I've never had a problem with weeds or grass in my phlox and it is near my lawn but I would be careful using any controls to eradicate anything because you could lose what you are trying to protect. It's hard to amend the soil with established plants like phlox but if you tried weeding would be easier.

Here is a link that might be useful: Propagating Perennials

I feel your pain. I've got endless grass in my Irish and Scotch moss that edges several of my ponds. I've been fighting it for years.

Personally, I would not use a weed/grass killer. I don't see how you can get the targeted grass without affecting the phlox, as well as perhaps the grass you want to keep.

If your phlox is right up along the lawn, then I suggest perhaps some kind of edging. Either a hard edging (that black pound-in edging, stones, etc.) or even a simple trench edge. That will help the grass from spreading. If your phlox is right along the lawn edge, sure as sugar the grass will keep spreading into the phlox, and by the same token, the phlox may spread into the lawn!

I have creeping phlox on a small slope that is about thirty feet long and about 3 feet top to bottom. About twice a year, once in spring and again in later summer, I get my little kneeling pad, plop it down, have a seat, and spend a few hours weeding. (Just did it Sunday, as a matter of fact!) It is actually fairly easy to weed phlox. You can pretty much lift the whole plant up or push it aside, and just reach in and pull the grass and weeds out. Then again, my phlox is in poor, shallow soil, so maybe that's why it's easy. Time consuming, but easy.

Good luck in whatever you decide!
:)
Dee

Thanks everyone for the replies. I'm not so in love with phlox that I would ever spend hours weeding it. I find it hard to get in and under it. I'll try the Grass Be Gone in a small patch and if that doesn't work....I'm yankin' it.

jospry - it's probably too late, but just in case - I deal with it with the electric clippers - I just shear everything back, grass and all. I take a few moments with the shears every time I'm out there (or every time I think of it) and cut back and grass that pops its head up. That pretty much allows the phlox to out-compete the grass.

I have used grass-be-gone with grass in phlox. It works in very warm weather. However, the grass always comes back the next year.

You could try painting Roundup onto the grass leaves if they are high enough with something like a sponge paintbrush. Don't do it when rain is expected. It's risky, but if you are at the point where you are ready to ditch the phlox, why not?