Answer: Most organic mulches are selected based on local availability and price, so there is no hard and fast rule about what is best. For example, larger particles last the longest, so large bark nuggets will last longer than "double shredded" bark; straw is less expensive than bark in some areas but does not last as long in the garden and so needs to be renewed more often. As the mulch breaks down and decays it helps to feed the soil, so an organic mulch is preferable to black plastic.
Mulch can be spread at any time. You might wish to control existing weeds prior to spreading the mulch, particularly if you plant to use a weed barrier fabric beneath it. If you do this with a chemical such as glyphosate, it is better to use it when the plants are in active growth. If you plan to smother them with several layers of newspaper or cardboard topped by a thick layer of mulch then the timing is not an issue.
Good luck with your project!
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