How do you plant a tree? As the old 
joke goes, green end up. In Tucson, it’s a bit more complicated, and one big reason for that is caliche.
Caliche is a hardened soil layer common to desert soils. It is made up of calcium salts and minerals (mostly calcium carbonate) which are naturally present in the soil. In rainier places, minerals and salts in the soil are flushed through by rainfall. In drier climates like ours, over time these salts and minerals build up and form a hard layer (also called hardpan) anywhere from several inches to several feet thick. Anyone who’s tried to dig a hole by hand in Tucson knows what it’s like to try to dig through caliche — a pick or a caliche bar will be your best friend. For larger jobs, you may need to rent a jackhammer, or even a backhoe.
This hard layer makes it tough on new plants — particularly trees — because their roots won’t get the drainage or the room they need if you leave the caliche in place. In addition, if a tree’s roots end up growing shallow due to the caliche layer, the tree will be in danger of toppling once it gets taller. This would not only kill the tree, but potentially be a hazard to property and people. The University of Arizona Extension Office has a helpful handout on managing caliche.
Guidelines for tree and site selection in the Sonoran desert
- Select a native or desert-adapted tree. These trees will be more adapted to our desert soils and will cost you less in watering and maintenance. You will also want to select a healthy tree from the nursery; here are some guidelines on what to look for. If you want a mesquite or palo verde, check out this helpful guide from the University of Arizona.
- Select a good site for your tree. Avoid areas where it will cause a potential hazard, such as near power lines, near property lines, or close to your plumbing or septic system. This handout from the University of Arizona’s Extension office has some great tips, along with great examples of what NOT to do.
- Plant the water before you plant the tree. The Watershed Management Group has a great handout to help you ensure that your new tree has all the water it needs throughout its long life. After the initial 2-3 year establishment period, native trees can thrive on rainwater only.