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

I planted candy stripe subulata phlox last year, AND this year on the west side of a very tall oak tree. I have created a raised bed around it and thought this would be the perfect plant to fill in the crevices. I have put in good dirt, mulched, planted my phlox, used Miracle Grow quick start, watered frequently and they are dying....again. I don't get it, they are supposed to LIKE the sun and they die. I did buy them in large pots from a good local greenhouse and divided them into 3x3 sections so they would spread...perhaps that's the issue? I am frustrated that my phlox won't grow. Does anyone have any suggestions as to what this newbie is doing wrong?