Please, can you keep your collected petunia seeds in an attached garage over the winter in Zone 5?
I keep them in the fridge, so I'd say yes, you can. However, they are probably hybrids, so there's no telling what you're going to get. I had some pink wave petunias that self-seeded in a hanging basket and the next spring they came up and were identical to the parent plant, but that doesn't always happen. These are apparently a stablized cross.
The seeds are very tiny, so put them in a small envelope if you can.
Thank you, eah.