In November, I covered four trachycarpus fortunei with just a double poly box and strung 100 miniature Christmas lights around each. The lights were connected to a thermocube 35F on/45F off. Total power was around 40 watts per strand, so you can see each did not receive a lot of supplemental heat. I relied on the sun's solar energy to provide the majority of the heating. Well, the cold weather came and it got downright COLD in the boxes. Temperatures ranged from 8F to 18F within the boxes on the coldest nights of winter when ambient air temperature was around -10F. During the heart of winter, the ground froze to within a few inches of the trunk in each box.

Now, that the snow has receded from two of the boxes, I removed the boxes and lights for the summer. The other two will require a few more days of melting before I can remove them. Here are the results from the first two boxes.

Closeup of spear.

No spear pull on these two. Fronds look in mint to near-perfect condition yet. Soil is soft and moist around the palms; no longer frozen. I did not provide any supplemental water all winter.

As I stated above, the other two have a few days to go yet. 55F tomorrow and 60s on Thursday should let me get at them! (grin)

I think they main key to survival on trachycarpus fortunei is:

1) Getting above freezing each day. The solar energy propelled the temperature in the boxes to between 45F and 55F even on days it was 0F to 5F outside.

2) Keeping the wind off the palms. Even though it fell into the single digits, the air was dead calm within the boxes.

3) Ensuring excess moisture does not sit around the palm. Snow melt was their only source of moisture all winter.

4) Sunlight. Palms do not go dormant, thus require sunlight in the winter. I've seen so many people lose their palms due to wrapping them up tightly in leaf enclosures or enclosures where light cannot penetrate.