With space at a premium on rooftop decks and terraces, the last thing on most home gardeners' minds is to keep withering plants around to harvest their seeds. It can, however, be quite fruitful, when these plants are herbs or vegetables.
These plants typically have four stages: they will bear "fruit", then flower, then set seeds, and then wither away. During the withering stage, these seeds dry up and fall to the soil to grow again the following year. The key to saving seeds is to wait until the seeds are fully mature and dried on the plant, and then to take them from their pods and store them in your home until you're ready to plant them.
This technique means leaving spent plants around a bit longer than usual, but the result is a harvest of seeds that can last you multiple seasons.