I would not do it. Joshua trees are desert plants  your zone is too cool. They also do not like transplantation. They thrive in their native soil, but the native soil can compact in pots. Too much moisture during the winter can destroy these plants.

Joshua tree national park is high desert. Here are some temps these plants thrive in:

http://www.joshuatree.org/visitor/weather2.html

Yucca brevifolia is rated hardy to 0F. Of course, they don't like to be wet in the winter and they won't grow very fast in borderline conditions. I think an rid zone 7 they would survive, Alabama zone 7 they won't last many winters.

Thanks for the feedback. So it seems that really my biggest problem is the wet winters we get here in Alabama? I have read that drainage is the most important for joshua trees.

Agaves & yucas seem to do well in this climate as do several palms. I live on a small mountain ridge and was thinking about planting the tree on a gradual slope in full sun. Maybe the only way to do this would be to build a little greenhouse around the plant until it got more mature - even then, it might not make it after it gets bigger and doesn't have the protection - just thinking out loud here. Thanks again!

What is your elevation, they need above 3500 feet. They need desert soil, they can take freezing temps. with snow on the ground at Christmas. They love the sun, and no water during the summer months, you must duplicate the growing condition that are in the high desert. They do not like humidity. They seldom survive out of habitant. Norma

Do you know where your seed was collected? Joshua tree habitat includes altitudes from 1,500' to 7,200' and winter lows as low as -15F in the northern part of its range, but in the southern part of its range, winter lows are much less severe. It survives out of habitat pretty well in Salt Lake City and Albuquerque, and if planted as a seed grown plant it will even thrive in Vancouver, WA so I think you have a chance. I also know of a small one in Bremerton. Plants dug from the wild are more apt to have problems out of habitat. With seed-grown, containerized plants I think you have a pretty good chance.

I'm not sure where the seed was collected unfortunately. I got them from J.L.Hudson Seeds (La Honda, CA) several years ago. The trees are doing really wonderfully in the containers even though we do have high humidity levels here in Alabama. The trunks are really starting to grow so I am getting worried about how to container them in the years to come.

I just checked the elevation here and I'm only at 990'. I may plant one in the spring so that it has all summer to acclimate and see what happens.

Thanks!

I wouldn't try planting a Joshua tree in Alabama because of the moisture and wetness there...But, I live in Denver, Colorado and I have a Yucca Brevolia v. jaegernia (didn't spell that right) growing in the yard. It's doing very well, and enjoys the high amounts of sun in Denver. Also, the dryness and the poor soils (similar to those of where the plant is from), and the elevation; 5,280' above sea level. You could try to plant them outside, but unless you don't want to kill your Joshua tree, I wouldn't.