Season of Planting:
Planting should be done with the first cool weather of fall, November 1st through the end of March.
It has been our experience that attempting to transplant the desert grown Joshua tree (yucca brevifolia) during hot weather is futile. A failure rate in excess of 90% can be expected outside of the cool months outlined. An important exception would be local plants that are fully adapted to Valley conditions.
Nearly any soil type seems to be acceptable although a rocky quick-draining soil is better. Joshuas do not have much ability to withstand continual wetness in the root zone. They can not be pushed into quick growth if water does not drain away. They can rot suddenly after years in the same location if kept wet.
We do not recommend modifying the soil as this tends to trap water between the well-drained root area and the non-draining area.
Size of Hole:
Obviously this varies with the overall size of the Joshua tree, but a general rule of thumb is to leave a small portion of the enlarged base exposed. If your tree does not have an enlarged base or your location has very high winds you may plant a little deeper. Leave a very shallow basin (1 to 4 inches deep) around the plant to hold some water.
Arizona Cactus Sales recommends that Joshua be planted in a muddy, almost soupy, consistency of the soil. Dig the hole you need a little deeper, add soil and water until you have a few inches of mush.
Push the roots down into this mixture being careful not trap any air pockets as a mildew-like fungus likes to grow in the cavities. Fill the balance of the hole with the dry soil and step it down gently.
Tamping at this point is utterly useless as the very wet soil underneath loses all consistency under heavy pressure and your Joshua will fall over. Your planting is relatively vulnerable to blowing over in the first few hours, but within a short time the mush firms up and holds like mortar.
Initially you can help your Joshua greatly by spraying the foilage with Superthrive. This supplement will be absorbed through the green parts of the foilage and works wonders to stimulate a newly transplanted Joshua tree. An easy way to do this is with a common household hand sprayer such as cleaning products use. The mixture is not rocket science and I mix it much richer than the one drop per gallon recommended on the bottle.
Later when you begin the deep watering you can add 4 capfuls of Superthrive to 5 gallons of water. Superthrive product is available at Arizona Cactus Sales. Water deeply the first calender year, 1 time per month in winter (November to March) and 2 times per month in summer (April to November). Then water summers only, 2 times per month. A bottle of Superthrive should last for the first calender year and then is not necessary after that.
Good Luck and Enjoy!