What is sprayed onto lawns, park areas, fields etc to keep the grass weed free?

Pre-emergent herbicides are used for prevention.

Broadleaf weed killers kill non grass plants in lawns.

Can you elaborate on why you're asking?

porkpal said: Pre-emergent herbicides are used for prevention.

Some thing I am heading out to do in about 20 minutes.

Alas, I should have done that too, but it's too late here.

RpR said: Some thing I am heading out to do in about 20 minutes.

I went out in shorts and a vest, but found out 35 degrees is a lot colder on April 19, thab it would feel on Jan. 19.