I brought along an interest in this subject after living in Washington, D.C., where microclimates allowed us to grow plants normally found in more southern climes like ficus pumila, southern jasmine, voodoo lillies, and a host of other plants 'that don't grow here'.
Of course, the hardy palms and agave were part of this tradition.
Last winter was pretty harsh with very cold temperatures and heavy snowfall. We had two trachycarpus fortuneii that wintered over with significant protection and several colonies of echincereus, the claret cup cactus, which did very well with no protection in pots.
Here are two Trachycarpus that wintered over with minimal trouble last winter. This season, I decided to try them with no protection.
The top specimen is a generic plant I picked up at a local garden center here in Maryland.
The lower specimen is smaller and is one of my original 'Bulgaria' Trachycarpus, originally one of 4 growing in my Georgetown garden, and the only one that survived multiple transplants before settling into its current location on the farm.
These Bulgarian palms are offspring of fortuneii palms growing in Sofia, Bulgaria, that have survived very cold temperatures. They are thought to have originated from Soviet-era breeding programs north of the Black Sea working to develop more cold-tolerant strains of palms, citrus, and the like.
As many gardeners who have experimented with these cold-hardy palms, one thing that tends to kill them off is fungal disease, in particular affecting the crown of the plant. So, in theory, they may perform better despite the cold with the better air circulation provided by NO winter protection.
Certainly, the larger the specimen, the greater their cold tolerance appears to be.
In addition to my original specimens, I added several Trachycarpus fortuneii and Chamerops humilis 'cerifera', the Morrocan form of the European Fan Palm, to the front garden of our house. Previously, the standard green European Fan Palm had proven NOT to be hardy in our protected Georgetown garden in Washington, but the cerifera (blue Morrocan) form is felt to be more cold-hardy.
While in Morroco on vacation, we witnessed 20 foot specimens of the Morrocan Blue Palm growing in the Atlas Mountains outside Marrakech at high altitudes in very cold conditions (maybe 20's with heavy snowfall and no protection). I love the blue color and found these to be very cold-tolerant in D.C.. The ones we have grown seem to be more of a clump forming palm, but all the specimens we saw in Morrocco, were single trunked and quite tall.
The last palm we have on the property, the needle palm or Rhapidophyllum hystrix, is native to the Southeastern U.S. and is one of the more well-known cold-hardy palms. It is definitely a clump-former and there are large specimens of this palm in the National Arboretum in Washington, D.C. I expect this species to be very reliably hardy on our farm.
It does require hot, humid summers and does well in our area. It has been reported to tolerate temperatures as low as -15 to -20 degrees celsius (~ 0 to -5 degrees F).
It is also commonly known as the porcupine palm for the 2 inch, needle-like projections around its trunk!
On the Agave side, we are trialing species including Agave parryi and a cultivar, Agave 'sharkskin' (shown immediately below).
So far, we have had a very cold December with lows hitting 14 degrees Fahrenheit. Many of these days have been associated with high-winds and wind-chills around 0 degrees.
I'll keep you posted as to how my little friends do this winter along with minimal temps and other findings.
If you're interested, many of these plants can be ordered through Yucca-Do Nursery http://www.yuccado.com/ and Plant Delights Nursery in North Carolina http://www.plantdelights.com/.