What is causing the leaves on my tree to turn yellow? ASK THE HORT AGENT
Question What is causing the leaves on my tree to turn yellow?
Answer Unless the foliage is variegated, yellow is not desirable. Changing color during the growing season is usually a bad thing for mature leaves. Leaves change color for one reason – they are dying.
Several scenarios can cause the death of a leaf. Forces outside the plant (insect, disease, freeze, vole, etc…) can cause leaves to die. These pests usually disrupt the cells within or the nutrient supply to a leaf. Typically, these situations are more isolated. In other words, it may only be happening in your yard.
Plants will also kill their own leaves to increase the chance of survival. This type of event is usually a response to stress. Drastic changes in light or water are the primary triggers of this response. Drought, flooding, excessive shade or sun will cause plants to start killing leaves. Mature leaves can not change their structure. Therefore, plants get rid of old leaves and grow new leaves which are more compatible to the stress factor. Sometimes it works and sometimes it doesn’t, but it’s the only option they have.
Not all plants respond the same to stress. Specific trees, like yellow poplar and wild cherry, are more sensitive to drought stress. Their leaves will begin to yellow much earlier than oak or magnolia leaves. However, if the yellow poplar and wild cherry trees are turning yellow, then the oak and magnolia trees are also being stressed even though they may not be showing the symptoms yet.
Newly planted or young trees are also more sensitive to stress than established trees. Other factors like low pH, fertility, compaction and nematodes can increase the severity of a stress like drought. Environmental conditions are broad and affect many plants over large areas. In other words, you can ride down the road and see other plants with yellowing leaves.
Most of North Carolina is experiencing dry conditions. If the mature leaves are turning yellow on your gardenia, mulberry, apple or some other plant, it probably needs water. You have three options – water it, pray for rain or watch it croak.
For great info on how to water a tree, visit http://warnell.forestry.uga.edu/service/library/index.php3?docID=164&docHistory%5B%5D=2
If you don’t have internet access, then call my office at 893-7533 or email me at gpierce@harnett.org.
If your prayers are not answered, the fall is a good time to replace dead plants. You may want to consider a different (more suitable) replacement. Have ya’ll noticed the yellow leaves on other various things like politicians, the United Nations and the search for Bigfoot?
Gary L. Pierce
Horticulture Extension Agent
Harnett County | 
