Answer: I had that same problem with my morning glories last year. Although water stress (or other stresses) can cause lower leaves to yellow, it may just be the natural progression of the growth of the vine. The plant sends its energy to young, vigorous growth and flowers. This year, I am going to plant some other, bushy, plants in front of the morning glories, to hide the yellowing lower leaves. Just be sure your plants are getting adequate nutrients and water--as long as the new growth looks healthy and vigorous, I don't think there is anything else you can do--but hide the problem!
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