Allelopathy is where a species of plant contaminates the ground with some secondary metabolite produced within it in order to 'poison' or hinder the growth of another species of plant which may decide to try and grow nearby consuming essential nutrients or shading it.

The chemical compound(s) the allelopathic plant produces to pollute the soil may only be repulsive to one species of plant, or maybe to several species. The toxins may have no effect on other species, they may be naturally immune (it is not toxic to them) or have, by repeated exposure to the compound, evolved immunity. The allelochemical may be released into the soil from the roots of the allelopathic plant, or it may just be present in the leaves or in the flowers and only contaminates the soil when these fall. Some allelochemicals may just inhibit the germination of seeds of some competitive species, or it may be toxic to the already growing competitor. In some trees the allelochemical is produced in the bark.

The allelochemicals may or may not remain long in the soil afterwards; some may decompose into relatively harmless chemicals, some may be water soluble and eventually wash away, some may be consumed or absorbed by other organisms or roots in the soil.

The whole topic of allelopathy has only recently [in the 1980's(?) been confirmed to occur in some species by chemical analysis and experimentation]. For unless you can identify the compound(s) responsible, you stand little chance of proving that it was the chemical(s) produced by the allegedly allelopathic plant which prevented or inhibited the growth of another plant. This requires experimentation to prove conclusively, otherwise it is just guesswork, as it has been in the past.

Airborne volatile chemicals produced by one plant to deter another does not count as allelopathy (???) [it may count as something else, though].

Species of plants either known or suspected to produce allelopathic compounds:

  • Cherry Laurel (Prunus lauroceraasus)
  • Garlic Mustard (Alliaria petiolata)
  • Black Mustard (Brassica nigra)
  • Bearberry (Arctostaphylos uva-ursi)
  • Rhododendron (Rhododendron ponticum) (and probably others?)
  • Elderberry (Sambucus nigra)
  • Purging Buckthorn (Rhamnus cathartica). In places where it is invasive, Purging Buckthorn forms dense monocultures and resists competitors, which is, some think, mediated by allelopathic compounds coming from the roots. Although the same source does not name the possible chemical culprits. Another source says that the allelopathic effects are overstated and fairly weak, but still does not name the agents responsible.
  • Forsythia (Forsythia)
  • Goldenrod (Solidago virgaurea)
  • Himalayan Balsam (Impatiens glandulifera) releases NaphthoQuinones into the soil which adversely affects other plants.
  • Fern () [some ferns]
  • Bracken (Pteridium aquilinum) contains 'several phenolic acid' phytotoxins which are released by the roots, rhizomes and dead fronds and remain in the soil inhibiting many other species of plants even if the bracken is removed (One notable exception is Foxglove which is often seen standing alone and proud in the middle of thick stands of bracken). Also reported are English Bluebell, Wood Anemone, Climbing Corydalis, Wild Gladiolus and Chickweed Wintergreen which all seem indifferent to Bracken toxins and many grow earlier in the season when bracken is over-wintering but your Author has not seen these plants amidst any dense bracken infestation. The high humidity under a full bracken canopy helps some mosses survive.
  • Perennial Rye (Lolium perenne)
  • Black Walnut (Juglans nigra) [susceptible species include Pine & Birch trees, Solanaceae and Azalea]. The tree contains the non-toxic HydroJuglone in its leaves, stems, fruit hulls, inner bark and roots. When either exposed to the air or to soil, HydroJuglone is oxidised to Juglone, which is the toxic allelopathic compound involved in the case of Black Walnut. Juglone poisons the soil and inhibits the growth of some plants, but not all. Shrubs and trees tolerant of Juglone number about 30 and include White Ash, Birch, Black Cherry, Elderberry, Globeflower, Hemlock, Hickory, Black Locust, Privet and Norway Spruce. Those which are damaged by Juglone include Apple, Azalea species such as Azalea (Yellow), Blackberry, Crocus (Autumn), Forget-me-nots such as Forget-me-Not (Great), Forget-me-Not (Changing), Forget-me-not (Early), Forget-me-Not (Field), Forget-me-Not (Wood), Lily-of-the-Valley, Mountain-Laurel, Peony, Potato and Rhododendron (which itself is allelopathic!).
  • Azetidine-2-carboxylic Acid (a non-proteinogenic amino acid and an analogue of L-Proline is the allelopathic molecule released by a number of plants:Lily-of-the-Valley, Solomon's Seal, plus certain plants from both the Fabaceae and the Chenopodiaceae families.
  • Similarly, the chemical meta-Tyrosine is secreted by various invasive grasses, such as:
  • Tall Fescue (Schedonorus arundinaceus)
  • Maple (Acer)
  • Pine (Pinus)
  • Pine (Bhutan), Pine (Lodgepole)
  • Eucalyptus (Eucalyptus)
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