By Nic Williamson

The urban forest is quite a different habitat than the natural forest. Apart from the numerous human-built obstacles that a tree’s canopy (e.g. utility wires) and roots (e.g. pavement) deal with, there are many land use practices which greatly affect the health of city trees. Such practices are often inspired by a landscape aesthetic, or ideas and practices involved in creating and maintaining beauty in an outdoor space. Mulching is one such practice that, when done with care, can enhance both the health and beauty of urban trees.

What is mulch? 

The word mulch refers to any material that is placed on top of the soil, usually surrounding desirable plants or trees. When you go to the garden center, you may be caught off guard by the dizzying array of mulch types. Simply, we may group mulch into two main categories: inorganic and organic. The makeup of inorganic mulches ranges from crushed stone to shredded automobile tires. As a rule, these inorganic mulches need to be replenished less frequently than organic mulches due to the fact that they decompose at a much slower rate.

Though inorganic mulches may save some human labor in the ability to outlast organic mulches, the tradeoff is that inorganic mulches usually contribute little to no nutrients to the soil and roots of living trees. Organic mulches made of chipped hard- or soft-wood trees are subject to the same bacteria and fungi that are essential to all soil nutrient processes, extremely important in promoting root growth of trees and other plants we use for food, fiber, and fuel on a daily basis.

Why use mulch?

Natural forests are full of litter. Go to a forest and you’ll find a seemingly endless supply of dead wood, bark, and leaves which cover forest soils. As time goes by, this messy layer is replenished by seasonal changes (e.g. falling leaves) and storms (e.g. windfall). This litter layer provides many services to tree roots. Functionally, the layer rests between the interface of the soil (belowground) and atmosphere (aboveground), and serves to buffer temperature changes and water availability in times of drought; in addition to protecting soils from compaction and erosion.

City trees can be just as messy as trees in the forest, but the main difference is that we (humans) are often less tolerant of this untidiness in urban areas (raking leaves in Autumn, anyone?). The irony is that by pushing leaves out to the curb or packing them into a trash bin, we are removing the very layer that provides so many benefits to trees and their roots.

What to use?

If you (or your neighbor) aren’t big on letting your leaves lay where they fall, mulch is the next best thing. A coarse, organic mulch like wood chips will stimulate soil nutrient cycles as it breaks down over time. A common concern is whether or not mulch needs to go through a composting process to reduce the likelihood of disease transfer from dead to living trees. So far this concern is unfounded, and especially when proper mulching techniques are used. The benefit here is that wood chips may be obtained cheaply (often free) from local arborists. If using a fine-textured organic mulch, you may check periodically for surface-sealing, which resembles a hardened crust where the smaller particles have become compacted. When using fine-textured mulch, it is best to spread a thinner layer (no more than 3 inches deep) and break up surface-sealing to encourage moisture infiltration. In Lexington, the Urban County Government usually has a mulch give-away at least one day per year. Otherwise, organic mulches can be purchased at many garden centers and plant nurseries.

Tips on proper mulching

• Mulch depth should be somewhere between 3 and 6 inches deep
• Do not pile mulch against the trunk or base of tree (this will hold moisture near the base, creating an ideal habitat for soil fungi and decay)
• Mulch ring diameter should be to the dripline of the tree (edge of the tree canopy when you look up)

Apart from supplying similar benefits that a forest litter layer provides, city trees benefit in other ways from mulch rings. One example is that an established mulch barrier reduces the likelihood of mechanical injuries to tree stems and roots caused by string trimmers by keeping these and other mowing equipment away from the base of the tree.   Seemingly simple, such injuries are commonly found on city trees. Additionally, we know that the impervious surfaces in cities lead to increased temperatures (urban heat island effect), which can worsen drought conditions. During these times of stress established mulch rings operate just as the forest litter layer does in retaining moisture, which aides in survival and growth of tree roots. Just remember, too much of a good thing can in fact be a bad thing. Always follow proper mulching practices, and happy mulching!

Additional Resources

Using Arborist Woodchips as Landscape Mulch (Washington State University Extension)

Video: How to Properly Mulch Around a Tree

Photography

1. Forest floor (Pixabay, CC0 Public Domain)

About the Author

Nic Williamson is the Arboriculture Superintendent at the University of Kentucky. Nic is an ISA Certified Arborist and is ISA Tree Risk Assessment Qualified. He can be reached at nwilliamson@uky.edu