Mulch is a layer of material spread over the surface of soil, primarily intended to benefit garden health. Organic mulch does absorb water, but its primary function is to prevent moisture loss from the soil beneath it. Understanding this dual nature—initial absorption versus long-term retention—is key to using mulch effectively, as this practice significantly reduces the need for watering.
The Dual Nature of Water Absorption
Organic mulch, such as wood chips or straw, inherently acts like a sponge, absorbing a portion of water from rainfall or irrigation. This initial absorption is due to the porous structure of the organic material, which holds water within its fibers.
However, the benefit of mulch lies not in its ability to absorb water, but in its ability to retain moisture in the soil underneath. A layer of mulch creates a barrier that shields the soil from direct sunlight and wind, dramatically reducing the rate of water evaporation. This retention effect far outweighs the small volume of water initially captured by the mulch itself, making it a net positive for soil hydration.
How Mulch Type Influences Moisture
The specific material used for mulching plays a large role in its moisture dynamics, primarily based on particle size and composition. Materials with smaller, finer particles, like shredded leaves or grass clippings, tend to have a higher surface area and can absorb a greater volume of water initially. This high absorption can be a drawback during light rain showers, as the water may be held in the mulch layer and evaporate before reaching the plant roots.
Conversely, materials with larger, chunkier particles, such as pine bark or large wood chips, generally absorb less water. These coarse materials allow water to pass through more freely to the soil below. Their structure is highly effective at creating the necessary barrier to slow evaporation from the soil surface. The degree of decomposition also matters, as well-composted materials often have a higher water-holding capacity than fresh, chunky wood.
The Critical Issue of Hydrophobicity
The perception that mulch “steals” water from the soil is often a consequence of a specific condition called hydrophobicity. This occurs when certain organic mulches, particularly wood products or peat moss, dry out completely and repel water. This water repellency involves the accumulation of waxy, organic compounds or the growth of fungal hyphae on the dried-out particles.
When the mulch becomes water-repellent, the water pools on the surface or runs off instead of penetrating through to the soil. This defeats the purpose of mulching, as the plant roots remain dry even after watering. This condition is more common in hot climates or during extended dry spells, and a compacted layer of fine particles can also contribute to the problem.
Practical Guidance for Application
Preventing the formation of a water-repellent layer requires attention to proper application techniques. To avoid hydrophobicity, gently turn or lightly rake the top layer of mulch occasionally to “break the seal” and prevent matting. If newly applied mulch is extremely dry, moistening it with water before spreading can help prevent immediate water runoff.
The recommended depth for most garden applications is between two and four inches to balance moisture retention and soil breathability. It is also important to pull the mulch back a few inches from the base of plant stems to create a “donut hole,” which prevents excessive moisture buildup that can lead to rot and disease.