Answer: If your geraniums are developing yellow leaves lower down on the stem but they're still producing healthy green growth at the tops of the plants, there's nothing to worry about - the plant has finished using the leaf for food manufacture and will drop it in favor of producing new leaves. You can snap the yellowing leaves off and new leaves should grow in their places.
Some petunias do not require deadheading but I find the plants look better and bloom more profusely if I pinch off the old flowers before seeds develop. I simply pinch off the flower along with some of the stem. The remaining part of the flower stem will die off. To make your plants bushier, pinch back a little further - to a leaf node (intersection). Wherever you pinch, two new stems will develop.
Best wishes with your annuals!
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