We are now spraying weeds in areas that were dormant seeded last fall, as well as areas that were seeded in early May 2014. We use Chaser Turf ester herbicide because it does not contain dicamba. It does not harm new grass or prohibit germination.
For lawns that were seeded in late fall 2013, and lawns that were seeded in early May, we are not applying any sort of preemergent herbicide. The oxalis, knotweed, etc that appears in the seeded areas >> we're spraying them. Usually using a backpack sprayer with flat fan nozzle (using Chaser ester).
pic 1) Dormant seeded December 3rd, 2013
pic 2) Seeded May 6, 2014
pic 3) Seeded May 7, 2014
All 3 properties were seeded with a 4-way blend of turf-type (dwarf) tall fescues
p.s. Those little white things in the photos ain't weeds --- they're fallen petals from flowering trees.