Any one have any experience growing Yucca brevifolia (Joshua Trees) in an area that might see 40 inches of rain a year?

As far as soil preparation goes I'm a minimalist. The 20 x 10 spot is along my driveway near the midway point of a long gentle hill. I've blocked off most of the water coming from up the hill with landscaping and have allowed the area to continue draining well.

Thanks for any help. Even a 3 foot tall Joshua Tree will make my Bradford Pear growing friends notice!

And St. Louis is too cold in the winter for long periods.

Any ideas on the most desert like plant I can expect to try in St. Louis?

You should try yucca schotti (available at Yucca-Do nursery).

Yucca baccatta may make it. High country gardens (not my favorite vendor) has Mora, county NM ecotypes that see temps colder than St. Louis and may seen even more snow. The Moreno valley is the coldest in NM - they saw -40 twice 2 years back further up in Angel Fire. Angel Fire, NM nearby was the coldest place in the nation a couple of weeks ago (in June!). They also have some cacti from Taos, NM that may make it.

There are native MO yuccas and cacti. Do a web search. You have some right in your backyard! They can probably take your wet if you duplicate their MO environment.

Opuntia imbracatta also may be worth a shot. Find CO seed or plants.

Good luck.