You should have a soil test done, mainly for acidity, soil composition, and basic nutrients. People might recommend fertilizer, but trees really don't need that - and most certainly not to maintain green foliage in normal conditions. Fertilizer may do more harm than good, especially if the soil ph is the problem.

There are several factors that could be the cause of the yellowing - IF that's really a problem. How was the soil prepared: how big a hole was this planted into, did the backfill contain amendments, or was it "native soil"?

There could be drainage issues. You can check the drainage by digging a hole near the tree, a foot or so from to the root ball, filling it with water, seeing how long it takes to drain. Explicit information is available on line, just google "drainage test" or something similar.

Also, why have you watered every week? Haven't you had rain in many of those weeks? Did you check to see that the soil was dry a couple of inches below the surface? Overwatering new stock is as bad as underwatering, and overall yellow leaves could be the result; underwatering tends to cause the lower leaves to drop, so the pattern can sometimes help you assess the situation.

Last, some trees DO start to change color as days get shorter - this can vary by species, variety or individual. You're smart to notice it, but you may not need, or want, or be able, to do anything about it.

I agree whole heartedly with dtds advise with the exception of; People might recommend fertilizer, but trees really don't need that - and most certainly not to maintain green foliage in normal conditions. as I too overwatered a newly planted shrub that started to yellow. I was advised to apply water soluble fertilizer (Miracle Grow) and it did help. Apply it by wetting the entire plant wait a week and you should see some improvement. If you are concerned about applying fertilizer to a newly planted tree this late in the summer you might apply it at half strength. Good luck it is a beautiful specimen. kt

It's more than likely not a problem. Newly planted trees don't have the extensive root system necessary to maintain a canopy of foliage. This stress can cause the tree to shed some of it's leaves. Quite often, this shedding process will take on the appearance of premature fall color. For magnolias, their fall foliage most of the time is a slight yellowing like you are seeing now. Only time will improve this problem when the tree has grown more of a root system. Fertilizing the tree at this point is not advisable. The tree doesn't really have much a a root system to take up the nutrients. If the root system that it has takes up any of the fertilizer, it may promote some new growth in the canopy that may not have time to harden off before winter. I would recommend that you keep watering it once a week unless we get enough rainfall. It should look much better when it leafs out next year.

I planted two deciduous magnolias in 1997. They grew great for many years. Then during the summer of 2006---9 years later---they weakened considerable and indeed the leaves turned yellow in mid summer. The 2 trees are about 50 feet apart and the same thing happened to both of them at the same time. So I suspect it might have been some common environmental factor, like dryness, as it was hot and dry that summer in my area. But I did decide to fertilize them the following spring with tree spikes. They returned to normal that year and are doing great again today.