You can tell winter is finally, truly over when your nose detects that unmistakable sweet-rot smell of mulch in the air. Landscape crews descend upon property after property, spreading six inches of mulch in a four foot ring. There is no more immediate proof that the maintenance contract you pay for is being executed. That's brown, splintery gold wrapped around your trees.
What is the point? In theory, a mulch ring is intended to retain moisture and keep roots cool, encouraging overall tree health and vitality. A secondary purpose is to cover exposed bare earth when a tree's shade precludes grass from growing below. But in execution, mulch rings are at best useless and at worst destructive, and lie to us throughout.
#1. Mulch rings can kill trees.
When mulch is piled against the trunk, retained moisture can rot the layers of living tissue that run behind the bark. The center of a tree is made of heartwood, but between heartwood and bark run the phloem, cambium and xylem layers. The tree uses those layers to transfer water, sugar, and nutrients between the leaves and the roots. If the layers become damaged, the flow is interrupted for the portion of the circumference that is affected. This is called girdling a tree. Impact enough of the circumference and the tree will die. Girdling can result from many causes; letting a seedling become rootbound in a container, failing to remove guy wires after establishment, vehicular or construction impacts, or even intentional acts.
The plant cells that make up the bark of a tree and those that make up the roots can handle vastly different moisture levels. Think of the skin on your nose compared to the skin inside your nostril. The skin types are much different despite their proximity, and prefer different conditions. If the skin on your nose was as moist as your nostril, the skin would swell and prune; just as dry nostrils resulting from a head cold are unpleasant. Imagine if you held a damp washcloth across your nose all day and all night- that's what improperly placed mulch does to trees. Beyond holding moisture on the bark, annual mulch replenishment eventually breaks down and becomes dense, compacted soil. This practice is constantly changing the conditions in which a tree grows, raising the finish grade higher and higher.
How does any of this promote tree health?
#2. Mulch rings are useless.
The photo shows a grove of trees behind my condo building. Mulch is piled in little circles around each tree, accomplishing nothing. When you get closer, you can see that the network of roots within the grove is extensive, expanding far beyond the limits of the freshly placed mulch. Further, the tree canopies create ample shade, so mulch is unnecessary to keep roots cool even if it had been spread over the entirety of the root system.
#3 Mulch rings lie to us by misrepresenting tree structure.
To reiterate, mulch is placed above tree roots to retain moisture and keep roots cool. When a new tree gets planted, designers usually dictate that a 4' diameter mulch ring gets installed, shaped into a saucer to aid in water collection. Sometimes the mulch is placed only as wide as the excavated pit. Every subsequent year, the mulch is replenished in the exact same location, regardless of how wide the canopy has grown. This indicates to a viewer that the tree roots only expand as wide as the mulch ring, which is untrue. The general public and even the construction industry has a poor understanding of how roots radiate away from a trunk, keeping within a few feet of the surface in order to conduct gas exchange. The general rule is that roots reach at least as far as the "dripline" (anywhere beneath a tree canopy onto which rainy leaves will drip), often much further. Mulch rings and urban tree boxes both belie the true location of roots, often to the detriment of tree health through compaction, excavation, or inadequate soil volumes. The photo earlier showing the grove of trees also shows vehicular tracks running between the trees. Repairs are being done to the building facade and the crews drove the machinery straight between the trees. No telling if they'd have done the same thing without the mulch islands to slalom between, but they certainly weren't dissuaded.
What would our landscapes look like if we placed mulch below the entire dripline? We would certainly have less lawn to mow. But is mulch even necessary after the first few years of establishment?
I think we should do away with permanent mulch rings, and reformulate the way we provide mulch to new plantings. Instead of topping off a newly planted root ball with a pile of mulch, I say let the top of the root ball remain exposed to the open air as it was in the container. Remove some sod and place a mulch "donut" around the root ball as a way to retain moisture in the soil into which you want the roots to grow. Create ideal conditions around the roots so they'll grow outward quickly, seeking moisture.
After proper establishment, trees don't need annual resupplies of mulch. But there is a disconnect between design intent and maintenance practice. To dummyproof the springtime ritual of maintenance crews piling mulch, mulch donuts around trees could gradually shrink in width. Every spring, a crew can maintain the outer diameter of a young tree's mulch ring but widen the inner diameter by a foot. After four or five years, mulch will no longer be added. Any new crew member or newly hired maintenance company can see at a glance where the previous year's mulch was placed and work outward from there.
Poor installation and maintenance leads to poor tree health and survival; and perpetuates the idea that landscapes are costly, fussy liabilities rather than high functioning, beneficial assets. The responsibility for enforcement lies with the property owners, whether single family residential, high density apartments or an entire municipality. A significant disconnect exists between the entities responsible for design, construction and maintenance of the urban forest. Property owners are the link and should demand better care, maintenance and protection of their forestry investment.
4. No mulch? Are you killing jobs?
There is a small elephant in the room; mulch rings go hand in hand with edging to keep planting beds looking neat and well-maintained. While my personal tastes follow a more naturalized style of gardening, I understand the aesthetic value of maintaining clean edges in the landscape. Lawn stays lawn, planting beds remain curved or straight or however else the designer intended. I say keep edging and keep using mulch as an acceptable groundcover within shrub and perennial beds, which can replicate the leaf litter in natural woodland settings.
But trees in a lawn don't need us wrapping them in a wet blanket for their own good. They can survive and thrive on their own without us smothering them with misguided care. If given the proper conditions, trees will thrive and deliver the ecosystem services on which our society depends.