Question
Hi,
I have a problem with a weed that is like a vine growing in my tall fescue lawn.  This problem started 2 years ago when I noticed a 2' x 2' area, I tried treating it with weed killer from Home Depot but it keep growing at a rapid rate.  I finally had to kill the entire area (10'x12')with a roundup product and let it sit all winter and into April before I laid replacement sod.  I notice a very small area in the front yard which I have been pulling out by hand.  The problem is the roots are very deep 10" to 12" and I have to follow them in order to try and get the entire weed.  I was told this is a witch grass but after doning some research it appears to be Cynodon dactylon Bermudagrass.  How do I kill this without killing my tall fescue?

Answer
I have previously noted with an appropriate degree of shame that I spent a great deal of time and money to control Cynodon dactylon in sugar-cane and failed in the long run. By all accounts you did things right; chemical, timing and allowing time for the product to be accumulated in the plant. Plants tend to store nutrients (and harmful products alike) in preparation for the dormancy period.
Cultural methods offer the best hope for control.
Mow the lawn at the highest adjustment your mower will allow. Cynodon likes full sunlight; so shade it out. Mow the weed infested area first and bag the clippings. Cynodon spreads by pieces so get rid of the clippings far away from the lawn.  
Cut down the irrigation to the barest minimum. Cynodon likes abundant moisture.
Reduce or eliminate fertilizing; at least for a year or two. Cynodon is a 'gross feeder' of nitrogen.
If by this time you are thinking that you should do the very opposite of what they do on golf courses, you are so right! Special strains of Cynodon are developed for the golfing industry.
And when we pursue our desire to have a lawn of golf course appearances, we are making it all too easy for Cynodon to proliferate.