In mild climates, such as Zone 10 where you are, nasturtiums can be planted in late summer/early fall for fall/winter bloom. They can also be planted in spring. Nasturtiums like the cool (not cold) season, but some survive during the heat of summer if properly cared for. In some mild climates, they may even act as perennials.
When collecting seed, keep in mind that if the seeds are from hybrid varieties, the seeds themselves may not produce plants that have the same features as the parents. If you'd like the same results each year, select an heirloom or non-hybrid variety.