The weeds look like crabgrass. What type of grass did you plant? Looks to me like fescue. If that's the case and you planted it 3 months ago that is why you now have an infestation of crabgrass.

The good news is the crabgrass will die out after the first frost. The bad news is it's already producing seed for next years crop. For now you need to mow it often and bag the clippings if possible. You don't want to let it form seed heads if you can help it. I would plan on overseeding so your turf will be thick enough to compete. Depending on your location the best time to overseed is sometime between now and October. Let us know where you're located and get ready to overseed.

I would highly recommend a good pre emergent come early spring to prevent the weed seeds from sprouting. I went with Prodiamine this spring and was really happy with the results. Main thing is applying the proper amount and getting it down at the correct time to prevent the weeds from germinating. For now, let us know your location and what type of grass you have so the folks here can help you.

I am in Pennsylvania, and yes it is fescue. What is the process for overseeding? I apologize if that is a dumb question, this is my first lawn.

Thanks for the help.

Ok, probably the next 2 to 3 weeks is going to be your prime time to overseed. I prefer to cut the existing grass shorter than normal. Typically I mow at 4" during the summer and will take it down to 2" before overseeding. I drop to 3" and then wait a couple days and drop down to 2" to avoid overstressing the exsisting grass.

For overseeding fescue the general rate is about 4lbs of seed per 1,000 square feet. I typically go a little more in the thin areas and a little less in the thick areas. Once you have the seed down it's a really good idea to roll it in with a lawn roller. This helps press the seed into the soil for better germination. Remember, the seed needs soil contact and moisture in order to germinate. If you have a small lawn you can just walk around on it to press the seed in but if it's a large lawn a roller will be much faster.

Once that's done you begin watering 3 times per day for for about 5 to 10 minutes each time. All you're doing is keeping the seed and the top layer of soil moist. Do this in the morning, at noon, and again in the evening. on extremely hot days you might have to add in a 4th watering but it is very important to keep the seed moist. If you get this right you should start seeing germination in about a week. You'll want to keep this up for 2 to 3 weeks until all the new seed has germinated. At that point you will start gradually backing off of the frequency of watering and increasing the amount of water you put down.

Some folks will put down some type of starter fertilizer when they put down the seed. I prefer to wait until I have cut the new grass a couple of times before I fertilize.