• Allelopathic Plants….what? “I want to be aloooone”

    by  • April 10, 2011 • Plant Profiles • 7 Comments

    Marlene Dietrich, in her strong German accent, said this in the old movie ‘Grand Hotel’, but plants say it too, in a silent deadly way.  Nature has a way of giving certain plants an advantage over the rest. They contain an unfriendly substance that prevents other plants from growing underneath. They want to be alone.

    Even in Spring the Manzanita has no weeds or grass under it, not even filaree.

    Even in Spring the Manzanita has no weeds or grass under it, not even filaree.

    The word allelopathy comes from two Latin words, allelon meaning ‘of each other’ and pathos which means ‘to suffer’. Allelopathy is the chemical inhibition of one plant to another. The chemical can be in the roots, any part of the plant or even in the soil where it affects the growth development of other competitors. Sometimes it means the plants are aggressive enough to crowd out the competition.

    Plants already compete for sun, water and soil nutrients and, like animals, even for territory it seems. Their allelopathic qualities are actually controlling or limiting their surrounding environment causing other plants to decline if they are unlucky enough to seed or be planted too close.

    Manzanita 'circle' can be easily seen and means "Don't plant here!"

    Manzanita ‘circle’ can be easily seen and means “Don’t plant here!”

    Most commonly, California walnuts trees, Juglans californica and Juglans hindsii , and any walnut tree really, have allelopathic effects on plants growing beneath them.  The substance produced by walnuts is called juglone. (Ain’t Latin great!)

    Other trees with allelopathic traits include Bearberry, Oaks, Sycamore, Manzanita California Bay laurel, Cottonwood, Forsythia, Tree-of-heaven, Black locust and Eucalyptus. In my garden, oaks and manzanita, Arctostaphylos viscida, are the ones needing their space, so I don’t plant anything under their canopies.


    Oaks have little growing underneath

    In the wilder area, oaks have little growing underneath..this was a eerie, foggy morning.


    California chaparral plants, like Brittlebush, Encelia farinose, Purple sage, Salvia leucophylla, California sagebrush, Artemisia californica, and Chamise, Adenostoma fasciculatum have been known to be allelopathic.  However, botanists now know that bare zones under chapparal plants are caused by animals seeking cover and, basicly “eating at home”

    In the garden, when you discover a tree or shrub is resisting any plants growing underneath, you can begin to work with nature.  Raking under a tree or spreading mulch to add neatness can help blend these areas into your scheme. You no longer have to fight it, but plant outside the drip lines of these difficult characters.

    Manzanita has only dried leaves and berries underneath

    Manzanita has only dried leaves and berries underneath


    Note: If you have eucalyptus and white fly problems you are in luck!  Covering the ground, underneath a white fly infested plant, with eucalyptus bark will clear up the problem like magic. After chopping down a Euc, and when the bark on the logs began to dry and flake off, I happened to use the bark as mulch under a hibiscus (this was in Fullerton…Southern California). It had had a bad case of white fly, but after a few months it was completely gone!

    garbo: “I want to be alone!”

    List of plants for around oaks and pines

    “It was concluded that allelopathy is likely to be a cause of understorey suppression by Eucalyptus species especially in drier climates.”

    “Plant ecologists generally view the entire concept of allelopathy with understandable skepticism because hundreds of investigations have produced little more than ambiguity. As demonstrated in Muller’s work, ecological variables are notoriously difficult to isolate. In commenting on the need for researchers to recognize this difficulty, John Harper (1975), the prominent plant population biologist from England, wrote, “It would be surprising if, among the variety of plant interactions in nature, there were not cases in which toxins played an important ecological role, but it does not help the discovery of such cases to preach their existence as though it were gospel.”

    “Hydroquinone and arbutin have been considered as allelopathic agents from the aqueous leachates of species of manzanita.”

    “This toxic affect(of Black Walnut) on surrounding plants appears to be related to root contact, as walnut hulls and leaves used as mulch have not shown toxic effects on plant growth…The juglone toxin occurs in the leaves, bark, and wood of the walnut but these contain lower concentrations than the roots.”


    Sue Langley, a passionate gardener and photographer lives and gardens with her husband and Corgi, Maggie on 7 acres just south of Yosemite, Zone 7 at 3000 feet. She also manages the Flea Market Gardening Facebook page and website.

    7 Responses to Allelopathic Plants….what? “I want to be aloooone”

    1. April 11, 2011 at 3:14 am

      I found this fascinating! I had not heard of allelopathic plants although I do know that here, nothing grows under gumtrees and, as far as I knew, the soil beneath them is rendered barren for years after the trees have been eradicated.

    2. April 11, 2011 at 5:25 pm

      Oooh…I may have to source some Eucalyptus bark this summer. We had a bit of a white fly problem last year. I’m curious to see if that would help.

    3. April 11, 2011 at 8:39 pm

      I used to be a firm believer in the allelopathy gospel until I ran across a review by Rick Halsey of the scientific literature on the subject in Torreyana. If you have access to JSTOR you can read it at: http://www.jstor.org/pss/4126940. (Even if you can’t access it, the abstract at the bottom of the page is a good summation.) I’m not a scientist, but I actually found it pretty easy going and incredibly eye-opening. There are lots of empirical observations that would support allelopathy, and you’ve shown lots of situations I’d consider evidence of it. Some authors even considered that some plants emit gases that inhibit competition. But Halsey’s paper shows that opinions are actually a little more mixed, and I’m trying to stay open minded and out of the way of the more heated debates. Maybe a safe place to hide out would be in the grass-free area under a spreading oak, whether the bare earth came about through allelopathy or some other form of successful competition…

    4. April 11, 2011 at 11:15 pm

      Thanks, Desiree, I had eucalyptus in Southern California where it grows widely and the grass slowly died underneath.

      Clare, I hope it works if you do this. It was a happy accident for me.

      Hi James, yeah, there is controversy. That paper, used as one source for this post, written to refute an earlier claim by botanist, C. Muller, that chaparral plants were allelopathic, would be interesting to discuss sometime. My personal observations here are that not much grows under eucalyptus (at my former home) and manzanita. I’m no botanist, though!

    5. April 13, 2011 at 9:27 pm

      Oops, missed the fine print citations at the end… Whatever the reasons for the barren patches, the patches are real!

    6. hank
      March 15, 2015 at 9:41 am

      So I have a native manzanita and a native black walnut growing (SF East Bay flats) side by side.
      Are they fighting each other, or allied together, while they’re both trying to beat the ancient juniper hedge they’re growing out of and trying to overshadow?

      • Sue Langley
        March 16, 2015 at 6:05 pm

        Hank, I leave my manzanita alone. it’s just an interesting phenomenon,…you don’t necessarily have to do anything. Plants don’t really fight. hahaha

        All three of your plants are pretty hardy,….Maybe consult an arborist if something must be done.

    Leave a Reply

    Your email address will not be published. Required fields are marked *