Another good reason to mulch

Posted by Bert Cregg

Researchers often get accused of concluding the obvious.  At some point we’ve all scoffed at headlines like, “Study finds cell phones and driving don’t mix” or “Researchers discover high heels make your feet hurt.”* But even when a study demonstrates something we already know, sometimes there is still value in being able to put hard numbers on the scope of the problem – and hopefully spur some action.

A case in point is a recent study by Justin Morgenroth, Bernardo Santos, and Brad Cadwallader at the New Zealand School of Forestry, “Conflicts between landscape trees and lawn maintenance equipment – The first look at an urban epidemic” Urban Forestry and Urban Greening 14:1054-1058.  Morgenroth and his colleagues surveyed over 1,000 trees in public greenspaces (parks, cemeteries, campuses) in and around Christchurch, New Zealand (pop. 375,000) to assess the amount of damage to trees by lawn equipment.  Their conclusion: Lawn equipment is hell on trees.  This conclusion, of course, surprises absolutely no one that has ever looked at trees near turf in a public place on this planet.

lawn mower blight 2


Morgenroth et al. claim their survey is the first systematic look at this issue and their data are staggering.  Nearly two-thirds (63%) of the trees they surveyed had at least one wound.  The proportion of trees with wounds was fairly constant regardless of type of planting (i.e., park, campus, cemetery, roadside verge), though trees in parks and campuses tended to have more wounds per tree than trees in nature reserves or roadside verges.

Not all the news was bad, however.  Morgenroth et al. found that grass cut-outs or mulching around trees significantly reduced the number of wounds per tree.

So, not a conclusion that should take anyone by surprise, but some sobering data to put some scale on the size of the problem.


*actual conclusions from real studies

8 thoughts on “Another good reason to mulch”

  1. Excellent news–thanks! UFUG is a good place for arborists to find highly relevant studies, and Justin does great work!
    I just looked at 150 trees in a town common and 98% had mower damage (anecdotal stats) My attempt to transfer mulching research into technology:
    Mulching is great for the trees, but it also creates maintenance issues. Here’s a proposed spec to create and maintain mulched areas with minimal headaches. I tried to make it simple enough to be followed by volunteers.

    1. Use hoe to scrape weeds away from trunk, out to >1’ beyond surface roots.
    2. Establish a weed-free area that conforms to the turning radius of mowers.
    3. Apply woodchip mulch to the area >2” deep, up to but not covering the trunk.
    4. Walk on mulch to pack it down to a consistent height.
    5. Use hoes and rakes to pack the edge to an easily mowable circumference.
    6. May and July: Maintain edges by hoeing or spraying with contact herbicide (such as glyphosate) to avoid invasion of mulch by grass and other weeds. Remove annual weeds within mulched area by hoe (preferred) or spray.

  2. Which lawn maintenance equipment caused the least amount of damage? I understand that mulching around a tree prevents damage, but it looks atrocious when the circle is small. Do I have to spec hand shears in close to trees or can I get away with a strimmer?

  3. Trees contribute more to cities than turf. Here’s the abstract; wish the conclusions were available for <$39.95.
    Urban forests are expected to provide numerous ecosystem benefits in challenging conditions that include environmental and anthropogenic stresses. Cities challenge the growth and survival of trees due to restricted growing space, highly modified soils, extreme soil moisture conditions, and climate that often differs from surrounding undeveloped areas. Compounding these stresses are the human factors, like vandalism – both intentional and accidental. Mechanical wounding of exposed surface roots and the lower stem by lawn maintenance equipment falls into the latter category. Anecdotally, lawn maintenance related mechanical damage is a major stressor to landscape trees, compromising their ability to thrive and thus, to provide ecosystem services. Unfortunately, no previous studies have formally quantified the incidence and extent of the problem. Here, we survey mechanical damage for 1018 trees across 308 randomly stratified plots in parks, nature reserves, cemeteries, educational institutions, and roadside grass verges in Christchurch, New Zealand. At least one wound was found on 62.9% of all surveyed trees. This was mainly driven by trees with exposed surface roots, of which 93.6% had at least one wound. This is in contrast to only 43.9% of trees without surface roots exhibiting wounds. Surveyed trees were subjected to repeat wounding with 17.8% of trees having more than 10 wounds. Maintenance activities (i.e. mulch, physical or chemical removal of grass from around the stem) reduced the incidence of mechanical wounding. In the absence of maintenance activities, 67.1% of trees were wounded, while this was reduced to 46.2%, 43.5%, and 64.2% for each of the three aforementioned maintenance activities respectively. While the reductions in mechanical wounding associated with maintenance practices are promising, alternative solutions are necessary to further reduce mechanical wounding, so that the ecosystem benefits derived from urban forests are not undermined by this blight on tree health and survival.

    How does counting wounds inform our knowledge of their effect?

  4. Guy, the goal of the study was to document the incidence and scope of the problem. Getting to the relation between wounding and damage would require a different sampling scheme. The implication in this study is that more wounds equal greater damage and more impact. Obviously it’s not that simple – a 2″ wide gash on a tree with a 30″ circumference is different than the same gash on a tree with an 6″ circumference. To get to the relation between wounding and impact, one would probably want to look at something like % of circumference affected and then correlate with indicators of tree vigor or stress (e.g., crown density, caliper growth rate, branch die-back, epicormic growth, etc.). Then there are other mitigating factors, initial tree size, vigor before damage, species, and so on. All that said, the finding that nearly 2/3rds of the public trees in a large city have some form of mower or trimmer damage is pretty sobering.

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