Its Time to Declare War on the Mulch Mounding of Trees!

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Q. I often see trees in my town and neighboring communities 'mulched to the hilt'—six to 10 inches of mulch packed right up against the bark. The majority (if not all) of those trees are diseased, dying, or dead, yet the "mound mulching" practice continues to be rampant--even at the college where I work! Why do landscapers practice this very damaging type of mulching? How can information on proper mulching be disseminated to landscapers, homeowners, and other property owners so that more trees do not have to die from this senseless practice?

----Lori in Lawrenceville, NJ

A. I've been telling people not to mound mulch up against their trees for decades now. But we're getting more emails than ever from people frustrated by this practice. So I asked myself: what more can I do?

How about recruiting a wide range of professionals to join me in condemning this practice and warning of its outcomes? First up: Andrew Bunting, plant curator at the famed Scott Arboretum of Swarthmore College, a very knowledgeable guy with an unusually wide range of experience, as he's also worked at the Chicago Botanic Garden, the Fairchild Tropical Gardens, and botanical showplaces in England and New Zealand.

He assures me that none of those places mounded mulch up against their trees, and explains that the Scott Arboretum has become so aware of the problem that when they mulch the ground around their trees, they actually glove up and physically move any mulch away from the trunk by hand, making sure the beautiful and dramatic root flare of the tree is fully exposed. "Keeping moisture against the bark of a tree," he agrees, "is guaranteed to shorten its life".

He adds that there is another danger caused by 'mounded mulching' that I haven't discussed much, but that several other experts I spoke with mentioned as well. Tree roots will always seek out the easiest source of moisture, and when you pile mulch up against the bark of a tree as opposed to spreading a light layer of mulch out to the edge of the dripline, it keeps that area right next to the tree moist—which encourages "advantageous roots" to grow in a tight circle under the tree, eventually girdling the tree and strangling it.

It also encourages those roots to come straight up through the mulch instead of growing outward. "Because they have very weak lateral roots, such trees dry out much more quickly", he explained; "and are the first trees to die during a drought."

Carrie Wiles, a Board Member of the Pennsylvania Landscape and Nursery Association (PLNA) expressed great frustration "that this is still even an issue" and cites lack of education as a prime cause. Their organization—and similar groups in other states--offer certification and professional training programs designed to eliminate these kinds of 'bad practices'.

So—why then does this still occur? Is it because of the oft-held notion that some homeowners want this kind of look?

Not according to Brad Groff of River Valley Landscaping, a PLNA Board Member who actually does a lot of installation work. He says, "I've never had a client ask me to do it"; and echoed Andrew Bunting's warning that these volcano mulched trees suffer from such pitiful outward root growth "that they typically die during the first severe drought year."

We'll finish up with one of the undertakers who has to deal with the tragic results: Steve Goin, an arborist with the international "Bartlett Tree Expert" company. He reiterated the dangers of girdling, adventurous roots, and rotting of the bark and flatly stated that "improper mulching practices account for the vast majority of tree deaths we see."

I asked, "really? More than insects like borers and all the new tree diseases that keep cropping up?"

Yes. He explained that pests and diseases often take the blame for the premature death of mounded trees, but that even when you can find evidence of such attacks, they often occurred because the tree was first weakened by the volcano mounds of mulch.

Which makes a lot of sense. We often point out that healthy plants are less attractive to pests and disease, so the opposite must also be true—that weakened trees make easy targets.

So—you may now be asking: How exactly SHOULD a tree be mulched?

The healthiest trees have a thin layer of mulch—one to two inches deep—beginning about a foot away from the trunk and going out as far as the furthest branches reach. Steve Goin from Bartlett explains that a healthy root system will often go out two to three times further than that, and using a light layer of mulch to keep weeds down and retain moisture in as wide a circle around the tree as possible will help support healthy lateral root growth, stabilize the tree and prepare it for a long and happy life.

And the way to stop improper mulching is TO stop it. Tree owners need to learn to "just say no."