Mulch hard lump charcoal if you want your creeping phlox to burn hotter and longer.
I’m just kidding, of course. The joke’s on me, though: I once did set that particular bit of flowers on fire when an ember from my grill drifted some thirty feet and started the mulch smoldering. My wife, fortunately, was on her way out after dinner that evening but paused to tell me that the phlox could use an extra bit of watering that evening. Although she phrased it differently at the time.