A Mulch Manifesto
Water Quality and Mulch’s Role
A light layer of mulch offers numerous water quality and ecological benefits. It protects soil from erosion by buffering against wind and water, reducing runoff, and helping soil particles stay in place. This is especially important on slopes and exposed areas where rainfall can quickly strip away topsoil, leading to sediment pollution in waterways.
Beneath the surface, mulch supports microbes, fungi, and soil fauna, which improve aeration, nutrient cycling, and water infiltration. It also creates a protective environment for worms and other beneficial organisms, allowing them to reach the soil surface without sun exposure. Their movement aerates the soil, forming natural pathways for water to penetrate more effectively.
Additionally, mulch reduces evaporation by shading and insulating the soil, conserving moisture and supporting aquifer recharge. By stabilizing soil, retaining water, and promoting biodiversity, mulch plays a crucial role in maintaining healthy ecosystems and improving water quality.
On-Site Mulching Techniques
On-site materials are the most sustainable choice for mulch. Allowing dead vegetation to remain on the soil surface and decompose slowly supports the soil microorganisms vital for plant health and allows for natural nutrient cycling.
Mulch in Place
How to Do It: Cut dead plant material into pieces up to 8 inches long and leave them around the bases of plants and in garden beds. For smaller projects, use hedge trimmers, garden shears, or pruners; for larger-scale projects, use mowers or brush hogs.
When to Use: Ideal for established plantings, especially those producing significant organic material.
Pro Tip: Native plants should be cut back in late winter/early spring. Leave 6-12 inch stumps of pithy stems to support native bees and insects that lay eggs and nest inside them.
Green Mulch
How to Do It: Use fresh grass clippings, garden trimmings, or freshly-pulled weeds as mulch in 1 - 2 inch layers.
When to Use: Suitable year round for great moisture retention and quick nutrient addition.
Pro Tip: Lawn clippings generate heat as they rapidly decompose, so be careful not to pile next to young plants. Keep green mulch layers thin to avoid matting.
Leaf Mulch
How to Do It: Use a mower to chop leaves into small bits. Run over the leaves 3-4 times, then collect and apply them in thin 2-inch layers to plant beds. Leaves can also be composted on site and used once they are broken down.
When to Use: Leaf piles provide winter habitat for wildlife, which emerges in spring. Wait until temperatures consistently reach 50°F before chopping leaves to avoid harming the critters that rely on them.
Pro Tip: Leaves tend to blow around, but the smaller you chop them, the better they stay in place.
Living Mulch
Nature’s Best Mulch: The best way to cover the soil is with plants! Native ground covers like violetes (Viola sororia) and sedges (Carex sp.) are excellent at shading the soil, helping to retain moisture while also supporting vulnerable wildlife like the Diana Fritillary- Arkansas’ State Butterfly.
Pro Tip: Nature arranges itself in layers. Taller plants provide shade for shorter plants that cover the soil, increasing moisture retention and improving nutrient availability. A layered garden not only looks more natural but is also more resilient.
Off-Site Mulches
Off-site mulch is often essential for new plantings, when there isn’t enough organic material on site, or when a polished look is desired. These are some of our favorite off-site mulches:
Pine Straw
Benefits: Locally available pine straw offers excellent moisture retention and weed suppression while being lightweight and easy to apply.
Sustainability Tip: Harvest responsibly by collecting only 20-30% of fallen needles to avoid harming ecosystems. When purchased, ensure it comes from a sustainable source.
Pro Tip: Don't worry about pine needles making your soil too acidic! When left on the surface, they have little impact on soil pH, as microbes naturally neutralize acidity during decomposition.
Leaf Mold and Composted Leaves
Benefits: Decomposed leaves are ideal for water retention and soil improvement. Mimicking a forest floor, leaf mold creates a fertile environment for perennial plant growth. Available at some municipal compost centers in the late fall.
Sustainability Tip: Create your own leaf mold or compost at home! Simply pile up leaves, keep them moist, and in a year or two, you'll have nutrient-rich leaf mold. For faster decomposition, add a nitrogen-rich material to produce leaf compost.
Pro Tip: Leaf mold is rich in fungi, while leaf and other types of compost are bacteria-dominant. Perennial plants thrive in fungal-dominated soils, while most annuals, including flowers and vegetables, prefer bacteria-rich soils.
Undyed Wood Chips
Benefits: Wood chips suppress weeds, retain moisture, and decompose slowly to add nutrients to the soil.
Sourcing: Obtain wood chips sustainably from local tree services, compost centers, or apps like ChipDrop. Bags of cedar and cypress wood chips are available at box stores for convenience. Aged hardwood mulch is a popular undyed dark mulch that can be purchased from landscape supply stores.
Pro Tip: Avoid mixing wood chips into the soil, as they can deplete available nitrogen. This makes them not ideal for annual gardens, like vegetable beds, where the soil is frequently disturbed.
Best Practices and What to Avoid
Mulch Depth
Best Practice: Apply 2 - 3 inches for most beds; 1 - 2 inches for seedlings or plants in thin soils.
Risks of Excess Mulch (more than 4 inches): Blocks water and air, leading to root rot and fungal problems.
Pro Tip: Dry prairie species like Little Bluestem (Schizachyrium scoparium) are highly sensitive to crown rot. To prevent this, avoid piling mulch, especially wood mulch, around the plant’s crown. It's best to gently clear the soil around the crown in early spring to prevent rot.
Mulching a Tree
What to Avoid: Piling mulch against tree trunks traps moisture, causes rot, encourages pests, and can cause girdling where tree roots wrap around the trunk of the tree essentially strangling itself.
Best Practice: Keep mulch 3 - 4 inches deep, with a 6-inch mulch-free zone around trunks. Aim for a "donut," not a "volcano." Keep the root flare of trees exposed.
Avoid Rock Mulch
Heat and Water Issues: Rock mulch retains heat, increasing soil temperatures and limiting water infiltration, which can harm plants and soil life.
Nutrient Deprivation: Unlike organic mulch, rocks do not decompose, depriving the soil of essential organic matter and nutrients.
Maintenance Challenges: Dirt and debris accumulate over time, encouraging weeds and requiring labor-intensive upkeep.
Reflecting on the Power of Mulch
Thoughtful mulching can restore disturbed landscapes, enhancing ecosystems, improving water quality, and supporting plant and wildlife health. By adopting sustainable mulching practices, we become stewards of the land, fostering resilience and beauty. Together, we can nurture healthier environments, one layer at a time.
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Have Questions about ecological landscaping? Reach out to the authors by email:
Taylor Hills, IRWP Horticulturist taylor@irwp.org
Annie Sales, IRWP Green Infrastructure Program Manager, annie@irwp.org
