I planted a pathway with stepping stones with 8 flats of Dymondia margaretae (Silver Carpet) about 6 months ago. 512 individual plants. The Dymondia is doing fine but there is now crabgrass all through it. In the springtime, I pulled out young crabgrass shoots as soon as I saw them. But it grew faster than I could keep up with it, and now in mid-summer there is about as much crabgrass as there is Dymondia. The Dymondia still looks good, but the spaces in between them have filled up with crabgrass. So now I'm considering a weed killer. Is there a narrow-spectrum weed killer that will kill the crabgrass but not harm the Dymondia?

Thanks!