I have been mulching for the past half-dozen years or more but I still don't feel I really know what I'm doing. The mulch I have available is chipper wood in various stages of decay which is provided by my local town.
I am pretty sure I killed off my asparagus bed by applying too thick of a layer of this mulch. Only later did I notice that the mulch had formed a rather particle-board-like dense layer a few inches down.
Yesterday I decided to pull it back away from my thornless blackberries to see what was happening underneath, but I could not really tell if there would have been a problem. Some areas had indeed compacted into impenetrable material but since the blackberry shoots are usually adjacent to previous growth they might have easily found an exit.
Is this the situation with mulch? Every year you need to pull it back and see what is happening underneath?