For some reason, I have more luck with parsnips that come up on their own than the ones I plant intentionally. The directions for planting parsnips make them sound very picky - not when the soil is too cold, not where you can't keep them watered until the sprout, not too late, not too early. The seeds have to be fresh, nothing from before last spring.
But I like seeing plants flower so I left one last year and the year before and some came up last fall, some this April. Some I carelessly threw out into a rather shady bed near the house flourished over the winter.
And I've decided I'm just going to let one flower every year and throw the seeds around. How intrusive can they be on broccoli or primroses? Easy to weed out as well. And much, much less trouble than carefully preparing a seedbed and watering it.