I'm afraid I hacked at the leaves and flowers before I pulled it out, but I'll go out and take a photo of what's left. Yes, it was a vegetable garden. In the area I found it it was planted with salad greens, most recently buckwheat to replenish the soil.

Becky: I think I see what the confusion is. The OP said he "just" had seed put down, and I had it in my head that this seed was still germinating, and in any case certainly is not long enough to mow. I reread his post and see that he said "in the last month" these weeds have been growing, which suggests the new grass is older than I thought. My bad. So given the grass isn't quite so new, I'd just pull the weeds out. I could get the ones in the photo in 10 minutes. I do hope the OP updates us on what happens. A little more on type of seed, etc. would be nice, too.

Yes! That has to be it. Thank you so much!

I think at this point I'd forego the manual pulling and hit it with Weed B Gone as morpheuspa suggested to ensure you got it all and minimize seed spread. The WBG contains triclopyr (amine salt) as the active ingredient and is effective on broadleaf weeds. You can also use any product that contains quinclorac, which will also kill the crabgrass. You may have to overseed/seed after treatment as it will most likely produce thin/bare spots. Early next spring hit lawn with a pre-emergent (Prodiamine) when soil temps at 50 degrees. Sulfentrazone and Halosulfuron can be used if you have a sedge problem. Will