I placed a tarp on the Dahlia's last night but they have frost on them this morning. The leaves are all mucky,is there anything I can do to save them?? Cutting off the shoots?
It sounds like you don't have much choice, but to cut off the damaged parts. Often they will send up new shoots for you. If they are not planted in the ground, one should consider taking them indoors to a garage for the night, and then back out the next day. They will be okay on nights that are not so cold.
When you mention the you "placed a tarp" on them, I hope it is not a plastic tarp. That doesn't seem to work well for me. I prefer to use an old sheet which breathes, if it is cool. On clear nights, when the temperature is really low though, I always take them indoors. I never plant in the ground until ALL danger of frost is over. Here that is the 1st of June, but the plants have had the benefit of a month and a half of growing outdoors in pots.