I understand that Crabgrass Preventives are supposed to be applied in
early Spring and that they work by sealing the soil surface so that
the seeds don't germinate.

I live in the SW corner of Missouri. My lawn is a mix of
fescue/bluegrass/rye. I applied Scotts with Halts in the spring. No
crabgrass had come up until the last 2 or 3 weeks - Now there is quite
a lot of it. Now I'm applying Weed B Gone crabgrass killer to it,
but that weems to really weaken and even thin the other grass.

Can pre-emergent like Scott w/Halts be applied later in the season?
If I applied it in early June would it prevent emergence in June and
July?

Thanks,
Dave in the Ozarks