Your gardenia may have received too much water, had too much fine root damage during planting or it may have just experienced sun scald.
I've had better luck planting flowering shrubs in the cooler weather of Nov and Dec.
I don't know that the foliage damage from insect/caterpillar has anything to do with the dahlia flowers being discolored before they bloom.
You might want to do a search here and see if you can get more info on dahlias specifically.
Keep up the enthusiasm and don't expect perfection. Plants have lots of leaves so they can afford to lose some without you having to resort to insecticides.

Thanks for your encouraging words. I am thinking about pulling up the gardenia and bringing it back to Lowe's. I heard they guarantee plants.

I have one more question...I have many potted plants on my porch that have done well but lately it looks as if they have a fungus. I want to plant some seeds that will do well through the fall. Any suggestions?

Although some old fashioned gardenias will tolerant sun, typically, they prefer shade, especially in the afternoon. If you want to save them, you need to move them to the front of your house where they will get morning sun, afternoon shade, or even to the north of your house where they will get nearly all day shade. They do not like competition from tree roots either, so it's best to keep them near a structure for their shade. Dig a bag of composted manure (no other fertilizer till they recover) into the WIDE hole, water and mulch them well, and with a bit of luck, they should recover.

Dahlias are notorious for failing in the deep south. They hate our heat, our humidity, and our hot nights. The only one I have grown successfully here (north of you) is Bishop of Llandaff. If these are the only two things you're having trouble with, congratulations! That's really good for a beginning gardener. If you only knew how many plants I have killed over the years! :)