Do some plants keep other plants from growing? ASK THE HORT AGENT
Question Do some plants keep other plants from growing?
Answer In 300 BC, a student of Aristotle named Theophrastus wrote of how chickpea “exhausts” the soil and destroys weeds. Theophrastus later became known as the “father of botany.” 2200 years later (in the early 1900s) two researchers finally isolated phytotoxic chemicals from plants.
When plants use those chemicals against other plants it is called allelopathy. The word allelopathy comes from two Latin words. They are allelon which means "of each other", and pathos which means "to suffer".
Some plants make other plants suffer by releasing chemicals. These chemicals can be found in leaves, flowers, roots, fruits, or stems. The harmful chemical is released into the environment where it affects the growth of nearby plants. Target plants (the victims) are affected by these toxins in different ways. The toxic chemicals may inhibit shoot/root growth or nutrient uptake.
Of course, some plants have other strategies for staying alive. Instead of allelopathy, they may block the sun by producing a dense canopy. They may also hog the water and nutrients by producing a thick mass of roots.
When plants compete with one another, they create the potential for resource limitations. Allelopathic plants prevent other plants from using the available resources and thus influence the evolution and distribution of other species. This battle for real estate has been going on for thousands of years. Only the strongest have survived.
Homeowners can use this natural dog fight to their advantage. Cover crops can be used on vegetable gardens to discourage weeds. Good examples include rye, vetch, clover and mustard.
On the other hand, homeowners fall victim to this phenomenon frequently. Grass doesn’t grow under the birdfeeder because the sunflower hulls are allelopathic. Rye grasses retard the growth of lawn grasses like Bermuda and centipede when they are overseeded in the winter. The best example of chemical warfare is the walnut tree emitting juglone. Pecans, walnuts and hickories are in the walnut family. They also play dirty.
Although there probably is no such thing as “harmony” in nature, the closest thing would be a good defense. Some plants have been able to adapt to these chemical attacks. More than likely, your expensive plants have not yet adapted. For more info on allelopathy, visit http://edis.ifas.ufl.edu/BODY_HS186 If you do not have internet access, call us at 910-893-7533 or email me at gpierce@harnett.org
Although plants can keep other plants from growing, people are typically a plant’s worst enemy. I’m convinced some people also have allelopathic effects on other people. I know folks that have stunted my growth.
Gary L. Pierce
Horticulture Extension Agent
Harnett County | 
