located in NC no siduron - I was just overseeding (yes I know I should do it in the fall, i did but had some extra seed and wanted to use it up) I do not have a crabb grass problem in my back yard, I have used crabbgrass prevented in the past, but I thought you have to wait 6 weeks or so because it will prevent your grass from germinating.

I mow mine after about 2.5 weeks if it's had plenty of sun...and watering intervals. If you're still having nice warm days and you're watering every day also (where the soil stays damp all the time and lots of fall shade), watch to be sure your grass doesn't pick up a fungal condition. Very easy to contract now. One of the problems I've had with letting the seedlings grow long and trying to wait for a complete germination is if you get caught with a .5-1" rainstorm, and that deluge starts flowing across areas of the new grass, it's going to mat the new seedlings down...and brother you will not get them back up again (they're to young and weak). I've tried several times and wound up having to reseed those areas that got washed across. So now I'll mow a little earlier if need be to prevent that situation.

There is another aspect that should be considered, and the only reason I mention it is because it caused me a lot of grief. No matter how much germination is still to be done, if you are expecting heavy rain and have areas that the rain will run over, don't let your grass get over 3.5-4" tall...and that might be too tall if it's a good tropical type storm. What happened to me was I waited too long to mow because I was trying not to mess with the new seedlings, the grass grew to around 4-5" tall and we got a cow-on-a-flat-roof type rain which quickly matted down the seedlings. When that happens those seedlings were too young to raise back up (even with light raking), and that area has to be re-seeded again...or mine did. Thought I'd pass that on if it hasn't happened to you yet.

Mow it. Don't do like I did a year or two ago. I let it get 5-6" high and a pretty good rain storm came through and dumped around 2-3". Next day I looked and most of my new seedlings were matted over and stuck to the ground where the rain flowed. I tried to rake them up to no avail. When that happens there's very little you can do to bring them back except to reseed all over again. Sooooo, mow it.