Do you like the grass you have and want to keep most of it and just thicken it with new grass, or do you want to see more of the new grass and you don't care if a lot of the existing grass dies?

If you like the existing grass, you should lower it two more times, the first time to about 2 inches and the second to 1.5 inches.

If you'd like to see more of the existing grass die or at least get really stressed, lower the deck all at once (maybe even lower than 1.5 inches) and mow it.

Either way, when preparing for overseeding, you should bag the clippings instead of trying to mulch them into the lawn.

Mostly just thickening is the goal. Back in April I had my front lawn extensively torn up from sewer line work, and I had to scramble to get something down at the end of April to prevent a mud-pit all summer. Most of it made it through the summer, but understandably there are still thin spots.

Fall snuck up on me, so after researching around the site a bit last night, (and finding out that average first killing frost for my area is Oct 15th), I decided to hold off until late March '09 to get started. I really wanted to dive into rejuvinating my lawn this fall, but it just slipped my mind the past few weeks with other projects.

Oh well, thanks again.

Another option would be to wait until it's too cold for the seed to germinate but before the lawn is snow covered and put the seed down. This is known as dormant seeding. The freeze/thaw cycles and the snow melt help bring the seeds into contact with the soil and the seeds will germinate as soon as conditions are right. This often gives an earlier germination than a spring seeding and gives the new grass more of a chance to survive through the summer.

hm, I kind of like the sound of that. I initially wondered about what if we get a warm spell of 5 consecutive 60*F days in January (unlikely, but who knows), but then I remembered that the soil temp is the key, and it'd be hard pressed to warm up enough for germination during a temporary warmup mid-winter.

The question is, do I still want to trim my lawn down to 1 1/2" when the growth is about to go dormant for months? Seeing as I have plenty of time to prep for dormant seeding, I could do it phased as you mentioned with no concern of rushing it, so time is on my side at least in gradually lowering it to minimize stress.

How much overseeding are you doing? Is it simply to thicken lawn or is it to establish re-establish a very thin lawn? I am also in SE Pa, and core aerated today, and will spread the seed in the next day or 2...i have a fairly well established lawn, looking to thicken,and the lawn has likley never been aerated..