years ago when i first started planting, mulching, i was a firm believer that weed barrier should be used under mulch. the people here told me no way, no how, never. what i noticed: the work we did, where a barrier was used, after a few months, you could see the plants deteriorating, just looking sickly, thinning, they lived, but looked terrible. the work we did with the barrier, looked good years after. the mulch breaks down, turns to dirt, then u have a layer of dirt, then the barrier which is now clogged and nothing can pass through it. water and air cannot get through, the plants health deteriorates. i believe this to be true, but i been wrong before.