Self-cleaning, no deadheading necessary, this is not necessarily true of all Petunias.
Fertilize often for best summer performance.
Late July and August is when plants should be at their best, but there is only one problem: The plants have run out of energy and the gardeners realize it too late to help the plants recover. This is especially true in raised containers and hanging baskets. Here is the best way to keep your Petunias in the Super category.
1. When you buy them in the spring, buy a slow release fertilizer to top dress the basket or container. If you plant your own basket, incorporate some of the slow release fertilizer in the soil as you plant. Follow the package directions.
2. In May fertilize with a water-soluble fertilizer once every two weeks. Mix up the fertilizer and water the plants just like you would if you were using clear water. It normally takes a good half of gallon of water to really water a 10" wide basket or raised container.
3. In June as the weather starts to get warm to hot, fertilize weekly, again with a good soaking. If the weather turns really hot as it sometimes does in Late June you may need to fertilize every third watering. By now the Supertunia is really growing and starting to tumble down from the basket or fill out across the flowerbed.
4. July is when the cutting back occurs. Around the 4th of July, (after your big party) get some of the slow release fertilizer that you bought in spring and re-apply across the top of the planter. At the same time, trim back some of the longer branches just enough to bring the plant back in line with the bottom of the pot or basket. Don't remove too much at the maximum cut back 20% of the branches or 1 in five shoots. You can also just give it a general light trim. Your plant will be out of flower for a few days, but will come back stronger than ever. By now with July's heat you should be watering at least every other day and begin to fertilize every other watering. I know it sounds like a lot, but if you want a plant to grow like an elephant and be the most it can be, you got to feed it like an elephant!
5. August is, hot, humid, & sometimes with monsoons. Keep up the water and fertilizing, and again, if the plant starts to look straggly remove a few more branches but never more than 20% or give it a general trim as before.
6. It is September and the plants should still look good, start to back off the watering and the feed, but shape the basket with the last pruning of the season.
I know it sounds like a lot of work, but it really is a labor of love. I fertilize my baskets and my garden every time I water, and I water daily from mid July through August. I recently invested in the WaterWise irrigation system and let me tell you it really works. You will find that your Supertunias, along with all the rest of your plants will keep looking fantastic through September and perhaps with the luck of a gardener and a late frost maybe even into October.