What do mulches do?

Question: I’ve heard that mulches can improve water quality—is there a connection? Aren’t they just for landscaping?

Photo by Susan V. Fisk

Landscape mulches come in wide arrays of colors and textures. Pine straw is common in some areas, shredded hardwood bark in others. Mulches can even be cocoa hulls or gravel. They help conserve moisture and suppress weeds, but how well they do those jobs, or how often they need to be replenished, varies from mulch to mulch.

However, mulches do more! After eyeing countless, unplanned, bare soil footpaths winding through urban landscapes, one of my recent graduate students, David Mitchell, wanted to study how mulches affect water runoff and sediment transport. Many rain simulations and muddy, field data collection days later, he had his results on eight different mulch types. His findings have really made me rethink the role of mulches.

First, all mulches have a huge effect on total suspended solids running offsite. Bare soil lost about five times as much sediment. Thus, mulches help control erosion.

Second, geotextiles underneath mulches (such as “landscape fabric” meant to suppress weeds) appeared to accelerate the production of water runoff in his studies.

Finally, each mulch wears differently and absorbs a lot of runoff on its own, independent of the soil beneath it.

Photo by Susan V. Fisk

Research reports are in preparation now with full details. However, all these results made me ponder. What really is the role of mulch? It’s not to provide acres of uniform, herbicided landscape surface.

Think of mulch as a temporary forest floor. Yes, its traditional roles are to provide aesthetics, conserve soil moisture, and eliminate competition for landscape plants. But it is also an important cog in the machinery of the hydrologic (water) cycle by keeping the soil surface receptive to water in areas where a natural leaf litter or herbaceous layer do not yet or cannot exist.

This improves water quality by allowing the water to get into the soil, instead of the stormwater control system. And soil is an important part of the water cleansing cycle.

In summary, the right mulch can:

  1. Suppress weeds
  2. Help soil retain moisture
  3. Reduce water runoff
  4. Reduce erosion of sediments
  5. Provide aesthetic value

Ideally, landscape plantings will fill in and cover the soil surface everywhere, including surfaces covered by mulches. People are starting to recognize the potential of “stacked” or “bundled” ecosystem services and having every piece of nonpaved land in urban areas provide multiple benefits. Mulch can play a role in making urban landscapes part of our “green infrastructure.”

–Answered by Susan Day, Virginia Tech University

Have a question for Soils Matter? Post it as a comment below, or email us at soils-matter@soils.org

13 thoughts on “What do mulches do?

  1. I knew that using mulch helps strengthen the soil, but I had no idea that it helps suppress weeds though. My wife’s biggest pet peeve is when she goes out to her garden, and there are a bunch of weeds all around it. I will have to let her know about this information, and go try using some to see how it works for her.

  2. My garden is really wet in places so I have avoided mulching it. I am putting down compost to help break up the really heavy clay soil. But would wood chips help with the erosion? What type of mulch would your recommend for this garden bed.

    1. Hi Elizabeth, we’ll ask this question to one of our soil scientists and get back to you! Great job with the compost in your clay soil. SF

      1. Thanks so much for your help. I am getting the gardens ready for winter and want to put down the mulch soon.

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