Windmills are solid 7b performers. Get it in the ground now to get it well established for the season; it'll do very well well in 8a. Still, in your area placing it about four to six feet from the house on a south facing wall will protect it from cold north winter winds. Perhaps this first winter you can put a frost cloth on it if you have some extreme weather below 15. I'm borderline area in coastal Delaware Andy I've never protected windmills. Friends Andy neighbors have specimens in the ground for 10 to 15 years unprotected that are 10 to 15 feet tall and produce flowers and seeds.

No fertilizer after you've planted it to give it time to aclimate. Plenty of water all growing season. Others can give tips on fertilizer, but generally a slow release like 10-10-10 is good. Others like more expensive palm specific fertilizers and epsom salts.

I would definitely plant it in the ground. It will do much better in the ground than in a pot. I have Trachys in a pot and while they do very well, they do not grow as fast or as green as the ones in the ground and I'm a zone 7. As long as you dont get below 15F there will be no problems and when established they can easily handle 10F safely. Mine went down to 7F it's first year with no protection and it survived with just a little spring damage.
Like wetsuiter said, plant it soon so it can establish for the season. Now is a perfect time to plant! It wont grow fast the first year because it will be establishing it's roots, but after a year or 2, it will really begin to fill out fast!
Good luck!
-Alex