Question
I have several areas in my St. Augustine grass lawn that is being invaded by carpet grass. How can I kill the carpet grass without killing the St. Augustine grass.  

Answer
I don't know what you are calling carpet grass, that is what we have always called St. Augustine.
I put carpet grass in my search engine, and apparently that is what they are now calling a type of grass that looks like centipede grass.
I read some pretty wild ideas of curbing it, most of which siad it also damaged the St. augustine and Floratam ( which is another vaqriety of St. Augustine).
I went to another site with a lot of grasses.
It describes carpet grass as a shallow rooted grass that requires a lot of water.
I water deeply to encourage a deep root system. This protects against out North Texas hot summers, and droughts. I water once a week, and when the top two or three inches are dry, there is still some moisture down where the roots are.
Seems to me if you water deeply and get your St. Augustine roots to grow deep, then next summer, watering deep ( to a depth of at least 6 inches, and lettine the top few inches dry well will kill out the carpet grass.
As soon as it looks like it is turning, it should be pulled or raked out, and you should win the battle when they St. Augustime grows over the bare areas.
That is all I know of to get rid of it.
My St. augustine tells me when it is getting into trouble and really needs a drink badly.
The baldes fold up lengthwise, so they are half a wide, so they expose less surface to the heat.
Wait for the blades to fold up before you water, then water and turn off and let soak in, turn one again, and keep repeating this until your soil is soaked at least 6 inches down. Then let those top few inche\s dry out before you water it again.
This should keep the St. Augustine going, and the shock of drown and drought should kill out the carpet grass.
I don't know if it is a weed grass, like crabgrass, johnson grass etc, but switching to organics got rid of my crabgrass johnson grass, dandelions, dollar weed and all the other weeds.
Or ganics builds a rich soil. Weeds will not thrive in rich soil, and those that do come up will start to die out right away.
Charlotte