Seed Saving from Cauliflower
Cauliflower, Kale, Broccoli, Cabbages, and Sprouts are all members of the same family (Brassica oleraceae), and will all cross with each other.
And they are mainly self-incompatible – which means that in order to get seed, insects have to carry pollen from one plant to another to pollinate the flowers. Because of this, you can’t simply grow your broccoli or cabbages for seed in an insect proof cage to avoid crossing. Only seed save from one member of the family each year and if growing other brassicas for food, don’t let them flower.
Fences, etc disrupt insect flight patterns, so as long as you don’t have any immediate neighbours with flowering crops in their garden, you shouldn’t have crossing. Plant in a block so that bees tend to move between plants, rather than away to other flowers elsewhere. Keep at least six plants for seed.
Brassicas have a tall flower stalk covered in lots of small yellow flowers. These will then form slender seed pods, which start out green, and turn a straw colour as they mature and dry. Once they start to dry, keep a close eye on them, as they tend to shatter and drop their seed. Its best to cut entire plants once most of the pods begin to look dry, and then leave them to mature further on a sheet indoors. Once they are thoroughly dry, the seeds will come out of the pods very easily; the simplest way is to trample the plants on top of a large sheet, and then sieve out the debris.
You can see more detail in this short film.
You should get lots of seed from even a few plants. The seed will keep well for up to five years so long as it is stored somewhere cool and dry.