after the head turns brown.... & has a chance to dry on the plant.... Will lose some of the seed but should have more than enough to start new bed + re-seed old one

Parsley is a biennial and does not set seed the first year. If it is located in well-drained conditions, and the crown doesn't rot over the winter, then it will over-winter. It blooms in umbels of little flowers and sets seed the 2nd year. I've got 1 flat-leaf and 1 curly plant that over-wintered from last year. They are seting seed now and it is easy to tell when they're ready. The seed heads turn brown and the seeds will come loose from the seed head.

Terrene, I have flat parsley and was going to ask the same question about harvesting the seeds. Are you saying I should just leave the plant in my garden over the winter. I live in Pennsylvania (Northeast area). Or, should I gather the seeds when the flowers turn brown.