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They will, definitely, grow in hanging baskets. You will only need one plant per container, though. They spread out to about 4 feet, and more than one would cause competition between plants.

This article will give you more information on their care: https://www.gardeningknowhow.com/ornamental/flowers/petunia/wave-petunia-plants.htm

I have several thoughts. One is that the weather has been cool for petunias. They like summer temperatures and sunshine. So don't throw them away; they will perk up. It will help to plant or repot them. Often the container they come in is too small. Each flower only lasts a few days then wilts and drops off to be replaced by more flowers. It is this unending supply of flowers that make petunias popular. Click on the link to see more tips for growing petunias. https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm

They will need a good 6 plus hours of full sun.
This article will refresh you on these plants.
https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-plant-no-flowers.htm
https://gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm

There is no trick. The issue is that Wave petunias are hybrids, a cross of the genes from 2 parents. Genetics determine the color and behavior of plants much like children; children rarely look exactly like either parent and their behavior is not the same. Each child has a different mix of parental genes. If you grew bunches of saved seed, you might get a few that wave. Many hybrids, including Wave petunias are trademarked; therefore it is illegal to grow it from saved seed or to propagate the plant through cuttings - even for your own garden.

Petunias are, not at all, picky. They will tolerate just about any range of conditions, and thrive just fine. Wetting the leaves will not harm them, and they will do much better outside in the rain, then watering with tap water anyways. I know that many places will state that getting leaves and flowers wet will harm them, but this is really more for hot and humid climates. These conditions paired with wet leaves will cause mold and other viruses. In most areas, the rain will not harm them. Your area is cool enough. This would really apply if you lived in a place like Texas. Even in Tennessee it is not hot and humid enough to be of much concern. This article will help: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-container-care.htm

I notice that it is one particular cluster that is doing this. This is completely normal as the flowers fade, so it is possible that these are only open for a very short time, and are completing their cycle early. Picking them off at this point will usually encourage more blooms. I don't think that there is an issue here, but I do know that some cultivars are more sensitive than others to certain environmental conditions.

This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm