34 degrees won't hurt your dahlias whether then are inside a cold frame of out in the open. However, forecasts can be a bit off target if you get a clear still night.

My own cold frame protection method is to throw old bed comforters over the cold frames on suspect nights. I have picked up a few comforters at fleamarkets and thrift stores for a couple of dollars and have found in side by side temperature observations that the covered cold frames stay 5-7 degrees warmer then uncovered frames.

However, on nights when the weatherman is actually predicting temperatures below 30 degrees I still tend to chicken out and get everything indoors for the night.

Well, tonight it's suppose to get 28. I piled 3 bags of oak leaf mulch over the frame and they are out there for the duration.