When it's warm enough.

Kentucky Bluegrass
I would say now and for the next two months. If your area is anything like ours, pring came a little late for us here. Once it gets going good, he'll notice the need to mow regularly due to increased shoot growth, and then rhizome activity will shortly follow.

Normally, it takes about two weeks for the grass to recover from something like power raking or mechanical aeration - practices that beat it up pretty well. Before making any determinations, tell him to give it a good two weeks from the time it begins growing good.

I noticed it thicken all winter.There was alot of horizontal growth happening in the sparce areas. The way it crept into the areas it had to spread by rhizomes.

Since that new lawn dosen't have a good root system yet feed it some starter fert that new grass will love the extra P&K this spring it will help speed up the thickening process.

I've read the cool-season turf does 75% of its growth for the year in the spring months. Taking note from my own Bluegrass lawn, spring and fall are both very active growth periods with late spring/early summer being the peak for above-ground growth.

Now. At least where I am--spreading has already begun in the warmer areas that are back. The colder ones, not yet back, should start in a week or two.

Assuming it has enough water, it slows but doesn't quite stop during summer. I was noticing spreading right through December of last year, as well.

My KBG blend from my lawn renovation last fall is just starting to green up now. Last week we finally hit 60 degrees again :)

I ran my dog over a portion of the lawn a lot over the snow in the winter. Nowt hat the snow is melted , i have quite a few matted areas already. i'm hoping they come back strong !! I put a lot of effort in last year.

I'm waiting for the Forsythia around here and then I will spread fertilizer with the weed inhibitor. Hoping the tillering will go quick I do have some shoebox-size missed areas that are around> i'll get some pictures soon.

I have to take the snowthrower off the riding mower this weekend to and reinstall the cutter.