Answer: When you deadhead petunias, remove each flower individually. Be sure to remove the bulge at the base of the petals, not just the petals. By midsummer you will find that the plants are beginning to look leggy. At that time, trim them back by about a third to a half. Then be sure to keep them well watered and fertilized and they will come back and continue blooming until frost.
To keep them healthy, water as needed to keep the soil slightly moist, never dried out. In the heat of summer and when they have grown large in relation to the size of the pot they may need watering twice a day.
Also fertilize regularly with a water soluble fertilizer such as 10-10-10 plus minors per the label directions. You may also use a slow release granular form of fertilizer, but I would suggest you use the granular type not a stick or spike form because the granules are more evenly distributed through the soil instead of concentrated all in one spot.
Finally, your petunias need to be in full sun or at least six hours of direct sun to stay healthy and bloom their best. Enjoy your petunia baskets!
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