Nasturtium seed harvest

Hi I have grown nasturtium this year. The flowers are now turning to seed. I would like to keep them for next year. At the moment they are green they look like triangle sort of shape can't work out whether it is one seed or three.  Anyway do I have to wait for them to dry out to the brown knarled seed on the plant. Or do they have to be harvested and dried in the greenhouse etc. Any advice welcome.

Thanks BB

Once you have enough seed for next year, perhaps consider collecting the remainder as they develop ie when green, as they can be pickled to use like capers, and are really tasty