The plants with the little purple flowers that are starting to make themselves known in home lawns are called henbit.

Henbit in Lawns
The plants with the little purple flowers that are starting to make themselves known in home lawns are called henbit.
If you are not sure this is what you have, check the stems. If they are square rather than round, you have henbit.
Though it actually comes up in the fall, most people do not pay much attention to this weed until it starts to flower.
Trying to kill it at this late stage with an herbicide usually is a waste of time and money. Though the plant may be burned back, it will rarely be killed.
So what do we do? Remember, this is a winter annual; it comes up in the fall, matures in the spring and dies as soon as it starts to get hot. All we can do now is keep it mowed until nature takes its course.
However, we can do something next fall that will help next spring. Henbit usually germinates about mid-October.
Spraying with 2,4-D, Weed-B-Gon, Weed Free Zone, Weed Out, or Trimec in early November can go a long way toward eliminating henbit next spring.
Plants are small during the fall and relatively easy to control. Choose a day that is at least 50 degrees F so the henbit is actively growing and will take up the chemical.
Spot treating will probably be needed in the spring to catch the few plants that germinate late.
Use Weed Free Zone, Speed Zone, Weed Out, Weed-B-Gon, Trimec, or one of the special henbit herbicides early before the henbit has put on much growth.
How Low Should You Go?
People will be starting to think about mowing their lawns before long.
We often are asked whether it is good to mow lower in the spring. The answer is yes and no.  
It doesn't hurt to mow lower than normal the first mowing or two. As a matter of fact, it can actually speed green-up by removing old, dead grass and allowing the soil to warm up faster.
But the mowing height should be raised to normal after the first or second cutting to discourage crabgrass.
Crabgrass seed must have light to germinate, and a high mowing height will shade the soil.
Also, root depth and mowing height are related on upright growing grasses such as tall fescue and Kentucky bluegrass — the higher the height of cut, the deeper the root system. A deeper root system means a more drought-resistant turf.
So, how low should you go on the first cutting? On tall fescue and Kentucky bluegrass, you should mow at about one to one and a half inches. Be careful you don't go so low that you scalp the turf.  
Normal mowing height for Kentucky bluegrass is two to three inches and for tall fescue is two and a half to three and a half inches.