I'd wait, the ugly brown could yet green up and you could be cutting away some of this season's blooms. You'll know if it's not going to regenerate soon enough. Phlox Subulata is such a vigorous groundcover and it needs very little attention. But you should prune or shear it only after flowering, cutting back the stems to encourage a denser, spreading growth habit. My gardens have had some for years (maybe 30 or so) and it rarely gets cut back; the "brown" always seems to regenerate and it cascades down a retaining wall.

It crowds like CRAZY. It grew up, inside and over the outside of a pine bush for me last year. It didn't kill it, but boy that felt aggressive for me. It's not terrible to cut back, but honestly a find a sharp shovel to be way easier. I just dig it. That way I get a nice clear edge line. I pretty much do this every spring or I fall behind and regret it. That being said, once a year with a shovel isn't too bad. I like the look both green and flowering.

It expands in mass at the base the stem...It won't do that in that weed cloth and river rock. In fact nothing much will do worth a darn in that combination for very long at all. My advice and the advice of many many gardeners is get rid of that felt stuff and the rock and mulch with organic matter...wood chips, sawdust. cocoa bean shells pine needles or whatever else you like...but the purpose of a mulch is to keep the soil cool and moist and rocks get very very hot and there is no exchange of air beneath that fabric stuff, no worms no composting of mulch going on and eventually the fabric will become full of silt and you will have a garden that can't get water, so the roots of your plants will grow on top of the fabric....and all the bits and pieces of stuff that falls will create a layer beneath the rocks....and in spite of it all you will get weeds growing in the rock. Ask me how I know!!! Sorry about the rant...but I spent a lot of time helping my daughter get some plantings in shape that had that stuff down for about 6 years before they bought the house. Lots of grunt labor, but we did it. Then they had another child and moved....and guess what!? There was more of that stuff in the new house...but this time it had bind weed growing in the fabric itself. Linda C

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