Maybe I missed it, but don't we need to know what the crop will be before we can recommend a pre-emergent herbicide? And, I'm just shooting from the hip here, but princep in the fall isn't going to be there come spring, will it? I'm thinking half life is 60-days.
I think I would skip the herbicides. Its almost an impossible situation from that angle. Its a biennial, perennial, that starts from seed or roots (maybe rhizomes?) in the ground. I was fascinated to see the stuff will germinate from seed anytime the soil temperature gets to 65-degrees, fall or spring - or winter in the deep south. So, if you going to use herbicides timing is a difficult proposition.
I've never had a problem with it. Like M-man said, just mow it and get your crop in a position to out-compete the dog fennel.