Stewardship Corner | Allelopathy – ‘The Chemicals Between Us’

Ever wonder how a single species can dominate an area and lead to a monoculture, where diverse neighbors are sparse? The answer might lie in allelochemicals.

Allelopathy describes how organisms produce biochemical compounds, known as allelochemicals, that influence the germination, growth, survival, and reproduction of nearby species. This process is essentially a form of chemical competition aimed at boosting the organism’s own survival and reproduction. Without getting too in the weeds, allelopathy is a form of chemical competition.

In allelopathic plants, these compounds can be released through leaves, stems, fruits, or roots. They enter the soil through root exudates, runoff from rain, or decomposing plant matter. Some plants also release allelochemicals into the air, adding another layer to this chemical competition.

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.” Now, 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