 10 F.. But again, duration of cold is as important as absolute minimums.

When I originally planted my palms(Trachys) I set a minimum temp of 10F as my goal to keep them above.....

The leaves were not beautiful in spring but functional and considering they hit the teens many times......they were fine.

Here's what it looked like April 19th, 23 days of teens and 2 times as low as 9F.

Keep in mind it was covered-in an umbrella greenhouse all winter.

Here's my unprotected Tracy in February of 2010. It was a wet and very snowy, windy winter, but minimum was only 14.0 F. It's protected by the house on two sides and faces directly south (unobstructed). There was hardly any leaf discoloration in the spring. I can't wait until it flowers! But am betting I need both male and female for seeds. I think the ability of the this palm to survive occasional arctic plunges depends on frequent winter thaws--so that subfreezing temps are not constant (day and night for weeks on end).

Here is a link that might be useful: