A bit of experiment this week.
In a corner of my plot I left a couple of parsnips to go to seed and forgot about them. It doesn't look tidy (see below). Now I notice that the seeds are dropping off so I reckon it's time to chop off the seedheads and collect them in a bag where the seeds can be rubbed off the stems without losing them.
collect the results in an envelope and pop them away somewhere safe and dry.
I wonder if they will be viable next March!