Yellow nutsedge (or perhaps Purple nutsedge, as the major difference is the color of the flowers).

You will recognize it by its height (sticks up above the grass) and color (a bit more yellow-green than your grass). Pull it as soon as you see it, before it grows a "nut" (small tuber). You also don't want to let it go to seed, as it spreads by seed as well as runners and tubers.

If you are dedicated about checking all your lawn and pulling the nutsedge before each mowing, it's possible to eradicate it -- I have -- though you may not get it all in a single season.

There is a treatment called Sedgehammer, but I don't know the details (like whether it kills your grass, or only the nutsedge!).

I put vinegar on a small part of my lawn to kill the crabgrass. The crabgrass is bright green and the wanted grass is dead. :)

Yes, that is nutgrass. Nutgrass is a swamp type grass which must be watered daily. If you can withhold water to weekly or much less, then you have a chance of it going away naturally.

Where do you live in SoCal? It can make a difference as to how to approach the issue. If you live in the coastal influence (west of the I-5 (or 405 in LA area)), then you can withhold water. If you live east of the coastal range, then you're going to be watering puh-LENTY just to keep the Marathon alive.

If you don't live in the coastal influence area, then you made a big mistake choosing Marathon. Your community is just about to clamp down on excessive lawn watering, and Marathon is the biggest water hog in California.

sjerin???????

Thank you for all the answers. I have pulled as much of the nutgrass as possible but I will continue to pull prior to mowing and will be doing. I have also reduced the watering and will further reducing it over the coming weeks too.

I live in Laguna Niguel, at the merge of the 5 and 73. We do get occasional coastal mists up from Dana Point/SC but not as regularly as when I was living in Costa Mesa.