By Brandon Gallagher Watson
In 1844, an Italian-born engineer working in France, Jules Dupuit, began to concern himself with an economic question. Working on a toll bridge project, Dupuit pondered what the ideal price for the toll should be. If the toll was too high, travelers would actively avoid the bridge they had built; too low and his company would be leaving money on the table and struggle to recoup their investment expenses. He realized there must be an optimal price where the consumption (crossing the bridge, in this case) was at a perfect equilibrium with what they paid to build the bridge and what consumers would pay. Though he likely didn’t realize it at the time, Dupuit had set the groundwork for one of the most important economic concepts of the modern world: cost/benefit analysis. While it can be described in as complex a function as you like, modern finance views it one way: projects that cost little to implement but reap a tremendous return on investment are the projects worth investing in.
In tree care, we do this analysis all the time. We weigh the price of a new bucket truck with how many more jobs we can complete with it. We weigh the cost of hiring an additional sales arborist with the potential revenue they can bring in. We also make value propositions to tree owners in tree healthcare, such as “protecting your tree for this issue would cost ‘x’, while removing your tree would cost ‘y’.” We also consider aesthetic and environmental cost/benefits of treatments and non-treatments as well. That said, I would submit that there is one service that we can provide trees whose cost is insignificant when weighed against the value it provides: mulch. Think of mulch as that low-budget indie film that grosses gazillions of dollars, wins tons of awards, and makes all the studio execs happy.
Roughly defined, mulch is any layer of material applied to the surface area of soil. We most commonly associate mulch as an organic material, such as wood chips, shredded bark, leaves or even grass clippings. Mulch can also be inorganic material such as rock, shredded rubber, fabric or plastic sheeting. In urban landscapes, mulch is often applied about the base trees or in bedding plant sites. Although few would argue the appearance of the landscape is enhanced with an application of mulch, what exactly does the tree get out of it?
The many benefits of mulch
Mulch can help trees in numerous ways. Intuitively, it makes sense when we picture the environments in which trees evolved. To my knowledge, there are very few tree species that we plant in urban landscapes that spent the last few million years adapting to the conditions found there. Most spent their time adapting to forest habitats in rich soils where years of fallen leaves and woody debris cover the surface. Although adding a few inches of woodchips around the trunk in a suburban yard is not quite the same as recreating a forest floor, it can help provide many of the same conditions. Mulch will help prevent erosion, keep roots cool, help retain moisture, and can keep competing vegetation such as weeds or turfgrass at bay.
Weeds are discouraged in a few ways. By forming a physical barrier between the surface and the top of the mineral soil, mulch basically makes it difficult for newly fallen seeds to take root. For weed seeds currently in the seed bank, mulch provides a sunscreen that inhibits light from reaching the seeds, making it difficult for them to germinate. Keeping turfgrass away from the base of the tree is key for a few reasons. First, grass is one of the most vigorous competitors for the same resources on which trees rely. Second, the maintenance of grass by means of mowers and string trimmers is one of the most frequent ways the bases of trees are damaged. Nice circles of mulch prove much easier to mow around, which can help lower turf maintenance costs at the same time it is aiding and protecting trees.
Organic mulches can also make a nutritional contribution to trees as they begin to break down. While mulch comprised of 100-percent wood chips does not provide significant inputs, mixing those wood chips in with peat moss, compost, or other organic material can create a multipurpose soil covering that can provide micronutrients on a slow-release basis throughout the growing season. These nutrients not only benefit the tree, but help feed the microorganisms in the soil as well. Due to the nutrient contribution, warmer temps, and increased moisture retention, root growth under mulch is significantly higher than root growth under turfgrass.
Optimal application of mulch
As mundane as mulch may sound for scientific inquiry, it is a surprisingly well-researched topic. Studies have been conducted on the different materials, such as organic versus inorganic, as well as the depth and width of application that have helped determine the optimal depth, width, material, application timing, soil temp at the time of application, moisture content, reapplication intervals, and even the optimal ultra-violet light reflection coefficient of said mulch. For practical use, three things are most important: material, depth and width. For material, all mulch will provide benefits over not having mulch but, as mentioned earlier, organic mulches provide more in terms of nutrient and microorganism value than an inorganic source material.
For depth, 2 to 4 inches seems to provide the most benefits, while a little more or less is still acceptable. The economics story we started with is interpreted in two ways: the cost/benefit analysis we spoke of and another theory — the law of diminishing marginal utility. Simply put, charge too much for the toll and the number of users will go down. Mulch has a similar law of diminishing marginal utility — use too much mulch, and the value of mulch goes down. We are all too familiar with the bright orange mulch stacked three feet high around newly planted trees in shopping mall parking lot. Long story short: don’t volcano mulch.
Recommendations for the optimal width of mulched area around a tree will differ from source to source. Although a ring of mulch a few feet in radius around the tree will provide benefits, most sources recommend mulching trees out to the dripline. While you can certainly over-mulch a tree in terms of depth, it is impossible to over-mulch a tree in terms of width. For the optimal width, I’ll defer to the quotable Dr. John Ball who once said, “How wide should the mulch ring be? Show me the property line and we’ll negotiate inwards.”
A shortlist of mulch downsides are mostly related to improper use, such as too deep or too close to the base of tree, both of which can be avoided at the time of application. Given the large number of known benefits, the short list of drawbacks, and relatively inexpensive start-up costs, it is easy to see why mulch is one of the smartest investments you can make for the health of urban trees.
Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).