This is a natural form of chemical warfare between plants to reduce germination of seeds and it can reduce the ability of other plants to compete. Allelopathic plants and plant material restrict germination or growth of other plants through releasing chemicals.

Allelopathy can be desirable, such as using mulch to inhibit the germination of weeds in a vegetable bed.

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But it is not always desirable, for example black walnut trees can kill nearby apple trees.

Lettuce seedlings can be suppressed by ryegrass mulch.

Technically allelopathy applies to more than plants because it is the influence one living organism exerts upon another by releasing chemical exudates that affect another population or individual. These effects are allelopathic effects.

Allelopathy is most commonly seen in farming as effects between plants, such as those that:
Barley can make a good "cleaning crop" to get rid of weeds in a cropping field, because it
Barley is high in allelopathic chemicals when it is growing and as the straw breaks down. Many other winter cereals also have allelopathic effects

You can get the benefit of barley's natural allelopathic effects to reduce weed germination and competition, particularly in later crops.

Certain plant residues provide exceptional weed control. For example, sorghum and Sudan grass straw can reduce weed weight by 90% and 85% respectively, where peat moss will provide only a marginal reduction. So you can suppress weeds with certain straws as mulches in orchards and on vegetable beds.

And the straw can be grown between the rows and slashed and thrown onto the trees or tomato plants rather than being brought-in straw. There is more about this in Green manures in orchards and vineyards and Green manures under sweet corn or tomatoes

Allelopathy is a common characteristic in cereals and other grasses as well as in many weeds and in pine trees and Australian eucalypts.

Effect of crop stubbles on weeds
. Dry weight of weeds (g/sq m) 47 days after applying crop residues
Residue type Wild oats Grass weeds Broad-leafed weeds
Nil 14 227 7
Field pea 90 100 3
Canola (rapeseed) 34 139 11
Sorghum 65 116 8
Sunflower 54 94 8
Wheat 138 71 2
Reference: Purvis, Jessop and Lovett (1985)

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This page was updated on December 27, 2007