I know all the recommendations are to NOT bag the lawn (assuming a mulching mower), however, given a crabgrass-infested lawn (approx 50% surface coverage is crab grass), is it better to bag the clippings and dispose of them rather than let them cover the ground? Our community has a weekly grass pickup.
Adding on to "winwaed's" answer:
Crabgrass is an annual weed (dies at first frost) and lives year-by-year by self seeding itself.
Therefore if you cut your lawn "regularly" enough so that the crabgrass doesn't grow large enough to form seeds-heads, you can:
The problem with the above is, crabgrass thrives in the heat, and you really don't want to be out there cutting your lawn (for the overall health of your lawn) when temperatures are above 90°F (32°C).
Video: Will the REAL Crabgrass Please Stand Up? from University of Illinois Extension
You way also wish to take a look at this, "What's an organic way to discourage crabgrass from a large “lawn”?" question here on SE.
From what I've read it propagates by seed rather than suckers/creeping, so you probably want to bag it if there are seed heads present. If there aren't, you are probably going to be okay.
This is what the crabgrass seed head looks like (photo from wikipedia, Forest & Kim Starr):
Note that "crabgrass" describes a variety of different species, so what you see may be somewhat different, especially depending on where you live.
Before you mow, patrol your lawn. Do you see crabgrass seed heads?
If yes:
If no:
From my experience here in Indiana (land of weeds), the only way to really get rid of the crabgrass is to pull them up by the roots, one by one and throw it in the trash. It will leave divots and the grass will fill those in over time.
