Allelopathy

Allelopathy is the scientific word for chemical warfare in the plant world.

There is plenty of New Age talk about ‘companion plants helping’ each other, but that is not true. Plants do not choose to help their neighbors. [We’ll get into the discussion about ‘plant intelligence' another day.] The truth is, life is a battle. Even in the garden.

Competition for resources

Most plant roots are constantly competing with neighboring plants for water and mineral nutrients. In the same way, most plants reach for as much sunlight as they can get, regardless of the needs of neighboring plants. That’s life. Its not a decision; its survival. Weeds compete with your garden plants by growing faster. Some plants use tendrils to climb other plants. And some plants use chemicals against neighboring plants and other organisms.

What is allelopathy?

Allelopathy [al-el-ah-path-ee] is a common way for plants to reduce competition in the immediate vicinity, and to reduce feeding by herbivores. Allelopathic plants actively discharge chemicals through their leaves, bark, and roots, as they decompose, and through other processes. These chemicals can stimulate or inhibit the germination, growth, development, reproduction, and survival of other plants and organisms. Autotoxicity is the flip side of allelopathy. In autotoxicity, plants generate chemicals to inhibit the growth of the same species in the vicinity. The chemicals used in allelopathy always impact other species of plants and organisms. These biochemicals are called allelochemicals.

​Allelochemicals

​Allelochemicals can interfere with another plant’s nutrient uptake, photosynthesis, pollen or seed germination, cell division, and even specific enzyme functions. These battles are being fought on the cellular and molecular levels! Because of these effects, allelochemicals are used in commercial agriculture as natural herbicides. For example, the lemon bottlebrush plant (Callistemon citrinus) produces an allelochemical called Leptospermone. Leptospermone is not strong enough on its own to be effective, but a synthetic version, nestorione, is. Nestorione is commonly used to control broadleaf weeds in corn, as well as the crabgrass in your lawn. Allelochemicals are also used as growth regulators, insecticides, and antimicrobial products. One advantage to using allelochemicals is that they tend to not have long term residual effects. Caffeine, and certain flavonoids, tannins, and phenols are all allelopathic chemicals.

Species distribution

Plants grow where their resource needs can be met. Because of this, allelopathy is an important player in species distribution and population density in the world and in your garden. In some cases, allelopathy gives weeds and invasive plants the upper hand. Nutsedge, garlic mustard (Alliaria petiolata), and spotted knapweed (Centaurea maculosa) all use allelopathy to beat out the competition. The allelochemicals used by garlic mustard have been shown to interfere with native tree roots and their mycorrhizal fungi, which help the trees gather mineral nutrients from the soil.

Allelopathy and incompatibility

Since some plants leave behind an allelopathic residue, it is a good idea to be aware of the potential for incompatibility when intercropping (succession planting), mulching, using green manure, planting catch crops, or when rotating crops. For example, decomposing straw has been shown to suppress weed growth, as well as reduce the number of pests and diseases found in an area, because of the allelochemicals contained in the straw. As it decomposes, it also improves soil structure and soil fertility. At the same time, decomposing straw temporarily increases the carbon-to-nitrogen ratio, so you may need to supplement the area with nitrogen.


Catch crops, planted to protect the soil from erosion between major crops, are an excellent way to protect your soil, but they also introduce allelochemicals that may interfere with the next crop. One sorghum-sudangrass hybrid (Sudex), in particular, can kill  50 to 75% mortality of your tomato, broccoli, and lettuce plants!

Allelopathic plants

Many common plants use allelochemicals. Black walnut is the most notorious, but the story isn’t (as is often the case) as simple as it is made out to be. Black walnut (Juglans nigra) produces several allelochemicals which are said to block everything from growing underneath. This is simply untrue. The chemicals involved, and the interactions, are far more complex than that. Onions, beets, squash, melons, carrots, parsnips, beans, yarrow, stonecrops, and corn can all be grown near a black walnut tree without any problems, according to the PennState Extension. At the same time, blackberries, raspberries, blueberries, asparagus, eggplant, peppers, potatoes, and tomatoes do not grow as well when planted near a black walnut tree, according to the University of Illinois Extension.

Allelopathy and agriculture
​
Most of the research regarding allelopathy focuses on commercial agriculture. It tells us that rice, some Eucalyptus species, and the Tree of heaven (Ailanthus altissima) all use allelochemicals. It also demonstrates that rye mulch significantly reduces broadleaf weed growth, and that growing mung beans in a corn field reduces crop yield, while growing tobacco in the same corn field increases corn production.

Allelopathy in the garden

This is a highly complex issue that cannot be watered down very far before everyone is simply guessing. That being said, I have been able to glean the following useful information for the home gardener:

• Rotating eggplant with garlic makes positive uses of allelochemicals left in the soil. (NCBI)
• Broccoli residue can interfere with other brassica crops, so space out the rotations. (IFAS Extension)
• Pea mulch can suppress lettuce, wheat, or sorghum. (USDA)

If you suspect that two plants are incompatible, simply conduct an online search, using both plant names and the word allelopathy. If there is any research available, please let us know in the comments section!

Most of the ‘rules’ associated with allelopathy have been developed with large-scale agriculture in mind. They are diluted by all the other factors occurring in your garden and compost pile, so take it with a grain of salt.

This information is not going to stop me from composting my healthy pea plants, but it will make me more aware of my plant choices, when it comes to cover crops, crop rotation, and catch crop planting.