I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Plants will do this when the plant is over or under watered. Container grown plants are especially susceptible. The older leaves (those farthest down the stem) are usually the first to go. The symptoms are often delayed a bit after the water stress conditions. So you may have recently watered but still see the problem. Overwatering contributes to root loss from lack of air and is often complicated by an ensuing root rot. This situation can result in a long term setback for the plants.
Lack of nitrogen can also cause a gradually loss of the green color in older leaves, but such symptoms are different from the sudden yellowing and dropping most often encountered.
Finally, evergreens such as magnolias often experience a spring leaf drop as the new growth comes on and the older leaves are cast from the plant. This is normal and varies in degree due to soil and site conditions, plant genetic variability and other factors.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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