Pre-emergence herbicide

I'm fighting a bad poa annua problem. The best advice I've found is preemergence application in late summer-early fall. Since this crappy weed/grass germinates in the fall but doesn't show growth until late winter/early spring, I'm confused about when to apply the preemergence. The instructions are generic and I'm sure different zones would have different application 'dates'. The instructions also state it needs 1/2" water within 14 days of application to activate the product. When activated, how long will the product remain active? In other words, I've seen the poa annua produce green sprouts over a 2-3 month period, so would one application of preemergence remain active long enough to cover that long a period?
Come on, Eggs, and jump in here.
Red
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fwiw I put down some dimension this week for just that purpose. I'm in north central Ohio. You have to be careful to not root prune your desirable turf if it's at all stressed, it's also nasty on bentgrass
Oh and forget about seeding this fall in the treated areas.
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