For the past several years, I've been experimenting with hardy palms in our area. At first, these were in our VERY protected Zone 7 garden in Georgetown, Washington, D.C. and then on the farm.
I found the windmill palms, Trachycarpus fortuneii, to be completely hardy in D.C. with minimal protection, even when young.
I started with FOUR T. foruneii 'bulgaria', a strain from Soviet era plantsmen looking to increase winter-hardiness in subtropical plants. The seeds were obtained from large specimens in Bulgaria that had survived very cold temps without protection.
After moving to the farm, I transplanted my 3 surviving trachycarpus fortuneii 'bulgaria' into a bed of Ilex verticulata in 2007. Of those, because of their young age and the colder temps, only one survived the winter of 2007 and 2008.
In 2008, I added a second Trachcarpus fortuneii that I bought at a local nursery, at about 3 feet of height. palms weathered the winter of 2008 - 2009 with significant leaf damage, but both refoliated quickly during the summer.
Winter protection was provided by bales of straw with a
rigid, clear plastic piece overlying as shown in the picture. Both palms were treated with copper-based antifungal powder in their crowns.
Lowest recorded temp without wind chill for both winters 2007-2008 and 2008-2009 was approximately 8 degrees Fahrenheit, as recorded by our weather station.
This year, we plan on using clear plastic umbrellas or cloches if you will for winter protection. I have added
both Trachycarpus fortuneii Tennesee form and Trachycarpus Wagneranius to our collection during the summer of 2009 and will keep you posted as to how they winter over this year!