This is a tough one. I would look at Image (Ammonium salt of imazaquin) and see if that would be an answer to this problem. Be sure that the species trees that you are planting are tolerant to this herbicide. If it's not listed on the label call the manufacturer and see if they have any ideas. Sometimes certain species get omitted on the list of tolerant material. You could also contact forestry or other Ag departments at various universities such as University of Tennessee, Auburn, etc.. Maybe the US National Arboretum can give recommendations. With something like Image you can possibly go ahead and plant your trees and then treat the weeds as they emerge.
