Answer: Some of that turning of foliage is a natural response to the arrival of new growth. A plant will often do this as it prepares to discard the older set of leaves. Other causes are drought or soggy soil, both of which result in root injury followed by yellowing and dropping of older leaves. Fertilizer burn can also damage roots. Finally, as new foliage covers the older growth, it excludes the light to those leaves. In response they can lose their deep purple or burgundy color and in time even turn yellow and drop.
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