Does Phlox subulata spread from the roots, or does it spread from where stems touch the ground?

I planted a few dozen small plants. They are getting "taller" but not spreading much (they are only out a few weeks). I don't know if I should move mulch away from where the trailing stems hit the ground, or if they only spread from the roots underground (in which case they won't have any trouble coming up through the mulch).

Thanks!