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Allelopathy is a natural phenomenon where one plant produces biochemicals that exert an influence over other plants. These biochemicals are known as allelochemicals and can have both beneficial or detrimental effects on neighboring plants, animals, insects and organisms.
Allelopathy can inhibit germination and or growth of other plant species.They serve as weed killers, but on the flip side they will also inhibit the development and growth of other garden crops. Allelopathy is heavily incorporated into the science of Companion Planting.
The allelopathic biochemicals of many plants are found in the foliage. As leaves drop and decompose these toxins accumulate in the soil and affect nearby plants. Some plants also release toxins via their roots, which are then absorbed by other plants and trees as well as insects and microbial organisms.
Black Walnut emits a phytotoxin known as juglone which destroys most other plants within its reach. Very few plants are resistant to it.
Marigolds emit a protective compound designed by nature to protect the plant itself, they are allelopathic. Their roots release alpha-terthienyl, a naturally occurring compound that is highly toxic. This compound helps to control nematodes, some insects and plant pathogens. Some varieties of Marigolds produce a much stronger version of alpha-terthienyl. Mexican Marigolds have the potential to hinder the development of neighboring plants. Once planted, the alpha-terthienyl accumulates in the soil and will protect for years to come. Don't expect immediate results as it takes time for this compound to build up in the soil, usually by the second season of planting marigolds in the same place, the effect will be noticeable.
Fennell is highly allelopathic to nearly all garden plants. It exudes compounds designed by nature to destroy any competing plants. Dill is one of the rare plants that can be grown with it.
Some varieties of Sunflowers are slightly allelopathic, they emit phytotoxins which inhibit the growth of competing nearby plants and will prevent their seed germination.
Asparagus has allelopathic properties which not only inhibits the growth of competing plants but also makes it more difficult to replant asparagus in the same fields it has been grown in previous seasons.
Bearberry is a short stout cold hardy shrub that contains multiple phytochemicals with allelopathic properties.
Elderberry - The allelopathic properties of elderberry are not that noticeable as they are commonly found growing in integrated landscapes surrounded by diverse neighbor plants. However Elderberry roots emit phenolics which prevent some trees such as Douglas Fir from growing. The foliage and roots are somewhat toxic to people and livestock as they contain a cyanide-like compound.
Garlic Mustard is an invasive plant species that releases biochemicals which hinder the growth of other plants. It will retard the growth of herbs and grasses.
Goldenrod will inhibit germination of competing plant seeds.
Forsythia has an allelopathic effect on Goldenrod and Goldenrod has an allelopathic effect on Forsythia - paper, scissors, rock as to which plant wins in this battle. Forsyntia also has negative effects on Cherry Trees, black cherry in particular and some other minor fruit bearing trees as well as Sugar Maple. It has negative effects on some grasses as well - the best documented is Kentucky bluegrass.
Garden lore assigns allelopathic properties to many other plants, but these claims remain unproven or undocumented so far as Science is concerned.
Various types of Beans, beets, brassicas such as Broccoli and Cabbage, Cucumbers, Soybeans and even the holy grail of backyard gardening, the Tomato is said to have allelopathic properties. It has been documented that certain volatile substances emitted by tomato plants, particularly their foliage will have negative effects on seed germination and seedling development of other plants. This is rarely a major issues due to the fact that the volatile substances degrade in nature rapidly. Unless large amounts of tomatoes are grown over extended periods - it is not something to be overly concerned with.