Weed Warfare: Investigating Allelopathy: Year IV



The word amaranthine means immortal but, more specifically, a flower that never fades.  These words take on a new reality in the plant Palmer amaranth. Imagine a class of weeds that will never fade, something that no herbicide can kill, a truly amaranthine weed. This weed exists and is making its way across the country. This is the reality of the growing problem of herbicide resistance.

Palmer Amaranth

At my school there is an area called Faith’s Forest, a three-acre wild area that is classified as a wetland. The rules about what herbicides can be used are very strict because the wetlands are important recharge zones for aquifers in Florida and must be protected. This poses a problem, as noxious invasive weeds in the area have to be controlled by hand; this means many hours of weed pulling and seed collection.

I already knew from my previous research that one invasive species can be used to control another, utilizing the plant’s own natural defenses. I had learned about allelopathy, a sort of natural herbicide that plants produce. Some allelopathic species like Lantana camera have crippling effects on other plants. I had experimented in the past with Lantana and was intrigued to see if other invasive plants might have a similar effect.

I decided to experiment with Dioscorea bulbifera, or air potato, as an extract because it demonstrates some unique qualities. It reproduces through bulbils that look like potatoes. It is native to tropical Asia, where insect pests abound. I suspected that it might have developed strategic chemical defenses over time. The bulbils of the air potato sit undisturbed for almost nine months before they sprout in late March. This would indicate that they have a way to protect themselves chemically. Using air potato rather than some other plant was a selection I made based on this evidence. Air potato’s noxious invasive status in Florida also made it a good plant to experiment with, as the potential to use one invasive plant to control others is intriguing.

My work with allelopathy began in 2010. In my sixth and seventh grade years, I investigated the effect of Lantana camara on Wisconsin fast plants (Brassica rapa) and on air potato, the invasive plant that originally got me interested in this area of research. In 2012, when I was in eighth grade, I worked with Palmer amaranth (Amaranthus palmeri) for the first time, and confirmed its susceptibility to a Lantana camera extract. I determined that the extract I created did in fact have a statistically significant impact on all these plants. This year, I created an extract from air potato and focused again on testing Palmer amaranth, due to the emerging problem of glycine resistance.

If invasive plants like Palmer amaranth become resistant to Round-Up® (glycine), they are extremely difficult or impossible to control without using harmful cocktails of herbicides. I see the potential for developing an herbicide that might be a new solution for the growing problem land managers face with resistant weeds like Palmer amaranth and the related water hemp. I thought that both glycine-resistant plants and invasive weeds on environmentally sensitive land might be managed with the same product. New, environmentally friendly methods of weed control are important to investigate.


Research Question

What is the effect of Dioscorea bulbifera extract on Amaranthus palmeri?



If air potato (Dioscorea bulbifera) solution is applied to pigweed (Amaranthus palmeri) plants during their life cycle, then the Amaranthus palmeri plants’ height, mass, and germination rate will be reduced compared to the control.



A comparison of treated plants versus the control.

  • Amaranthus palmeri seeds acquired from a university researcher
  • 120 nine-inch black growers pots
  • Mechanical potting machine
  • Commercial mist watering system
  • 3 eight-foot white plastic folding tables
  • 24 feet of water mat material
  • Metric ruler
  • Coin
  • Cheesecloth
  • 260 grams of air potato bulbils
  • 400 mL distilled water
  • 2 spray bottles
  • Garden hose

    Preparing the air potato solution.

  • Trimming shears
  • Triple-beam balance
  • Heavy garden gloves
  • Non-latex gloves
  • Large graduated cylinder
  • Ten clear petri dishes
  • Germination paper
  • Spray cleaner with bleach

2013-14 Experimental Setup

Field Study

First, I acquired Amaranthus palmeri seeds by mail from the University of Florida Weed Science laboratory. A researcher there, Dr. Jason Ferrell, was very kind to send me seeds and give me advice about growing Palmer amaranth. Then I visited a local nursery and obtained 120 nine-inch black growers pots that were potted mechanically, ensuring that they contained equal amounts of identical commercial potting mix with equal aeration.

Back at school, I set up eight-foot growing tables in full sun with a water mat, ensuring that the plants received identical amounts of sun and water. I planted the seeds and allowed them to grow for 14 days. Palmer amaranth takes in enormous amounts of water, so it was essential to continue to provide a constant, equal supply.

The Palmer Amaranth was sprayed with the air potato solution

Next, I thinned the seeds and allowed them to grow for another seven days. After the growing period, I randomly assigned the pots to control and experimental groups by flipping a coin. Then I thinned the plants to one per pot, choosing the strongest, healthiest-looking plant. Next, I made the extract with 260 grams of Dioscorea bulbifera bulbilsthat I gathered from the school wetland. I macerated the bulbils in a blender with 400 mL of distilled water and strained them using cheesecloth to form an extract. Then I placed the extract in a spray bottle for application. I applied 5 mL of air potato extract to each of 60 pots using a spray method (seven pumps of the sprayer, which had been previously calibrated). I applied 5 mL of distilled water to the other 60 pots.

Julia measures the mass of each plant.

After allowing the plants to grow for eight weeks, I harvested and analyzed each Palmer amaranth plant, which had grown to an enormous height. I measured the height of each plant from the soil line to the tip in centimeters. Then I cut the plants at the soil line using trimming shears. I measured the mass of each plant in grams using a triple-beam balance.


In the control group of seeds germination was evident.

Laboratory Study

I was also interested in how the air potato extract might impact the germination of Palmer amaranth. I placed ten seeds on germination paper in each of ten petri dishes, spaced equally apart. They all received the same amount of light and were maintained at the same temperature. To randomize the experiment, I assigned five petri dishes to the control and five to the treatment group by flipping a coin. I applied 2.5 mL of air potato extract to the treatment group and the same amount of distilled water to control group. I observed and recorded the germination rate by counting the seeds germinated each day for five days. I repeated the experiment three times.



Data indicates that the hypothesis was supported in both the field study and the lab study. Dioscorea bulbiifera extract reduced plant height, mass and germination at statistically significant rates. The most important thing I came to understand is that the P-value (probability) indicates how likely it is that the data could have occurred by chance. Statistical analysis is very important to understand the significance of the data and to be able to see the bigger picture. For this study, a P-value of less than 0.05 indicates statistical significance. To determine significance, I ran a paired T-test using the statistical software program Statistix.


Palmer Amaranth after treatment.

Plant Height:

Data indicates that Dioscorea bulbifera extract reduced the height of Amaranthus palmeri. The P-value of 0.0000, is statistically significant, with a confidence ratio of nearly 99%. This means that there is less than a 1% chance that the difference in height was due to chance. The control plants also seemed to have longer seed plumes than the experimental plants; this was an interesting observation that merits further research.

Height comparison field test



Plant Mass:

Data indicates that Lantana camara extract reduced the mass of Amaranthus palmeri. The plant mass value had a P-value of 0.0017, meaning that the difference is statistically significant, with a confidence ratio higher than 99%. This means that I am very certain that the difference in height was actually due to the independent variable, the air potato extract.

plant mass


Plant Germination:

Data indicates that Dioscorea bulbifera extract reduced the germination rate of Amaranthus palmeri. In the laboratory germination study, the experimental group showed little to no growth and less than 20% germination, while the control showed up to 80% germination. This is statistically significant, with a P-value of 0.0000 and a confidence ratio higher than 99%.

Germination rate


Learning from the Data

The data paints a picture indicating that Dioscorea bulbifera is in fact allelopathic. This property is the result of a chemical in the plant that developed as a defense mechanism over time. Exactly what chemical it is and how it works is something I hope to investigate in the future. The potential that this natural herbicide shows is both interesting and important.

Thoughts for the Future


Could we create a more sustainable way of land management through the use of allelopathy? Can natural weed control solve the problem of herbicide resistance? In the future, I hope to investigate these important questions, as well as use mass spectrometry to determine the chemical composition of Dioscorea bulbifera bulbils. Understanding the chemistry will help me determine exactly what component of the extract is the active ingredient. This will allow me to isolate and concentrate the active chemicals for further testing.

It is also possible that I could develop a granule that could be applied to the soil. These are just some of the many possibilities to be explored in the world of weed warfare. My hope is that these herbicides of the natural world would make a difference in the lives of everyday people by creating a more sustainable way of land management.



Achhireddy, Nagy R., and Megh Singh. “Allelopathic Effects of Lantana (Lantana camara) on Milkweed Vine (Morrenia odorata).” Weed Science 32.6, Nov. 1984: 757-761. JSTOR. Web.

“Air Potato.” Center for Aquatic and Invasive Plants. University of Florida Institute of Food and Agricultural Sciences Extension, n.d. Web.

Bezuidenhout, S.R. “Allelopathy as a Possible Cause for Crop Yield Reductions.” Department of Agriculture and Environmental Affairs, Province of Kwa-Zulu Natal, South Africa, n.d. Web.

Callaway, Ragan, et al. “Natural selection for resistance to the allelopathic effects of invasive plants.” Journal of Ecology 93.3, June 2005: 576-583. Wiley Online Library, 19 Apr. 2009. Web.

Charles, Dan. “Farmers Face Tough Choice on Ways to Fight New Strains of Weeds.” National Public Radio, Morning Edition, 7 March 2012. Web.

Cohen, J. “Impacts of plant invasions become less robust over time: Invasive plants are more likely to be replaced by other ‘invasives.’” Science Daily, 20 Nov. 2013. Web.

Daniell, H., et al. “Containment of herbicide resistance through genetic engineering of the chloroplast genome.” Nature Biotechnology 16, 1998: 345-348.Nature.com. Web.

Ferguson, J.J., et al. “Allelopathy: How Plants Suppress Other Plants.” University of Florida Institute of Food and Agricultural Sciences Extension, March 2013. Web.

Hager, Aaron, Loyd Wax and Christy Sprague. “Identifying the Enemy.” The Bulletin: Pest Management and Crop Development Information. University of Illinois Extension, 7      Sept. 2001. Web.

Hirschburg, Joseph and L. McIntosh. “Molecular Basis of Herbicide Resistance in Amaranthus hybridus.” Science Magazine, 23 Dec. 1983: 1346-1349. Web.

Hollis, Paul. “Cotton weed control has changed radically due to resistant pigweed.” Southeast Farm Press, 20 July 2010. Web.

Jasieniuk, M., et al. “The evolution and genetics of herbicide resistance in weeds.” Weed Science 44.1, Jan.-March 1996: 176-193. JSTOR. Web.

Langeland, K.A. and M.J. Meisenburg. “Natural Area Weeds: Air Potato (Dioscorea bulbifera).” University of Florida Institute of Food and Agricultural Sciences Extension, n.d. PDF.

Ma, Rong, et al. “Distinct Detoxification Mechanisms Confer Resistance to Mesotrione and Atrazine in a Population of Waterhemp.” Plant Physiology 163.1, Sept. 2013: 363-377. Web.

Macías, Francisco, et al. “The Use of Allelopathic Studies in the Search for Natural Herbicides.” Journal of Crop Production 4.2, 2001. Taylor and Francis Online, 20 Oct. 2008. Web.

Morichetti, Sergio, Jason Ferrell, and Ramon Leon. “Amaranthus palmeri, Palmer amaranth.” University of Florida Institute of Food and Agricultural Sciences Extension, Sept. 2012. Web.

Neuman, W., and A. Pollack. “Rise of the Superweeds.” The New York Times, 3 May 2010. Web.  

Ridenour, Wendy and Ragan Callaway. “The relative importance of allelopathy in interference: The effects of an invasive weed on a native bunchgrass.” Oecologia 126, 2001: 444-450. Springer Verlag, 1 Dec. 2001. Web. 

Robinson, E. “Designing the Perfect Weed—Palmer Amaranth.” Delta Farm Press, 24 Dec. 2008. Web.

Sajjaphan, Kannika, et al. “Novel PsbA1 Gene from a Naturally Occurring Atrazine-Resistant Cyanobacterial Isolate.” Applied and Environmental Microbiology 63.7, March 2002. Web.

“Specie Profiles: Air Potato.” National Invasive Species Information Center. U.S. Department of Agriculture, National Agricultural Library, n.d. Web.

Stalcup, Larry. “Pigweed Will Strangle Yields: Early Weed Control a Must for Corn, Soybeans.” Corn and Soybean Digest, 1 Jan. 2012. Web.

“Where Weedkiller Won’t Work.” The New York Times, 2 May 2010. Web.