That is good to know about the onions surviving the cold as I will be planting them soon under the cold frame. When the petunia flowers are pollinated by the bees a small teardrop-like seed pod will form. When they turn brown you can pick them off the stem and save them in an envelope. I start my very tiny seeds by surface sowing and then mist them with water and keep a dome on them until they germinate. Start them in late February or early March if you plant them by May.