Answer: I apologize for the delay in answering your question. By now you have probably spread the mulch and seen what happened. Generally, mulch spread over prepared soil does not need to be underlaid with anything. However, some gardeners will use a water and air permeable landscape fabric under a stone or large bark chunk mulch placed in a shrub bed. This will limit the depth to which seeds can root through the mulch and makes them easy to pull.
Keep in mind too that a natural mulch such as shredded bark will need to be renewed periodically because it rots away over time. This rotting process helps feed the soil so it is actually a good thing to have happening in the garden.
Mulch alone placed over weeds or grass will not usually be sufficient to kill out that growth all by itself. If a layer of several sheets of newspaper or a piece of cardboard is placed under the mulch, and the mulch is layered on quite thick, eventually it can smother the plants beneath it. How long this takes depends on what the plants are and the time of year the mulch is applied. Annual weeds are much easier killed this way than tough perennial weeds.
You can also use the newspaper topped with mulch in between desirable plants. It will help greatly to control crabgrass and other annual weeds in a flower bed. I hope this anwers your question.
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