Mulch retains soil moisture and suppresses weed growth, benefiting plant health. On an incline, however, gravity and water runoff from rain or irrigation easily displace the material, leading to erosion. Keeping mulch in place requires a multi-layered approach: selecting the correct material, modifying the soil, and installing barriers to counteract water and gravity.
Selecting Mulch Materials for Slopes
Choosing the right type of mulch is the first step in ensuring stability on a sloped area. Materials with a high degree of interlocking capability are significantly more effective at resisting downhill movement. Shredded hardwood or bark is preferable because the long, stringy fibers weave together, creating a matted layer that grips the soil. Lighter, non-cohesive materials, such as straw or larger wood chips and nuggets, are not recommended as they tend to float and wash away easily during heavy rainfall. Shredded pine bark is noted for its superior matting quality, helping it remain stable on steeper inclines.
Preparing the Soil Surface
Modifying the bare soil surface before application is a simple, non-structural way to improve mulch retention. Creating small, horizontal interruptions in the slope’s contour helps to slow water flow and provides anchor points for the mulch. This technique, sometimes referred to as “notching,” is achieved by making shallow, horizontal ridges perpendicular to the direction of the slope.
Anchoring Techniques
For areas where the mulch meets a flatter surface, digging a small “mulch trench” at the base or perimeter can prevent displacement. This trench, typically three to four inches deep, catches any mulch that migrates downward, allowing the material to be easily redistributed. When planting, forming a small soil berm around the base of each plant creates a water-holding basin that prevents water from immediately running off and carrying the mulch.
Installing Structural Edging and Barriers
Physical barriers provide necessary infrastructure to support the mulch layer and intercept runoff water. Placing landscape timbers, logs, or stones horizontally across the slope, following the natural contour lines, creates a series of low retaining walls that physically trap the mulch and reduce the velocity of water as it moves down the incline, forcing it to slow down and soak into the soil. For steeper or larger slopes, biodegradable erosion control wattles, also known as fiber rolls, can be highly effective. These dense, cylindrical logs made of coir or straw are placed in shallow trenches and secured with wooden stakes or rebar pounded through the center. The wattle acts as a sediment and mulch dam, breaking the slope into shorter segments and stabilizing the area until vegetation establishes itself.
Applying Surface Stabilizers
Once the mulch is applied, securing the top layer with stabilizing agents provides an extra measure of protection. One common method involves laying down physical netting, such as biodegradable jute or coir mesh, over the mulch surface. This mesh is then secured to the ground using U-shaped wire landscape staples, holding the mulch layer in a cohesive mat. The netting slows the impact of raindrops and prevents the shredded material from shifting.
Alternatively, liquid mulch binders, often called mulch glue, can be sprayed directly onto the applied material. These non-toxic, polymer-based adhesives cure to form a clear, flexible film that binds the individual mulch pieces together. Application is done using a garden sprayer, and the resulting bond anchors the top layer while remaining permeable, allowing water and air to reach the soil below.