Does anybody know a good chemical to put in an already established garden to control it?
The best method of control of Bindweed, wild Morning Glory, is to pull out the roots and keep any new growth cut to deny any roots the plant growth needed to manufacture the foods needed by the roots for regrowth.
Many people have used various "weed" killers over the years and found they do not work.
If you go to this thread and read the info posted by Bry84, you will find good instructions. Good luck!