The Complete Handbook of Mulch

Chapter 1: Understanding Mulch

Mulch is decomposing wood fiber, bark, or other organic material (and sometimes inorganic material) spread across soil to protect plant roots and manage moisture and temperature. That clinical definition is accurate but incomplete. To understand mulch as a professional, you need to see it as an engineered layer operating within a specific soil and climate system, not as a static decorative product.

Start with what mulch actually is at the material level. Most mulch sold commercially is either bark—the outer protective layer of a tree stripped during timber processing—or wood fiber recovered from tree grinding, chipping, or sawmill waste. Some mulch blends both. A few products incorporate hardwood, softwood, or recycled wood specifically, each with different decomposition rates and structural behavior. A smaller category includes inorganic materials like rubber, stone, or plastic. But the majority of what landscapers, grounds managers, and gardeners use is some form of wood-based organic mulch, and understanding how wood decomposes and behaves under different conditions is the foundation for everything else in this handbook.

When you apply mulch to soil, you are creating a buffer layer between the soil surface and the air. That layer performs three primary functions: it slows water evaporation from the soil below, it moderates temperature swings at the soil surface, and as it decomposes, it contributes organic matter that improves soil structure and nutrient cycling. These are genuine benefits, not marketing claims. But the extent to which they occur depends entirely on what you apply, how