I don't have much good news for you. If your grass is just now 1.5 inches high, it will likely be dead by the end of next month. The new grass is too tender for the summer heat in most of the grass growing world. The time to plant grass is in the fall. Yes, I realize grass seed hits the shelves in February, but all they're trying to do is sell stuff.

Where do you live?
What kind of seed did you put down?
How many inches is "lots of topsoil?"

Good news is you can use a string trimmer to take out the tall weeds.

What I would do is nurse what you get from this seeding as long as possible and accept the crabgrass that comes with it. Then in August get ready to reseed at the right time of year. By this time next year you will have a full lawn and the weeds should be gone.

Hello,

I do realize what you said about spring seeding but I live in Ottawa, Canada and it has not been very warm, in fact, it has been raining almost every week and the temperature is around 54 degree F.
I had 4 inches of topsoil and I used the Scotts Sun & Shade Coated grass seed.
I will have to wait until it stops raining to use the string trimmer like you advised and will try to remove as much weeds I can. And coming August, reseed the lawn.

Cheers
Joyce

There are herbicides you can apply after seeding to kill grassy and broadleaf weeds but they require precise mixing and application rates. You can look up "Q4 plus turf herbicide" for example. Don't know if it is available in Canada.

Thanks Kimpa but unfortunately this product is not available in Canada.

Un-huh! See why I practically insist that people say up front where they live! Made me look foolish. That's MY job.

You will probably be fine with your seeds up there in the Arctic.

Was your lawn rolled down after it was seeded? If it was done right, you should be able to walk on it without leaving any foot prints.

Four inches of topsoil? Really??? Did you have a wash out and lose your soil to erosion? Adding top soil can be a huge mistake. Why? Because adding topsoil ALWAYS changes your drainage. In the worst case it diverts runoff into your house or basement. I just talked to a guy about that topic today. He had a river running in the front door and out the back while he was away for 2 weeks. In the best case the added soil diverts runoff where it belongs. But unless you had that in mind, usually a somewhat worse case scenario usually evolves.

With 4 inches of new soil, the weed roots do not have much to grip to. You might do better to pluck them out one by one.

Yes, we used a roller. We did not know how much topsoil was required, so we went with 4 inches. We will pluck the weeds like you said and reseed in August

You Can Use Weeds Killer on roots of Weeds or you can do manually digging or pulling weeds. There way you can control on Weeds without harming your lawn

Here is a link that might be useful: Care2