Are the petunias done when they turn brown or can you cut them back?

You can cut them back and see if any new growth starts.

They are annuals here in Minnesota. All you can do is try. Is it the flowers you are talking about or the whole plant? If it is the flowers let a few completely dry for the seeds that you can plant to keep them going longer with new plants. If it is the plant, most likely the roots may be dead. If you dead head the flowers when they are spent, they will try to bloom more.

Petunias should still be blooming and growing if you have had no frost. They turn brown when they do not have enough water. You may still be able to save them if you water them every other day and put mulch around them to help hold the moisture.

If they still have color towards the top I would try cutting them back and then fertilizing once a week. It is most likely that they might be done but worth a try.

I cut them back...the flowers when turning brown, the greenery stems when yellow or brown. I cleans them up and I get more blooms. They usually last well into the fall, for me.

I heard that Busch Gardens trims them back, but cut no more than 1/3 of the plant at a time to keep it from shocking.