I've harvested my cauliflower, so do I rip the plant roots and all out or cut it at the base? Also, I had one that went to flower. Can I get seeds from it?
Yes, once the Cauliflower head is harvested you can dig up the entire plant and compost it.
You can harvest seed from Cauliflower plants and there are specifics to doing so.
I have also included a link with this information.