When you spray the weed try to use a product that does broad leafs and crabgrass, like Trimec plus by PBI Gordon. Don't apply this product or any other herbesides to a new lawn you must mow it at least 4 times before you apply.

I would spray out the weeds then wait a few weeks then over seed and fertilize with a starter fertilizer. Good luck

Here is a link that might be useful: do_it_your_self_website

1. Get a Weed Hound tool at any of the box stores. That tool will pull a bunch of tap rooted weeds as fast as you can step on them.

2. Reset your mower for the highest setting and take off the bag. Most weed seedlings cannot penetrate the shade provided by tall grass. Even established dandelions can disappear when the grass grows up tall around them (it pushes up their big broad leaves and they don't get enough sun). By recycling the cut grass to your yard, you recycle a lot of nitrogen.

3. Water deeply and infrequently. Deeply means for an hour or so each time. The normal recommendation is to water an inch per week. That depends on your soil but 1 inch per week is a great starting point. Infrequently means you only water once a week during the hottest part of summer. Now you should be watering more like monthly or every few weeks. This gives the soil surface a chance to completely dry out. Weed seeds need nearly continual moisture to sprout. By breaking the water cycle the soil surface and weed seeds dry out.

3. Fertilize with your favorite fertilizer regularly. If you need to spray weeds (and you may not after doing 2 and 1 above), use a spray on product like Weed-B-Gone to spot-spray each weed rather than spreading the granules.

See if you can pawn off the weed-n-feed you already bought. Those products do not work like you think they do. They appeal to our desire for all-in-one solutions to multiple problems, but in this case it really doesn't work. Here's why.

Weeds need to be growing and healthy to absorb the herbicide in the wnf products. The herbicide goes to work quickly but if the weeds were not already fertilized, they don't absorb the herbicide. The fertilizer may or may not go to work quickly but it doesn't go to work fast enough to affect the weeds before they herbicide dissipates. So basically the timing of the product is all mixed up. Since you already have the stuff, and you probably don't hate anyone enough to give it to them, you can make the best of the situation by fertilizing your yard with a cheap organic material 3 weeks prior to using the wnf. That will give your weeds (if you still have any) the nutrition/health they need to really absorb the herbicide when it hits the soil. You don't have to worry about any organic/synthetic interactions or over fertilizing. The two materials work on different principles. You won't technically "need" the fert from the wnf but it should not hurt anything. The organic fertilizer I would suggest would be ordinary corn meal. The application rate is 10-20 pounds per 1,000 square feet. A free alternative is to apply used Starbucks coffee grounds (they give them to you in huge bags). The theoretical application rate is the same except the coffee grounds will be wet. For wet coffee grounds use 20-40 pounds per 1,000 square feet.