Determining the Right Mulch Depth Ideal Thickness for Different Plants The optimal mulch depth varies by plant type and material used. For most organic mulches, a 2- to 4-inch layer suffices. Delicate perennials and annuals thrive with a thinner 1- to 2-inch covering to prevent suffocation, while trees and shrubs benefit from a deeper 3- … Continue reading How to Apply Mulch Effectively