How to Winterize Your Cold Hardy Palms
For the Trachycarpus :
Douse the growing spear with a copper based fungicide and powdered sulfur and then put clear plastic around the plant. Fill the bag with crumpled up newspaper or leaves, which act as an insulator. Place mulch on the bottom of the plastic bag that touches the ground so that no wind can pass through the bottom. Then wrap burlap around the top of the palm to keep the sun from baking the plant. Do this when the temperature is expected to drop and remain below 25 degrees Fahrenheit.
The center spear is the most vulnerable part of the plant. When temperatures rise above freezing, the fungus diseases will destroy the center spear. Therefore, place an air vent hole within the side so that it doesn’t overheat during the day. Do this if the temperatures during the day rise significantly above freezing, but nights tend to drop below 27 degrees Fahrenheit.
These palms are hardy to about 3 degrees Fahrenheit if foliage and center spear are kept dry over winter.
For the Rhapidophyllum Hystrix, Nannorhops Ritchiana and Sabal Minor Varieties:
Make a cage around the palm. Line the cage with clear plastic. Leave extra plastic at the bottom and anchor with soil or mulch. Stuff cage with dried leaves or crumpled up newspaper and pack tightly. Completely cover the palm with the packing. Fold excess plastic over top and seal using any method.
All cold, hardy palms must be planted in the ground. The more established the plant, the more tolerant of cold it will be. An extensive root system helps enable cold tolerance. During growing season, complex carbohydrates are stored in leaves for cold tolerance. During winter, dry soil is good for the palms, helping to maintain high levels of complex carbohydrates.
The healthier you keep the plant during growing season, the healthier it will be for winter. Fertilize during growing season.