The Southern Magnolia is not common in the Tucson area, but if you see them in a plant nursery and wonder if they are a wise choice in the desert southwest, just think about where they are from and you will have your answer.
Magnolias do well in the deep south, where humidity is high and soils are acidic. This alone should tell you they will not like living in the desert. Our dry, hot temperatures are tough on them, as is our highly alkaline soil. Magnolias suffer in our very cold winter temperatures as well. Keep in mind the limiting factor for plants here in the desert is not just the high heat, but the winter low temperatures. In fact, hard freezes are more limiting than hot summers.
If you have a Magnolia and notice yellow leaves with green veins, your plant is suffering from iron chlorosis. This is a condition caused by iron deficiency. It’s not that there isn’t any iron in the soil; it’s that the high pH prevents its absorbtion. While you can add iron to the soil to help correct the problem, this is a difficult if not expensive amendment to add on a regular basis. You must use chelated iron to get results, and you must treat the entire root zone. If you choose this route, it will take one to two years before you notice any difference.