Answer: I have used Grass-B-Gone (by Ortho) and it can be effective. But it takes persistence. Thoroughly spray the grass on a warm sunny day, once a week, for about four weeks running. It won't harm your flowers, and it will get rid of the grass, but may not do so completely.
The best way, however, is to lift your pernnials up with a shovel and pull out all the grass's roots. Just follow the stem down to the roots and remove it. It's work, but it's the most effective way...and next year, put some Preen around your flowers to prevent weed and grass seed form germinating.
Best wishes with your flower garden!
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