Raddichio is biennial, which means that it may not produce seeds in the first year. You may have to mulch a few plants so they survive over winter and then allow them to bloom in the second year. In order to save seeds, leave the head on one or two plants and allow to bloom. Collect mature seed pods as soon as possible on a dry day. Radicchio seed develop mildew very easily. You can harvest as early as two weeks after the flowers open. You can pull the whole plant, roots and all and allow it to dry upside down in a well-ventilated area protected from the elements for further seed development. You can also cut the seed stalks off as they mature and place them in a paper bag for drying. If you don’t need to collect large quantities of seed, you can pull off the individual white tufts, to which the seeds are attached. Put the tufts into an envelope or jar so you won’t lose the tiny seeds. To clean the seeds, shake the seed heads into a large bowl or bag to loosen the seed. The seeds are small and light, just barely heavier than the chaff so the best way to clean them for long term storage is to rub them through seed screens. The right size will have the chaff fall through and the seeds will stay on top. If you are only storing until the following season, you can probably get away with storing your seeds with the fluff, and just planting the seeds and fluff together the next year.