The largest species of magnolia tree, the southern magnolia, can grow up to 80 feet tall and 50 feet wide. Magnolia trees can be deciduous, semi-evergreen or evergreen, depending on the species. They may lose leaves year-round. However, if the tip leaves are falling off or the leaves are yellowing and dropping with no new growth showing, the tree may be experiencing one or several problems. Proper diagnosis is essential to stop the leaves from yellowing.
Magnolias are particularly susceptible to iron chlorosis or iron deficiency. This deficiency can occur when they are overwatered. The iron in the soil leaches away from the tree and is no longer available to the tree. Severe iron chlorosis turns the magnolia leaves a pale yellow, sometimes with brown spots. Installing a tensiometer helps to monitor soil moisture and to prevent overwatering. Fertilising the magnolia tree with an iron-based fertiliser may help the problem.
High alkaline soils with a pH of 8.0 or higher can also cause iron chlorosis. There may be enough iron in the soil to sustain the magnolia tree, but the high alkalinity of the soil blocks iron absorption by the tree. The whole tree may have pale yellow leaves or just one side or branch may be affected. Amending the soil by acidification may improve the iron absorption rate.
Magnolia trees can become deficient in nitrogen. This nitrogen deficiency causes the magnolia leaves to turn fully yellow. Test the soil around the magnolia tree on a regular basis to determine the nitrogen levels in the soil. Yearly applications of a nitrogen-based fertiliser at a high rate when the tree is young and at a lower rate as the tree matures may rectify the problem of yellowing leaves.
Verticillium wilt is a magnolia tree disease caused by a soil fungus. The leaves wilt, turn yellow or brown and drop before new foliage growth has a chance to appear. Pruning off the infected leaves and raking up the dropped yellow leaves from under the tree should clear up the fungal problem. Clean all pruning and raking tools with a solution of two parts hot water to one part bleach to prevent a recurring fungal infection.