If you didn't kill the existing lawn, you can expect it to come back. Scalping it doesn't kill it, it just gives you some time for the new grass to start before you have to mow the existing lawn. If you mow it before the grass germinates and starts to put roots down, you will probably not see any of the new grass survive, especially if you're using a rotary mower. There's now way to mow gently. The mower creates a partial vacuum so it will suck the seeds off the ground and if they have just sprouted but the roots aren't set, it will pull them out of the ground.

Your best bet is to wait as long as possible before mowing.

Here's an older thread with a few comments on the subject....

Here is a link that might be useful: click this link

I think mowing the lawn is far less detrimental to the new seedlings than overgrowth from the existing lawn--that is the kiss of death.

Some would say overseeding kbg into PRG is a lost cause due to alleopathy. Let us know next year how it turned out. You may be better off because of the core aeration you did.

"I did a scalp/core aerate/big-time thatch/tear-up/rake job on my old lawn (mostly TTPR) and reseeded with 30% America KBG/40% Moonlight KBG/30% Oxford Hard Fescue. Had added starter fertilizer to soil 7 days before reseeding. "

I don't understand why you dethatched and raked if you were also going to core aerate.
When I overseed I follow a bit different procedure.
First I scalp the lawn as low as possible. Second, I core aerate, and do so very thoroughly by running over the lawn several times in different directions. That kicks up 2" cores to the surface with dirt, thatch and grass attached. Then I broadcast the seed and starter fertilizer and don't rake at all. The seeds find holes to fall into and the dirt and thatch helps to cover the seed when you water.
It sounds like you gave the established grass too much of an advantage by dethatching and putting down the starter fertilizer 7 days before the seed. It liked it too much!

That is the double edged sword of overseeding...the aeration, fertilizer, and water will do wonders for your existing lawn before too long. Not really a problem if you are trying to make a bad lawn better, but really a problem if you are trying to make a good lawn better with better seed.

"I don't understand why you dethatched and raked if you were also going to core aerate. "

I dragged a dethatcher (multiple times) after the core aeration to both break up the plugs and scratch up the surface - it was a case of 'use the tools you have'

I wouldn't cut until you get KBG germination and some good growth, if your able to tell. Keep watering to a KBG schedule. There will be enough light for the KBG. Positive thinking here - just let mother nature work. At about 30 days would be a better time to worry.

Please let us know next week how it goes.

It is now Day 15, and the Oxford Hard Fescue is really up and growing well. Does anyone know a way to reliable way to tell whether the Moonlight and America KBG is beginning to germinate just by looking at the seedlings, or do I have to just trust that it is starting?

Also, I'm getting people asking why I added the Hard Fescue to my mixture. I did it to have a variety that will compete well where the tree shade and tree roots make life tough for the KBG. Some people question whether the KBG will outcompete the Hard Fescue even in the shadier spots before fading out in a year or so. Opinions?

the Kbg is slow to germinate, and slow to 'establish' after germination, which is why I made my post on the thread I referred you to.

Stay off it as much as you can. Keep it moist.

I know you want to mow.

I know you do.

Don't.

Nah - not feeling the urge to mow. The scalped grass seems to have stopped growing much in most spots - and is 1/2 inch with lots of bare spots. As anyone would expect, I'm just looking for a hint of results with all the work put in. I'm also second-guessing the Hard Fescue decision because I have the time to, while I wait (and water and water). Any feedback on the mixture I used?

OK, I waited a full month to mow. Mowed today. Observations:

(1) The new grass plus the (recovered) scalped TTPR make an emerald-green lawn that is nice at this point, but I'm sure that the color will darken as the varieties are selected for darkness. It's thin in a few areas, but I know enough about KBG to wait for next May-June.

(2) Directly under the maple trees where I used the hard fescue, the seedlings got "flattened" a week ago by runoff after TS Hannah. It's been a week and they're still flat against the soil. I've been patient, but after a week I'd hope for a bit of recovery. Ideas?