Answer: I think the different grasses are trying to compete with each other and total eradication may not be possible. Fescue lawns grow best in cooler temperatures; Bermuda loves hot weather. So, as the fescue is going dormant or at least is not actively growing in heat of summer, the Bermuda is just coming into its own. Digging out the Bermuda and reseeding with fescue is one approach, using a Bermuda grass herbicide is another. Reseeding will be necessary once you've gotten rid of the Bermuda.
Bermuda in flower beds presents a whole new problem. You first want to put up a good barrier between your beds and your lawn. I recommend at least a foot of hardwood mulch with plastic edging buried about six inches. Keep the lawn trimmed back to the edge of the mulch line so stolons don't cross over it. It will be like a "demilitarized zone between your lawn and flower beds and if you see Bermuda grass stolons starting to cross it you can kill them.
If Bermuda grass does cross over into the garden bed you need to remove the stolon root and all. Wet the flower bed down well and using gloves pull up the runners, root and all. Any root left whatsoever will start an new Bermuda grass plant. If the runners are very deep in clay soil and cannot be pulled out get a painters wool glove and wearing a second rubber glove place the painters glove on your hand and douse with Roundup week killer at the recommended dilution.
Run you painter's glove with weed killer on it across the shaft of all the grass in your flower bed, careful not to touch garden plants.
After the grass has died pull what remains out. Repeat the process for any new shoots that crop up.
Best wishes with your project!
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