If the stem is triangular it may be some type of sedge, perhaps one of the nutsedges (Cyperus spp.). Is it extremely wet or damp where this occurs?
hortster

There are damp AND dry areas of my yard, but none extremely damp or boggy. The stem is triangular - long and spindly. I'll compare it w/ nutsedges.
Thanks!

I say it is the dreaded nut grass. I never had any till I used some Wal-mart cheap soil. Never again will I make that mistake. I am using a small paint brush and non diluted round up. I just brush it on and hoping it kills the nut also.

Thanks for the tip plantcrazy. I had been looking for something that I could spray on that wouldn't damage my valuable plants - an easier way. (See Link URL) However, I realize the paintbrush with Roundup may be my only option for eliminating the stuff at this point. It will take a lot of patience, I know, because nutsedge invaded most of my beds over quite a few years. And yes, I've been helping most big box stores stay in business with my soil, manure and other purchases. So I wouldn't be surprised if there was one tiny nutsedge rootlet lurking in a bag of topsoil that started all this!

Here is a link that might be useful: Nutsedge Management Guidelines

What has worked for me is just after rain go out and pull all of the sprouts you can see. You can pull after it sends up it's pollen shoots but at that time it is also forming nuts under the ground. This year it has been so wet that few sprouts have shown up.

Most herbicides do not kill sedges as they are not a grass. You must use one that is for sedges.

Thanks, maifleur... I found a product at Home Depot called IMAGE. It's kind of expensive (around $20 for smallest bottle), but on the label it says specifically that it kills nutsedge. So with hopes high, breath held and eyes squeezed shut I bought it and sprayed every nutsedge-looking plant in my yard - believe me, there is plenty! So I'll wait and see. If this product works (that is, kills the nutsedge and not my valuables), I'll be soooo relieved. Thanks again for the response. I'll post results in a few days...

I recently read about Image in another forum here. The poster said that nutsedge would probably need to be sprayed more than once for a complete kill (just as with poison ivy, horse nettle, greenbriar, etc.).

Thanks for the tip, missingtheobvious. It's been a few days since the first spraying, and the nutsedge is looking almost as strong as before I sprayed. So I will give it as many additional sprays as it needs for the complete kill.