Mulch is a material (such as decaying leaves, bark, or compost) spread around plants to enrich or insulate the soil. It can help to retain moisture, suppress weeds, and more. But, not all mulch is created equal!
Mulch should always be an organic substance that is derived from some living (or formerly living) matter. This can include grass clippings, leaves, hay, straw, shredded bark, whole bark nuggets, sawdust, shells, wood chips, shredded newspaper, cardboard, and animal manure.
The longer mulch is aged the better so that it is ready to do its job as a soil improver. Wood can rob soil of its Nitrogen while decomposing, so the longer it has aged, the least likely this is to happen.
Another thing to look for is a medium-textured compound that will break down into the underlying soil but is substantial enough to hold its own against the elements. Big hunks of bark or wood will take forever to break down, so it is best to use something that has already been composted, but isn’t is fine-textured as sawdust.
Ideally, gardens should be mulched twice a year–once in the spring and once in the fall. It’s not an exact science: when it starts to feel warm, mulch your garden; when it starts to feel cool, mulch your garden.