crab grass??

I removed the double post, no problem.

Is the plant putting our runners with a pair of leaves every few inches? That's the best description of Crab Grass I can come up with.
And from what I have read, once you see it in your yard in the spring it is too late to treat it this season. But in the fall you need to put down a pre-emergent to keep it from coming up next Spring.
This season you could try spraying each tuft of it with a weed killer or pulling it up. A general weed killer sprayed over the yard will also take care of the grass, I would think so you don't want that.

 

It is a spray that kills the weeds before they have a chance to emerge above soil level.

Timing is most important, if you already see them it won't do a thing to them and you waste money and time on pre-emergents that's when you need to use the post-emergent sprays. My fingers got faster than my brain in the first post when I said it was too late after you saw them, I should have added too late to use the pre-emergent. OOPs

Down here there isn't hardly any time of the year when weeds are not growing so I am not sure a pre-emergent will ever work here. But both my front and back yards have been changed to total gardens, weeks are still a problem but not like when you have a lawn.

If you have snow on the ground then as soon as the snow is gone, before the soil starts warming up that's the time to apply, but someone at a garden center in your area can give you help with figuring that out.

If you have to use a post-emergent then make sure it is selective and only kills the weeds you are trying to get rid of. There will be a list of weed targets on the label. If it isn't selective, then it will kill anything and everything green that it touches....grass, plants, etc.

 

If you go with a total killing, be sure to do it on a perfectly still day. Your neighbors might not appreciate the death of their grass and plants from over spray.

And use something that has a short half-life so you can reseed in a reasonable time.

Something I just thought about, if you have a couple of years to work on this it might be easier in the long run than widespread killing then having to reseed and hope the weather cooperates for the seeds to take hold and grow.

Most weeds are in lawns because there is some nutrient lacking in the soil that causes the grass to weaken and the weeds to take over. In other words if the soil is dead or dying then the weeds rule.
Start improving the health of the soil with compost and organic fertilizers. The grass will love it and spread, overtaking the weeds that are dying from living in healthy soil. When the weeds start dying you can cover the bald spots with plugs of the grass you have there.