Answer: Sounds as though the water stress shortened the life of the flowers. I'd prune off the droopy flowers. Pruning will encourage your plant to develop more flowers - this time they should be healthy and remain for several weeks. Mums appreciate a deep soaking once or twice a week, depending upon weather. Watering every day will keep the soil too moist and will drive out all the oxygen - roots need to breathe! So, water when the top inch of soil dries out and water thoroughly so the entire rootmass gets wet. Then wait until the top inch of soil dries out before watering again.
Best wishes with your mums!
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