Summer 2024

The Magic of Mulch

by Lori Palmquist

What if I told you that mulch could cut your water use by at least a third, provide abundant soil biodiversity, making your landscape thrive like you could never imagine, AND contribute to the reversal of climate change? Could you even fathom a more magical scenario emerging from a bunch of chopped-up tree clippings on the ground?

Well, it’s true, my friend. And so much more is riding on the coattails of the bold statement above.

How do I love thee, mulch? Let me count the ways:
You keep water from escaping the soil.
You feed the abundant soil organisms.
You modulate soil temperature.
You suppress soil-borne diseases.
You reduce weeds.
You contribute to carbon sequestration.
You invigorate ailing plant material.
You mend and restore compacted and hydrophobic soils.
You help facilitate the plants’ uptake of water and nutrients from the soil.
You improve the landscape’s ability to retain stormwater.
You harvest and store rainwater.
You improve soil structure.
You prevent soil erosion.

Okay, so it’s not exactly the sonnet written by Elizabeth Barrett Browning. Where is Shakespeare when you need him, anyway? Obviously, there are so many reasons to love mulch.

So, what is mulch? It’s material applied to the surface of the soil to act as a cover. Do you remember the 1989 film Field of Dreams that brought us the adage “Build it, and they will come?” Just apply a layer of fresh arbor chips, and a host of micro and macro organisms appear out of nowhere.

When mulch is applied to the soil, it turns the landscape into a compost factory. The soil micro and macro organisms go to work, consuming the mulch and moving it down into the underlying ecosystem. Here we find a universe of cooperative beings, a “soil food web”, if you will, who carry on processes of nature that require no further action on our part other than leaving them alone. And that compost factory is turning your landscape into a water-holding container that rivals any cistern, tank, or barrel you could buy.

It is commonly quoted that increasing the soil organic matter content by 1% will help the soil hold 20,000 gallons more water per acre. Brought down to a smaller scale, for every 1,000 square feet of landscape, if you were to put on a mulch cover of at least three inches, that 1,000 sq. ft. area will be able to store 460 more gallons of water than before. That’s nine fifty-gallon drums of water! This is how we can leverage that free water falling from the sky to provide water to our landscapes well into, or even through, the summer!

Take a walk out in one of our beautiful regional, state, or federal parks. You’ll see stunning, thriving landscapes that have zero water provided to them in the summer. How is this possible? Mulch and plants that are adapted to dry summers. Who’s mulching out there? The trees are mulching themselves. We call it duff or leaf litter. Do you think that emulating that process in our gardens would allow us to use less water? You bet!

At this point, you might be asking what kind of mulch is best. Dr. James Downer, a researcher with the University of California, who has been studying mulch for 35+ years, tells us that freshly cut and chopped arbor chips are the best. 

Now that you know what mulch is, let me tell you what it is not. Mulch is not weed cloth. Please do not lay weed cloth on the ground, pile mulch on top of it, and think you’re providing the beneficial properties listed above. Actually, more harm than good will result from this.

Weed cloth only provides for temporary suppression of weeds. I say temporary because weed seeds that blow in will germinate on top of whatever you pile on top of the weed cloth, and will root into the cloth making it even more difficult to control weeds. And over time, the holes in the weed cloth silt up and air and water are no longer able to pass between the soil and the atmosphere. Plants and soil organisms are harmed by weed cloth. Just apply mulch at least four inches deep for weed reduction. It’s not perfect weed control by any means. But it makes weed control more manageable.

Mulch is also not gravel. Mulching with gravel contributes nothing beneficial to the soil. A good rule of thumb is to consider gravel mulch for fire safety within five feet of structures, or for plants whose natural habitat is the desert or the coast. These plants are not adapted to having leaf litter and decomposing wood around them. Refer to the following News to Grow By article on lowering the risk of wildfire in your landscape https://ccmg.ucanr.edu/News_to_Grow_By/?story=2700

And what about this claim that mulch is magical for water conservation? This is based on two studies known of by the author, and the anecdotal claim of a 35-year UC researcher. A study conducted at Texas A&M University found that just one inch of mulch covering a planted area lowers the plant water requirement by 25%. Another study conducted at the University of Florida: Mulch reduces soil water loss to evaporation by 33%. And the UC researcher (Dr. Downer) said in a webinar in 2020, “If you put mulch on the soil, you can just about skip every other irrigation.” Sounds pretty convincing, right?

If we’re talking about the ultimate in water conservation, mulch is a huge contributing factor, but it can’t do it alone. Our pristine and magnificent landscapes in our parks and open spaces also use the strategy of low-water plants to be able to survive six months of the year without water. So, if you truly want the benefit of using the least water possible, consider the one-two punch of mulch and low-water plants. Check out the Calscape website for a listing of plants native to California that would thrive in your neighborhood and provide a sense of place—a California aesthetic that’s undeniably compelling and beneficial to us and to local fauna.

July is Smart Irrigation Month. It’s the month of the year in California that the most water is applied to the landscape because of the high temperatures and long daylight hours. It’s the month of the year that our federal, state, and local government agencies and water districts are blasting us with information and inspiration, imploring us to reduce our water use in the landscape. Please do your part in preserving our precious water supply by applying at least four inches of mulch in your landscape.

In recognition of Smart Irrigation Month, we’ll be holding a Master Gardener webinar on mulch in partnership with the Contra Costa County Library on Tuesday, July 16th, at 6 pm. It will have the same title as this article and will provide much more expanded information about the magical benefits of mulch and low-water plants. Click here to register for the webinar.

Resources

Dr. Arthur James Downer, Ph.D. https://ucanr.edu/?facultyid=553

Mulches for Landscapes, UCANR publication