I just bought a 3gal windmill palm from ebay and I was wondering how to take care of it regarding winter protection for it in zone 7 nyc and when to put it outside for good.
I also recieved a 3 gallon windmill palm off of ebay about a year and a half ago in March of 2009 and planted it in the ground in early march soon after I recieved it. It saw one or 2 nights below freezing and of course had no problems! It didnt grow too much until this past summer. I protected it with some Christmas lights followed by a frost cloth and a plastic garbage bin over it to retain some more heat and keep it dry. It kept the palm about 20 degrees warmer than the actual temeprature outside which was more than enough to keep it happy. Its easy to apply and works well. Its also not that hard to find the supplies. The only downside to this method of protection is that your palm will eventualy out grow it, but not for at least 4 years or more from a 3 gallon.
I grew mine as containerized plants until they got larger than my largest tub/containers then planted it in the ground. I protected the trunks with frost cloth and X-mas lights for the first couple of years. Last Winter they were not protected at all but they are in a good microclimate (southern exposure not far from the house). They're about 4-5 feet. Larger palms gain in cold hardiness as they get big. Protect from hard frost but try to keep COOL as a containerized plant over Winter then plant in early April. These palms are VERY cold hardy but appreciate well draining soil, and some wind protection.