I have two organpipe cacti, one very tall and weak-stemmed (support by cord from the top), the other low and squat and budding. Both have decided to turn rather yellow toward the tops. Can they be coaxed to turn green again, or should they be pruned down to the green. They are kept in a very dry part of my yard in San Francisco, facing east, and get good sun about 1/2 the day.

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CORKER