The windmill palm, a general name for all members of the botanical group Trachycarpus, is regarded as the most winter-hardy palm that grows a trunk. Four species of windmill palm can survive temperatures as low as 5 degrees F without any protection. Placing a windmill palm on the warm side of a building, or wrapping the fronds or trunk with cloth, can protect the plant to infrequent bouts with -5 to zero degrees F.
Plant your windmill palm in a protected microclimate that is sheltered from cold winter winds and receives some supplemental warmth from a building facade. Gardeners are familiar with their yards, knowing which locations are warmer in winter, as seen with lack of snowcover, remnant green turf grass, or the earliest emergence of bulbs or shrub leaves in early spring. These microclimates are a good choice for the windmill palm.
Water and fertilize your windmill palm during the growing season so that the plant is healthy and vigorous. Healthy plants are better adapted and more resilient to adverse weather or pest conditions.
Sprinkle a granular fertilizer that is high in potassium around the palm trunk base, and water it in early autumn. Potassium is known to improve a plant's tolerance of cold.
Reduce watering of the windmill palm once daytime temperatures fail to get about 65 degrees F in autumn. Only allow natural rainfall to water the palm. Overly wet soils in winter cold can lead to increased palm injury.
Wrap the trunk of the palm with a cloth blanket when temperatures are expected to drop below 5 degrees F. If the palm is in a location that is exposed to winter winds, the wrapping should be done when temperatures dip below 20 degrees F. Wrap and gently tie rope around the cloth over the trunk to secure it.
Lift the fronds in the canopy of the palm so that they all are bundled and point vertically. Tie the fronds together in this formation. Wrap these fronds with a cloth blanket (as in Step 5) to protect them from cold when temperatures are expected to drop below -5 to -10 degrees F. The frond bundle should not be weighted down or bent over by the excessive weight of the cloth blanket.
Allow snow cover to mound up on the blanket-covered trunk, as it will help insulate the plant from cold air temperatures. Remove snow or ice from the wrapped fronds as needed so the bundle is not weighted down or bent over. If snow or ice accumulates on unwrapped fronds, brush it off so fronds are not weighted down or stressed so much as to break.