I'd go with the Blue yucca rigida or the rostrata. The rostrata seems to be more cold hardy but being in a zone 7, i don't think you should have any problems with either.

I am a landscape designer/contractor in Boise, ID. I just finished a subdivision called Saguaro Canyon. It is our experiment for introducing some new varieties of plants to our valley. I will tell you what we planted. In the large plants we installed 25 yucca faxoniana and Joshua trees, both in the 12-17' size. We also planted a large number of yucca rostrata, yucca ellata, yucca thompsoniana?, sotos(desert spoons), cholla and ocotillo all in the 5-9' range. I have got to tell you we have had more attention with these trunked yuccas than anything we have ever planted. They just came through their first Idaho winter. We had a 4 week inversion in which the daily temperatures never reached over 32 degrees and about 5 nights in single digits. I am happy to say that all of our plants came through great. (especially since it seemed like a big gamble at the time). Timber Press puts out a great book called yuccas and agaves, very informative.

Wow. that is right up my alley. your story has inspired me do what i set out to do. I just wanted to hear some success stories from places that have a similar climate as my own. What zone is Boise? I am very familiar with yuccas having studied them extensively in college. i really want to find a small rostrata and rigida. Also i'm half tempted to get a joshua tree but i'd need a large specimen and i'm not sure where to find a decent one for a fair price. Where did you pick up your LARGE plants.
Thanks
Don

How long have you been growing Windmills in the St. Louis area? How big are they?

Mike

Windmills don't take that much to be happy..
I planted 2 last spring they were about 2 ft tall ,
I would bet they are 5 ft tall now .
been through several frost and several nights of 20 temps.
Cold does not bother them !

In my opinion they will be fine in colder climates
The main problem is not cold it is heat : )

Don ..... I grow half and dozen or so Yucca trees here on the east coast and I actually like the look they give more them palms. The grand-daddy of them all in size is the Yucca faxoniana ... huge barking trunk in diameter and very cold and moisture tolerant ..... the Rostratas are nice too, the Yucca elata in my opinion is the most cold and moisture tolerant maybe only second to the Rostrata.
Make sure you plant them in well drained soil, do not over water, and you may want to raise the bed a good 6-12 inches .... make sure you identify where the root and trunk lines are and do not plant the yucca above the trunk line, it will rot at the base if you do so ... do not heavy mulch either ... Yuccas are very cold tolerant, more so then palms but they can not stand long period of moisture in the winter months, mulch holds to much moisture around the base, they are better off taking the cold at the cost of keeping as much moisture away from the base. Calplants mail-order in Vegas has large size Yucca trees for sale and ship. Good luck

Like another post here ..... Windmill palms, do you protect them in the winter???

I would definitely advise against trying a Joshua tree except in a VERY well-drained location in an arid environment, zone 6 or warmer. Jrroma's advice above about mulch and drainage is good, but my own Yucca faxoniana still died from too much moisture, even in well-drained soil and a dry climate, because I had unwisely planted (and watered) annuals around it. The base of the trunk rotted and the whole huge plant just keeled over.

Yucca rostrata might be better, and Yucca schotti is also good with more moisture, and has grown fairly quickly for me. 'plantdelights.com' is a good source of small yucca plants.