A follow-up to Linda’s post about a recent study in Arboriculture and Urban Forestry that indicated mulch may not reduce evaporation of water from soil as we generally assume and which suggested that landscapers may want to re-evaluate their mulching practices. As Linda noted, we have some real concerns about this conclusion and believe that proper mulching of landscape trees and shrubs has well-established benefits.
First, I don’t question the results present in AUF article. As my grad students frequently hear me say, the data are the data. But we need to bear in mind the paper is looking at one aspect of one of the impacts of mulching. As Linda notes there are plenty of data to suggest that mulching does conserve soil moisture and she included some data from one of my studies which demonstrated this. But there are other benefits to mulching beyond improving soil moisture. I’d like to mention two here; one is a practical observation, the other is based on data from another study.
Preventing lawn mower blight and string trimmer trauma
From my observations and experience, one of the biggest and perhaps least appreciated benefits of mulch is keeping mowers and weed whips at bay. I’m not aware of specific data, but damage by trimmers and mowers has to be near the top of the list of causes of mortality and long-term damage of new landscape trees. A mulched zone around trees provides a buffer and simplifies maintenance operations. It’s an easy, simple and effective way to eliminate a major cause of tree distress. A no-brainer.
Young, thin bark is no match for mowers and trimmers
Mulch protects trees and simplifies maintenance operations
Reduced soil temperatures
We conducted a trial a few years ago to look at the impacts of plastic mulches to improve establishment and early growth of Christmas trees. http://www.hrt.msu.edu/assets/PagePDFs/bert-cregg/cregg-et-al-2009.pdf In a bit of serendipity we added a treatment at the last minute because our research station had some leftover wood chips from another project. The trees mulched with wood chips were not irrigated but grew almost as much as trees in the plots that were irrigated. What was especially impressive was the insulating effects of the wood chip mulch. We installed probes to measure soil temperature 2” below the soil surface that we logged continuously. During a July heat wave we found that soil temperatures were up to 10oC (18oF) cooler on the wood chip mulch plots than on the bare-ground plots. Reducing soil heat load has profound implications for reducing root respiration and improving overall root function.
Soil temperatures were continuously logged on bare ground plots (foreground) and plots with wood chip mulch (background)
Soil temps peaked at around 38C (100 F) on bare ground without mulch (green dots); max. soil temps were about 28C (82F) with wood chips (purple squares).
As Linda noted in her comprehensive review article, mulching has a myriad of benefits for landscape trees and shrubs. It is important that we continue to look at all the various aspects of mulching and understand how to maximize the benefits (or reduce negative impacts) of mulching but in doing so it’s important to not lose site of the bigger picture. While individual studies may yield conflicting data, on the whole, the preponderance of evidence and practical considerations come down strongly in favor of mulching