How Thick Should I Apply Organic Mulch?
A layer of mulch 3 to 4 in. deep will keep most weed seeds in the soil from sprouting and increase moisture retention. However, more isn't always better. Limit the depth to 5 to 6 in., especially around shallow-rooted plants. And pull back mulch from the base of plants so it doesn't cause rot.
If you want to use organic mulch on slopes, apply a shredded type about 6 in. deep. It'll mat together and stay in place better than a thinner layer.
Note: Cocoa bean mulch is popular in some areas because of its deep brown color and chocolate odor. But it's a bit tricky to use effectively. Apply it no more than 1 in. thick, because thicker layers tend to retain too much water and become moldy. You may also have to replenish it more often because it blows away easily when dry. Also be aware that dogs can get sick if they eat or chew on this mulch.