When is the germination period for giant ragweed there?

Here, most if not all germinate in March.  We have lots of them growing down along the river, but them being a problem in a field is rare. Did once have a couple of fields that had an area of them that seemed to have spread from next to a waterhole, and was a common problem in corn along the river.  Seems like the problem went away around the time we started adding dicamba to the first burndown in the spring trip. Maybe spraying 2-4d and dicamba together would be best for controlling them. I am guessing that is not an options since you either plant before ragweed emergence or they germinate over a longer period of time then they do here.