The best time to TRY to kill the Bermuda is at the height of the growing
season.If it is well established you can spray Round-Up forever and not
kill it off.Round-up will do nothing to kill the abundant "runners" the
Bermuda grows from and it will come back relentlessly.
I have watched my neighbor spray Round-Up for six years on the Bermuda patch
in his fescue lawn and it never works.So he has finally succumbed to the
fact that he will have to live with it.
Also,if you try to burn it off,it just gets stronger form the nitrogen. Good
luck!