Brassicas are insect pollinated and cross within species only (see following table) so if you want to try growing your own seed, you must plant a minimum of 300 plants, rouge out the100 least true to type, and save the seed from the rest. “Seed to Seed” says you must have 1 mile between flowering cultivars of the same species but I’ve found the distance can be far less, depending on hills and valleys and what is in between.