Question
I live in Southern California, and have recently planted 10 carolina trees along the side of my property. They are about 8 feet tall, and are planted on ground that is surrounded by a two foot retaining wall. (Causing the trees and the soil to be slightly elevated)

My soil is very hard clay, which tends to drain very slowly. After planting them 3 weeks ago, the weather turned very hot, in the 90 to 95 degree range. I set my sprinklers to water them every day, for around 5 minutes, because I was afraid they would dry out in the heat. Now all of them have developed yellow leaves. The yellowing started at the center of the tree, near the trunk, and has slowly worked it's way out over the past week.

I'm looking for some advice on what to do next. I have stopped watering them completely, in hopes that the soil will dry out, but it seems to be taking a long time. The yellowing is just getting worse, and I am really afraid of losing all of these beautiful new trees! To make me even more confused, I read on one tree-planting site that yellow leaves near the center of the tree is a sign of LACK of water. So now I'm at a loss. The soil still seems slightly moist a few inches below the surface, so I doubt the problem is too little water.

Is there anything I should be doing to help these trees immediately? Is there a way to help get oxygen to the roots in my clay soil? Other than simply waiting for it to drain out? Any direction you could provide would be greatly appreciated.

Answer
Hi Chris, Internal yellow foliage is usually a sign of a lack of water, which oddly enough can be caused by too little or too much water.  No plant wants to die, if you under water it, it sheds foliage in an effort to survive.  If you over water it, the feeder roots die and now the plant can't absorb water so it dries up and dies even though it may be sitting in water.  The result is the same. Water thoroughly when you water to saturate the rootball.  Check each rootball the next day.  Stick your finger about knuckle deep into the rootball, not your surrounding soil.  If it feels dry, water it, if it feels moist, wait, if you're not sure, wait.  You have clay soil which the plants should do quite well in.  The elevated bed should help improve your drainage.
Here are some other things to check out.
Are you sure that the retaining walls aren't holding water?  Could there be concrete or plastic in the bottom that causes the bed to hold water?  Did you over-improve the soil?  Was the clay loosened and mixed with a soil amendment or was soil removed and replaced which would create a "clay bowl" effect?
I think if you adjust your watering, the plants will respond.  If you don't see some new growth soon or the leaf drop continues, I'd consider lifting one to see if there is water underneath.  If there is, them lift them another couple of inches to improve their drainage.  Let gravity do the work.
In the meantime, try this...
In 1 gallon of water, add 1 tablespoon of a water soluble fertilizer, 1 tablespoon of Epsom salt and 1 teaspoon of Superthrive.  Apply 1 quart to each plant weekly for 4 weeks.
I really believe you initially under-watered the plants which caused the yellow leaves and I suspect you may have over watered since.  Water with caution.  Jim