Mulching is the practice of covering soil with a protective layer of organic material to improve landscape health. Fallen leaves are a natural source of organic matter, while traditional landscape mulch, such as wood chips or shredded bark, offers a durable, aesthetically pleasing top layer. You can put mulch over leaves, and this combination is recommended by many horticultural experts. Using a layer of processed leaves beneath wood mulch creates a highly effective, dual-purpose barrier that improves soil quality and moisture retention better than either material used alone. This method mimics the natural decomposition cycle found on a forest floor, where foliage slowly breaks down to nourish the earth below.
Preparing Leaves for Mulch Application
The primary step in combining leaves and mulch involves properly preparing the leaf layer by shredding the leaves first. Whole leaves, particularly large varieties like maple or oak, tend to form a dense, impermeable mat when wet. This matted layer repels water, preventing it from reaching the soil and plant roots, and can also suffocate the ground by blocking air circulation. Shredding or chopping the leaves solves this problem by reducing the particle size and increasing the overall surface area.
Shredding is easily accomplished by running a mulching lawn mower over the leaves or by using a leaf shredder machine. The resulting small fragments create a lighter, fluffier material that allows water and air to pass through easily. Increasing the surface area also hastens decomposition, meaning the leaves break down faster to release stored nutrients into the soil. Processed leaves are also less likely to blow away, helping them stay in place as the base layer for the heavier mulch application.
Proper Layering Techniques
The physical application of the combined materials requires a specific sequence and thickness to ensure maximum benefit. The shredded leaves should be applied first, directly contacting the soil surface. This layer should be kept relatively thin, generally no more than one to two inches deep, as it is prone to compaction.
The traditional landscape mulch, typically wood chips or bark, is then layered on top of the shredded leaves. The total depth of the combined layers should not exceed three to four inches for established trees and shrubs, or two to three inches for perennial beds. The most important rule is to avoid the “mulch volcano.” All mulch material must be pulled back several inches from the base of tree trunks and plant stems to expose the root collar or crown. This prevents moisture from being trapped against the bark, which can lead to decay, rot, and insect infestation.
Benefits of Combining Leaves and Mulch
The dual-layer approach provides advantages over using a single material. The shredded leaf layer acts as a food source for soil microbes, increasing the organic matter content as it breaks down into nutrient-rich humus. This process improves soil structure, enhancing both aeration and the ability of the soil to hold moisture and nutrients.
The top layer of wood mulch functions as a protective cap, suppressing weed growth by blocking sunlight and moderating soil temperature. Together, the layers create a superior insulating barrier that keeps the soil cooler in the summer and warmer in the winter, reducing stress on plant roots. A common issue with high-carbon wood mulches is that decomposition can temporarily draw nitrogen from the soil, known as nitrogen tie-up. The nitrogen-rich leaf layer acts as a buffer, providing the microbes with the necessary nitrogen to break down the carbon-heavy wood chips, mitigating temporary nutrient depletion in the underlying soil.
Avoiding Common Application Mistakes
Even with the correct layering technique, homeowners can make mistakes that diminish the benefits of the combined mulch. One of the most frequent errors is applying the total layer too thickly, which can create conditions detrimental to plant health. A combined depth exceeding four inches can restrict the exchange of oxygen and carbon dioxide between the soil and the atmosphere, leading to anaerobic conditions.
Another common pitfall is failing to break up matted, old leaf or mulch layers before applying new material. Hardened, compressed mulch can become hydrophobic, meaning it repels water, causing rainfall to run off rather than soak into the soil. It is also important to monitor for pests, as overly deep and consistently wet layers can create a favorable habitat for rodents like voles, which may tunnel and gnaw on the bark of nearby trees and shrubs. Regular light raking of the top layer can help prevent matting and ensure proper water penetration.