Mulch Obliged

Southwest Yard & Garden 

By Marisa Thompson

The crocuses in this photo are beautiful, but the mulch is what really catches my eye. Photo credit Marisa Thompson.

Few people are as excited about mulch as me. Maybe it’s the name. Maybe it’s time for a rebrand. Whatever you call it, mulch can be defined as any material (organic or synthetic) that’s added on top of the soil surface to benefit the soil, the plants, the gardener, or, in some cases, all of the above. Organic mulches are made of natural plant materials, like woodchips, shredded bark, pine needles, leaves, etc. Synthetic mulches consist of human-made materials, like recycled rubber bits or sheet plastic. Each mulch type has its list of pros and cons. Once applied, mulch may look like it’s just sitting there, existing, but that’s the beauty of it. Behind the scenes, mulches perform a variety of proven benefits—when applied correctly.

When I recommend mulching, I’m referring to the natural kinds, but both organic and synthetic mulches can provide the following benefits: inhibiting weeds, retaining soil moisture, slowing erosion, moderating soil temperature fluctuations (in winter and summer), and others. In addition to all of that, organic, natural mulches can add nutrients and organic matter to the soil as they break down and thereby help the roots of surrounding plants even more.

When we list the benefits of mulch, we tend to breeze through them collectively, but each benefit has a whole story with decades of research behind it. This week we’ll focus on weed control. It’s a doozy.

 

Question: Why is a mulch depth of 4 inches such a common recommendation?

Answer: That 4-inch rule is the minimum mulch thickness needed for controlling annual weeds when using coarse mulching materials (like woodchips or bark). The reason is that a thick layer inhibits weed seeds from germinating and growing by blocking sunlight. A much thicker layer of mulch may be needed to control perennial weeds fully, but we don’t have an exact number yet. In my own yard, sections mulched with 4 or more inches of chipped and ground woody materials have almost no annual weeds and only a half dozen or so perennial stalks poking up. The mulch was applied almost 2 years ago, and before that, it was a weedy nightmare cycle of London rocket in the cold season and an itchy, sneezy blanket of kochia and Palmer amaranth the rest of the year.

As mentioned above, there are pros and cons to each different kind of mulch, and these depend largely on particle size.  In the recent University of California Agriculture and Natural Resources publication “Mulches for Landscapes” (https://anrcatalog.ucanr.edu/pdf/8672.pdf), authors Drs. Jim Downer and Ben Faber include a helpful table titled “Benefits and problems associated with various mulch materials used in landscapes.” Comparisons are made between bark mulches; sawdust or finely ground wood products; shredded wood products; recycled greenwastes; fresh tree trimmings; compost; and stone, rock, or gravel. For the most part, all are considered to be good at weed control (when applied thickly enough), except compost, for two main reasons. Firstly, depending on the source, compost may contain viable weed seeds, which can be a very frustrating problem. Secondly, mature compost tends to be much finer in texture than other mulching materials, which means that it might  even support weed growth. Furthermore, that fine texture is a problem because, “For moisture savings, mulch must be coarser than the underlying soil. Mulches that are texturally finer than the soil underneath them will conduct water to the surface and can lead to increased moisture loss and drying.”  

We’re comparing plots with woodchips, gravel, and bare ground in a study on soil health and landscape tree establishment at the NMSU Agricultural Science Center at Los Lunas. You’d better believe I’ll share the results ASAP. And we’re gearing up to study other mulch-related questions for New Mexico environments in the coming decades. For now, my favorite advice when deciding what kind of mulch to try in your landscape is to use whatever you can find locally. As Ben Sears posted in a recent Facebook gardening group discussion: “The best mulch is what you can get for free.” If you and your neighbors have a good pile of pruned tree branches, it could be worthwhile to rent a chipper. Instead of bagging and hauling them off, keep your leaves and pine needles—and pecan shells too!


Link to educational webinar series: https://desertblooms.nmsu.edu/ready-set-grow.html

 For more gardening information, visit the NMSU Extension Urban Horticulture page at http://desertblooms.nmsu.edu/ and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/. Find your local Cooperative Extension Office at https://aces.nmsu.edu/county/.

Marisa Y. Thompson, Ph.D., is the Extension Urban Horticulture Specialist in the Department of Extension Plant Sciences and is based at the New Mexico State University Agricultural Science Center at Los Lunas. 

Comments

  1. I bought a house with my partner in Tularosa NM in August 2019. Our dream is to create a sustainable food forest using Geoff Lawton's permaculture techniques and greening the desert. He also emphasizes on using mulch, maybe it's due to covid, but it seems like mulch is really hard to come by around here. I've asked locals and businesses and I've been renewing my getchipdrop.com request since purchasing my property. We've been walking around our neighborhood w/ cart and wheelbarrow gathering as much organic material as we can. My partner got a job with a local landscaper and is allowed to take stuff as we need. It's too expensive to go buy handfuls of mulch for the prices at home depo etc. So my question is, if we weren't so resourceful, what resources do we have for mulch around here? My main reason for asking this is, as much as I wanted it, I wasn't able to get a house on the wonderful ditch irrigation system here. So I have to install irrigation. I wish they could expand it to where I could get it on my property. I'm sure you see where I'm going with expenses but honestly I thought with the forest so close It wouldn't be an issue. I'm kinda shocked. I didn't go to school for any of this, I just picked it up after my mom passed away cause she loved gardening. I'm very relieved to find this, nmsu is somewhat local, and this topic specifically due to how very important mulch is in my process. I'm just so happy to see local specialists talking about these subjects. I'm hoping to attend some of the webinars you have listed coming up.

    Thanks, Summer

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    Replies
    1. Thanks, Summer! I know resources for mulch vary from town to town. Here are some ideas: local garden centers, various solid waste departments, tree pruning and landscape companies, and even debris from existing plants. Rather than bagging up and removing leaf litter, keep it on the ground. That's why we call them "leaves." ;)

      I know some resourceful gardeners who pick up their neighbors bagged leaves in the fall and use them in their own yards.

      I hope that the mulch materials industry grows to support sustainable landscaping techniques as the need increases each year. Seems like a clever entrepreneurial opportunity to me! - Marisa

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