Answer: It's a bit late now, but the best time to gather seed is in the fall, when the seed has had time to ripen but before it has been dispersed by the plant. If you have let some flowers go to seed on the plant, simply pick off the seed pod, cut it open and dry the seeds for a week or so. Then you can store them in a sealed container in the refrigerator until you are ready to sow them in the spring. Incidentally, most petunia varieties available commercially are hybrids, which means they will not reproduce true from seed gathered from the plants. To grow these types, you need to start with hybrid seed, available from seed companies.
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