A.P.
Take heart darling. My first attempt to plant roses 10 years ago resulted in nothing but dead sticks coming out of the ground that my husband still teases me about. Now I have a yard full of flowers. Are the mums completely dead, or just the flowers? If it's just the flowers, did you buy them in full bloom? If so, they are just ending their bloom cycle. They'll fade and come back next year, fuller than ever. It's best to buy mums with only buds that are ready to open. They last longer that way. When they do come back next year, by May you should start pinching the tips back. This will prevent them from blooming too early and produce more branch ends for more blooms in the fall! Mulch them well through the winter and water well through the fall to get them established and they should do great. Water about twice/week, when the ground is dry to about 3-4 inches down. We've had a lot of rain, so be careful not to overwater. Good luck! And check out www.savvygardener.com. She's a blogger in KC who puts out a weekly newsletter with great gardening tips!
PS I've never had any luck with petuni