Mulch Volcanoes: Another Bad Gardening Idea

Root Simple reader Donna, in response to my post on decomposed granite as mulch, alerted me to a related phenomenon: the infamous mulch volcano. For whatever reason, I don’t see this viral gardening phenomenon much here in Los Angeles but it’s really common elsewhere in the US.

Mulch volcanoes are generally considered to be a bad idea. It’s thought that the lack of air circulation at the base of the tree can lead to disease problems and you don’t want roots to grow up into the mulch so close to the trunk. When applying mulch you should keep it a few inches away from the base of a tree.

herber bayer grass mound

Artist Herbert Bayer’s EarthMound, 1955. Image: GardenHistoryGirl.

How strange gardening practices, such as mulch volcanoes, get started is really interesting to me. Mulch volcanoes remind me of miniature versions of minimalist art earthworks or Native American mounds. Is the mulch volcano a kind of outsider landscape art? Is the mulch volcano a misguided attempt at putting a human imprint on nature, what landscape architects call “clues to care?”

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  1. I’ve only seen this (on the East coast) in the past 15 years or so. We got rid of the ones that the previous owner had made. When our neighbors had new trees planted, it seems as though the landscapers barely dug a hole; then they placed the tree and mounded up some soil and mulch. I don’t get it.

    • I haven’t done much traveling within the US, so I have no insight as to whether mulch volcanoes are strictly an East Coast thing, but I certainly can attest to their popularity in the Northeast, particularly in newly-built, higher-end subdivisions. My theory is that their height makes them more visible from the road – and what good is landscaping if you can’t impress passers-by? I kind of doubt that really, really rich people have a lot of wood mulch on their properties, but it seems that the aspiring-to-upper-middle-class sort equates large quantities of natural color wood chips with quality landscaping.

  2. The mulch piled like that is effective when done correctly. There is supposed to be a moat around the tree, not stacked/dumped on/against the tree. The people who just dump it against the tree are not doing it incorrectly. The idea is to trap water for the tree.

  3. Having mulch up against the crown of the tree is an invitation to sucker growth, mildew and even worse, particularly if your tree is grafted.

    Our fruit trees are in 5′ rings mulched about 3″ deep in coconut coir, pulled away to bare soil a few inches around the tree crown. The roots still do grow very near the surface there, and particularly in the Lapins cherry and Fantasia nectarine, sucker strongly to a foot around the crown. Maybe I should pull the mulch back more there.

    But the coir does an excellent job of holding the water in – the top dries out into a crust that holds the water at the soil surface (the worms love it), but when it gets disturbed (cats and birds), it dries out pretty quickly. But the trees with rings do seem to do better than ones mulched in wood chips.

  4. Its an oft repeated garden law not to mulch against the stem, to avoid collar rot.
    And I follow that rule.
    But I wonder if this is true of all species, especially larger trees.
    Just to be devils advocate, I think a mulch volcano would rot down fairly quickly.
    I have large 6 foot high woodchip piles rot down to less than 2 foot in a year or so, if I keep them moist.
    And the nutrition and grass/weed suppression that a thick volcano mulch supplies to a tree, may more than make up for any increased risk of disease.

    • If you follow the link on the image you’ll see a short post on a study of mulch volcanoes. Initial results (from several years ago) showed no detectible problem with mulch volcanoes. But the author of the study still thought it’s probably a bad idea. I couldn’t find the completed study, unfortunately.

  5. Hi: Mulch volcanoes represent a Victorian fear of Life/Generative organs; they hide the roots and the trees’ connection to its source of life,and the trees turn into inanimate Yard Furniture. My son calls them Tree Underpants. The most diligent Mulchers are the ones most afraid of anything actually growing in their yard, they also seem to use a lot of chemical controls.
    Also: thanks so much for your earlier article on how to make rye sourdough bread; I made a successful sourdough for the first time ever using your instructions, and am very tickled.

    • I love this explanation. And I’m glad the rye recipe worked for you. There’s a new bread book out, by the way–Josey Baker Bread. I’ve been baking out of it and it’s pretty good.

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