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Why do we even use mulch? Besides looking pretty, mulch serves many purposes in our gardens.
First and foremost, mulch is a blanket that conserves moisture in the soil. This blanket slows evaporation by shielding the soil from the harsh drying effects of direct sunlight (which also regulates the speed of warming and cooling of the root zones of plants) and also by physically trapping most of the moisture which evaporates naturally form the warming of the soil.
Mulch is also erosion control. We have all seen the effects of even a light rain on exposed soil. Rain drops hit the ground with quite a bit of force, throwing soil hither and yon. After just one rain fall, the contour of the soil surface will have changed, and lower leaves of plants will have accumulated soil on the undersides of the leaves - potentially spreading soil-born disease! Mulch- typically heavier than the soil it protects- serves as a water break. Falling droplets are disbursed more evenly and the soil is given a better chance to absorb the water slowly. This also helps prevent pooling and eventual run-off!
I list weed control last because so many people have the idea that mulch STOPS weeds. It does NOT. Mulch is certainly indispensable in the ongoing battle against weeds, but it alone is not a solution.
Just about any material you want to use to cover the bare ground in your garden beds can be used as mulch. Almost any mulch is better than no mulch at all. I only recommend against plastic (and other impermeable materials which impede or prevent exchange of gasses and lead to anaerobic soil) or un-composted garden waist (this can spread pests, diseases, and the processes of rot and decay put a demand on valuable nitrogen in the soil. It is available again when composting is completed.)
Personally, I think the number one choice for mulch is GRAVEL, since it never rots, is not easily displaced by rain or traffic, doesn't hold any water itself, and doesn't rot away - so it doesn't require annual replenishment, as do organic mulches.
But...gravel doesn't always fit the look of a bed (I use shredded hardwood mulch in 85% of my personal beds at home). Use what your eye likes best.