Stewardship Corner | Allelopathy – ‘The Chemicals Between Us’

Ever wonder how a single species can outcompete other species to take over an area? And in some cases, the dominant species lacks diverse neighbors – resulting in a monoculture? If so, it could be the effects of allelochemicals.

Allelopathy is the term used to describe the biochemical process an organism produces to influence the germination, growth, survival, and reproduction of surrounding organisms (known as allelochemicals) to enhance their own survival and reproduction. Without getting too in the weeds, allelopathy is a form of chemical competition.

In the case of allelopathic plants, allelochemicals are produced in the leaves, stems, fruits, or roots (or all the above) and then enter the soil via roots, washed off leaves during rain events, or released from decaying plant matter (known as residues). Some plants may also volatilize allelochemicals, releasing them into the air.

Examples of allelopathic species (not an exhaustive list):

  • Exotic invasives
    • Tree of heaven (Ailanthus altissima)
    • Garlic mustard (Alliaria petiolate)
    • Water primrose (Ludwigia)
    • Purple loosestrife (Lythrum salicaria)
    • Japanese knotweed (Reynoutria japonica)
  • Natives
    • Maples (Acer )
    • Hackberries (Celtis)
    • Sunflowers (Helianthus)
    • Pines (Pinus)
    • Sycamore (Platanus occidentalis)
    • Cottonwood (Populus deltoides)
    • Black cherry (Prunus serotina)
    • Red oak (Quercus rubra)
    • Sumacs (Rhus)
    • Black locust (Robinia pseudoacacia)
    • Sassafras (Sassafras albidum)
    • Goldenrod (Solidago)
    • American elm (Ulmus americana)

 

The most famous allelopathic example is the black walnut (Juglans nigra). Walnut trees (and hickories!)  produce juglone (5 hydroxy-1,4- naphthoquinone), a biochemical compound. Juglone is present in all parts of the plant, with the highest concentration located near the roots. While it’s not a good idea to plant tomatoes or peppers (Solanaceae family) near a walnut tree, native species (including most of the list above and many more) have coevolved and can tolerate juglone and other allelochemicals. This helps explain why some exotic invasives are so “good” at what they do since native plants have not coevolved with those species to adapt to those specific allelochemicals.

Often, allelopathic effects carry a negative connotation, but we can use these unique traits to our advantage. For instance, consider using residues as green manure. Cover crops such as cereal rye, wheat, sorghums, buckwheat, sunn hemp, and the brassica family (rapeseed, radishes, mustards) can suppress weeds via allelochemicals. “Cereal rye and its residues are quite active against weed species of pigweeds, lambsquarters, purslane, and crabgrass and far less against ragweed, sicklepod, and morning glories. Sorghums can suppress bermuda grass and nutsedge (Schonbeck 2015).”

So, whether you’re a home gardener or a restoration specialist, don’t let allelopathic plants deter you. Learn to coexist with them and even use them to your advantage! Remember, there’s always a “right plant, right place.”

Cue Bush’s ‘The Chemicals Between Us’ (1999).

Resources:
https://eorganic.org/node/2535
https://extension.psu.edu/allelopathy-in-the-home-garden
https://extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants
https://www.nrcs.usda.gov/plantmaterials/mopmstn2825.pdf