Stubby wrote:

A pre=emergent control must be applied BEFORE the crabgrass seeds
sprout. This means March or early-April.

Later in the year you can kill the plants with crabgrass killer. This
takes about 3 applications 10-days apart. The idea is to kill the
plants before they spread seeds which will sprout in the following
Spring.
So, there are two very different approachs. I use both and stay
barely ahead of the crabgrass infestations.

Makes sense to me. Once established, that stuff's a bear to deal with.
When does crabgrass go to seed?