I have the same problem with a large planting of Fountain Grass and Purple Love Grass. Since I can grow them from seed so fast, I'm probably going to dig and bareroot the best of the Fountain Grass next spring, and then RoundUp the bed, killing everything.

Barerooting them is about the only thing I can think of for your 'Red Baron.' As far as I know, there isn't any herbicide that will do the trick on the weedy grass and not the ornamental. You could try direct application of RoundUp, painting it carefully on the weeds, without touching the 'Red Baron,' but that's no less tedious than barerooting the Imperata.

I will agree w/ donn on the removing of "weed" grass in your grass bed(s).

We have used this method w/ some success but again it is tedious.

Get a chemical retardant "rubber" glove, not a synthetic glove, put it on your hand and then put a cotton glove over the rubber glove.

Dip the gloved hand in your weed-be-gone, round up, and wipe the designated weed(s) or weed grass w/ the treated gloved hand.

This method allows better control of chemical placement w/ no over spray and minimal amount of chemical getting on desired plants.

It has worked in our perennial beds when the beds are in their full bloom and weeds get ahead of the bed care taker.

Jake