Contact your county agriculture agent. They will better advise you.
I think the grass has to be actively growing and or the temps be above a certain degree..

It will kill grass only if there's green on them. They need to be actively growing to be able to absorb the poison to be killed.

Why can't you just mow them?

I'd be hesitant to spray roundup on the lawn since the grass may not be totally dormant. Since weeds are primarily broad leaf why not spray a broaf leaf weed killer on them? Perhaps that is what you used already. It may take a few days to a couple of weeks for them to die, but I find that it works well. The big ones you can pull up. Try to not let them go to seed.

Spray very carefully with the sprayer set to emit large droplets and not a fine spray so it won't drift over to any shrubs you may have.

You may need to apply a pre-emergent weed killer to keep seeds that are still in the soil from sprouting. There are organic and chemical kinds you can use.

It will take diligence to keep the weeds down until the lawn gets thick. Before you get the weedy lawn beat you'll be pulling and spot treating so many weeds you will see them in your sleep! Most of us have been down that trail before we had a weed free, or nearly weed free lawn.

I know you want the weeds gone now, but it takes time to build healthy soil which is the real key to a weed-free lawn. With any weed-killer, you risk killing your grass. Hand-pulling is work, but it is really the best method. Can you hire a teenager in your neighborhood to help you? I would also invest in some compost to build the soil. Good luck!

I concur with roselee...your lawn may not be totally dormant. I'm north of DFW and my buffalograss is actually beginning to green up. Not by a huge amount, but there's a definite green cast to the lawn with greening coming up from the base of the grass, which means it's not totally dormant.

Hello-Welcome to the GW. Well, first off whatever is growing in your yard now is probably henbit & other assorted cool season annual weeds. Roundup will not help because they are broadleaf weeds not grassy weeds. The easiest solution to your problem is to mow everything down low & if you had Bermuda grass before it will definitely come back, You cannot kill Bermuda grass by any means, I have tried!! Spread 1/2 to 1" of good compost on your yard and water when needed. Once you figure out what kind of grass you have I would go with a Natural & Organic program, especially if you have children or pets that will be walking on the lawn, you do not want to expose them to chemical fertilizers or weed-killers. If you Google, Howard Garrett you will find a wealth of info. Good Luck!
Dianne

I try to keep my weeds mowed low until the grass starts (in warmer weather which will also destroy most of your weeds), and then the grass will take over from there.

Ditto.

Yep, I keep my weeds mowed down too and use weed & feed in the spring and fall. It has really helped a lot to get rid of the mainly weed lawn we had when we moved in. The grass is starting to fill in better since I used it.

fool4flowers-

Do you realize just how much poison weed and feed contain? Way too much just to kill a few weeds. Very toxic stuff. Do google for it even county extensions warn against it.

Roundup kills broadleaf weeds as well as grassy weeds. It is true that if a plant is dormant roundup is not going to kill it because it won't be taken up into the plant where the killing occurs.