Answer: Removing the spent flowers on petunias is optional. Most people don't do it because it is too tedious. However, if you feel it would help their appearance, it won't harm the plant a bit.
Petunias are usually quite tidy about dropping their old blooms quickly and neatly. If you are watering the plants daily with a sprinkler, you may be creating some disease problems on the blooms causing them to look bad. Also, the flowers usually go downhill quickly after getting wet from rain or a sprinkler anyway, so try to water less often by applying more when you water, and then giving them a few days before watering again. Better yet, drip irrigate to avoid wetting flowers and foliage.
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