University of Florida study: Mulch reduces soil water loss to evaporation by 33%

I was just in Fargo for the North Dakota Urban and Community Forestry Association conference – our pantywaist -12 deg. F wasn’t cold enough – I wanted to experience some real winter. Dr. John Ball from South Dakota State University, whom I enjoy listening to, was also on the program. As an aside, if you are ever invited to speak at the same conference as John, DO NOT allow yourself to be scheduled after him. He is hilarious and you will sound like a boring moron by comparison. John is an excellent and entertaining speaker and I usually agree with 90+ % of what he says. In this case he was talking about mulch – a subject near and dear to my heart – and lead off by mentioning the recent study by Gilman et al. (2012) as a reason why we should be concerned about possible negative impacts of mulch.

Oh boy. Let’s go through this and see what the paper does and doesn’t say about mulch.

The study was done in collaboration with Richard Beeson, who is widely known for his work using weighing lysimeters to estimate water use by container-grown trees and shrubs. In this case lysimeters were used to measure water lost from containers filled with soil or container media (60% pine bark : 30 % peat: 10% sand) and then covered with pine bark mulch or left uncovered. The containers were watered and allowed to drain. Water loss due to evaporation (there were no trees in the containers) was measured for three days. For the first day after watering there was more evaporation from the containers filled with container substrate that were mulched than the containers that were not mulched. However, if you add up the evaporation over the entire 3 days there is no difference between mulched and non-mulched containers. This is not too surprising since the mulch and container media were largely the same thing – pine bark.

Now here’s the important part. If you look at evaporation for the soil-filled containers – which are the ones we really care about from a landscape perspective – total evaporation was 3L for non-mulched containers versus 2L for mulched. In other words, there was much greater water loss from soil when the pots where not mulched. A much better title for the article would have been, “Mulch reduces evaporation from soil by 33%”!

gilman pic

It’s easy to nit-pick articles but this isn’t pit-picking. There is only one table in this article and they got it wrong. For whatever reason people are prone to hysteria when it comes to tree care and I can already see this morphing into “Oh my God! Did you hear? Ed Gilman says if you mulch a tree you’re going to kill it!” A misinterpreted 3-day study has everyone criticizing mulch while ignoring a vast body of long-term studies.

13 thoughts on “University of Florida study: Mulch reduces soil water loss to evaporation by 33%

  1. Prof. Cregg,

    If you’d like to take a break from the frozen North (and much of the South), would you consider looking at California where we’re being brought to our knees by an awful drought. Any thoughts on how to keep our trees alive through the summer and fall until the winter rains, hopefully, arrive.

    Many of us are caught between wanting to mulch our trees to limit evaporation but are scared that mulch will become fuel for fire. I suspect my neighbors would call the fire police if I were to start spreading wood chips around my trees. Any thoughts?

    Ed Morrow
    Carmel Valley, Ca

    1. Hi Ed:
      Fire-prone areas are a situation where rock mulch can be appropriate. It doesn’t provide some of the benefits of organic mulches in terms of adding organic matter, but can reduce evaporation from soil.

      You may need to get creative in finding ways to get some water to your trees. For example, during the Midwest drought in 2012 people kept buckets by the shower to collect the water while waiting for the water to warm up.

      Here are some additional tips.
      http://eden.lsu.edu/Topics/Hazards/Drought/Documents/FAQs%20for%20Helping%20Your%20Trees%20Survive%20during%20a%20Drought%20-%20H%20Stelzer%20and%20B%20Cregg.pdf

    2. think about installing a greywater system. In 2009 CA adopted sensible regulations governing greywater. It is much easier to install a legal permitted greywater system than it used to be. It used to be that you had to spend thousands and thousands or dollars on an overly complex system. I’m currently installing plumbing for greywater in my house and I have only a few hundred dollars in materials invested.

  2. Prof. Cregg.

    Thanks for the information. Accurate, scientifically based guidance like this is sometimes hard to find. The attachment is especially informative.
    Thanks again.

  3. Just 33 percent?
    In my experience it’s more than that. Here in New York we have about 800 mm of rain per year so last year I didn’t use any supplemental water on mulched perennial beds. There were doing just fine.

    1. Lucy, one of the limitations of this study is that it studied pots of soil. There were no plants. So the practical usefulness of this study are pretty limited anyway.

  4. I may have missed it, but how deep was the mulch? I’ve always told clients that mulch deeper than 4 inches is not worth the increased cost. What do you think Bert?

    1. Robert, it depends on what you’re trying to do with the mulch. Thick layers of coarse mulch can pretty much eliminate all but the most aggressive perennials weeds. That won’t happen with a thinner mulch layer. A thin layer will also need to be reapplied more often to maintain optimum depth. When mulch gets to be less than 2″ in depth the weed problem gets worse.

  5. With relevance to the subject of correctly interpreting scientific data, I didn’t see any standard errors on the graphs from Gilman et al. Was the 33% difference they measured statistically significant (what was the p value)? How many pots (N =?). I’m not seriously doubting that mulch is effective at preventing moisture loss, and I would continue to use it for its other benefits anyway, but if we are being scientifically rigorous about this….

    1. DavidO: the table in the article presented water loss by day for three days. I summed the across days to calculate the cumulative loss but there is no way to estimate the S.E.’s or determine a p value from the data given in the table. The replication (N=9) is reasonable for this type of project. If a one-third difference in the main thing you’re measuring is not statistically significant, the next question would be are the data worth publishing. The main point of this post is to point out that authors, myself included, have biases. The article abstract states the differences in evaporation were “minor”. I don’t think most people in California right now would consider reducing water loss by 33% to be minor. Moreover, as I saw in Fargo and in Jim Urban’s blog http://www.deeproot.com/blog/blog-entries/against-mulch , people are already using these results as a rationale against mulching. My interpretation is just the opposite.

      1. Hi Bert: Thanks for the clarification, and I agree with your points and interpretation. Right now my garden in Southern Ontario is 18 inches deep in another form of mulch, aka snow. So I’m not even thinking about moisture loss ….but if I were in California or Southern Ontario in July, or in my native Australia now, it would take more than this article to put me off mulching! My bias, (and I agree, we all have them), would be to opt for the slow release nutrients mulch provides and to consider any positive on moisture loss to be a bonus. The original article suggested mulching was ineffective at reducing moisture loss, not that it was detrimental in any sense, am I correct? Will you and/or your colleagues be replicating these experiments to clarify the interpretation of these data?

        1. DavidO: Actually conservation of soil moisture is a huge benefit of mulch. I just wish there were some way we could say it loud enough: The Florida study was conducted in a highly artificial system, was very narrowly focused, and reflected water loss over a period of three days. I have no interest in trying to repeat this, nor do I think there is anything to be gained by doing so. We have conducted field trials with REAL trees and shrubs growing in REAL soil where we have found REAL benefits in terms of plant survival, plant growth, soil moisture and plant water relations over the course of YEARS.

          For more see:
          http://blogs.extension.org/gardenprofessors/2012/10/19/somededtrees-update/
          http://blogs.extension.org/gardenprofessors/2013/11/04/for-mulch/
          http://www.hrt.msu.edu/assets/PagePDFs/bert-cregg/Cregg-and-Schutzki.pdf
          http://www.hrt.msu.edu/assets/PagePDFs/bert-cregg/cregg-et-al-2009.pdf

  6. Good clarification, Bert. We had Dr. Gilman present at the Indiana Arborist Association Annual conference and was presented with the studies. There was quite a buzz with the controversial message and many questions regarding the application of mulch in our landscapes.

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