Where are you located, what major city are you near?

First off- some types of crab grass grow roots 6 feet under ground. So don't expect to be rid of it in a single season.

We planted No Mowing Grass (a naturally short growing grass) on our lawn and we use TriMec to keep down the crabgrass.

Usually with weed control, it's best to mow the weeds before they go to seed, but with crabgrass and dandelions, they will just keep coming back unless you do regular maintenance (TriMec).

True crabgrass is a summer annual plant. It would not matter if the roots were 60 feet underground because the entire plant dies in the fall. Being an annual, it has to come back from seed every spring.

With that in mind, spring is the worst time of year to start a new lawn by seed. Well, maybe July is worse but the spring is the worst for the reason you are trying to stop. Crabgrass seed is trying to sprout now. If you try to sprout grass seed, the crabgrass will sprout first, fill in fastest, and choke out the grass you really want. It is much better to nurse your weeds through the spring and summer until fall, the best time of year to seed new grass.

If you have real creeping Charlie, then you might want to take the last few months of summer to try and get rid of that stuff. Supposedly it can be done by persistent use of RoundUp. One time will not take it out. You would spray once, continue to water and see what lives. Whatever comes back with the watering needs to be sprayed with RoundUp again. Creeping Charlie is the single worst weed we see discussed here in the forum. It forms a dense mat of roots right underground, and you do need to kill all of those roots. If it has escaped into the neighbor's yards, it will escape right back into yours if it is not stopped everywhere.

Contrary to popular belief, weeds can be controlled better with proper watering and mowing height than with chemical sprays or dry herbicides. Dense grass is necessary to keep weeds out because it stops sunlight from falling on the weed seedlings. And a dry soil surface helps keeps weed seeds from germinating in the first place. Water deeply but infrequently to achieve this. Deeply means a full inch all at one time. Infrequently means monthly in the cool months and as much as every 3-5 days if you live in Las Vegas in the summer. For the rest of the world you probably only need to water every 7 days in the summer.