Depth of Rock or Wood Mulch Depends on the Size of Mulch

Q. I have been replacing gravel with wood mulch and I have to put the gravel someplace. What thickness of gravel can I place around the plants, none at the trunk, without causing damage?  Also, what is the maximum amount of wood mulch can be place around plants without being detrimental to the plant?

A. The depth of rock mulch depends on the size of the rock; the larger the rock used the deeper rock mulch can be applied. Roots need to “breathe” so there must be an exchange of gases between the roots and the air. This happens through the pore spaces in the soil.

Rock of a larger size can be used as mulch and applied more deeply than smaller rock

            Generally speaking, rock mulch is applied somewhere between 2 to 4 inches deep. If this rock mulch contains “minus” material (e.g., 1/2 inch minus, ¼ inch minus) then apply 2 to 3 inches. If the rock mulch does not contain minus material then apply it 4 inches or more.

            Course mulch without minus material can be applied right up to the trunk. Rock mulch with minus material should not.

            Most information out there tells us not to apply more than 3 to 4 inches of wood chip mulch around trees and shrubs. Where this recommended depth came from I don’t know but it is repeated over and over.

            I have no problems applying 6 inches of wood chip mulch or more around trees and shrubs provided this mulch is kept away from the trunk of the plant a distance of 12 inches during its first four or five years of growth.