Weed Warfare: Investigating Allelopathy

Part of the Young Naturalist Awards Curriculum Collection.

by Julia, Grade 8, Florida - 2013 YNA Winner


I have always been interested in the natural world and love to spend time outside. At my school, my favorite place is 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. I noticed that invasive plant species like air potato (Bulbifera discoria) were a serious problem in Faith’s Forest and was motivated to do something to help.

I began to wonder if one invasive species might be used to control another, utilizing the plant’s own natural defenses. Through research, I learned about allelopathy, which is a way that plants sometimes protect themselves from predators—a sort of plant chemical warfare. Some allelopathic species—like black walnut, which contains a chemical called juglone—are well known, but black walnut trees don’t grow in central Florida, where I live. 

Close-up of a Lantana camara plant with green leave and yellow flowers.

I decided to experiment with Lantana camara because of its distinctive pungent smell, toxicity to animals and the fact that it is native to the West Indies, where insect pests abound. I suspected that it might have developed strategic chemical defenses over time. Using Lantana rather than some other plant was a hunch based on evidence. Lantana’s invasive status in Florida also made it a good plant to experiment with, as the potential to use one invasive plant to control others was exciting.

My work with Lantana camara began in sixth grade. In my sixth and seventh grade years, I investigated the effect of Lantana camaraon on Wisconsin fast plants (Brassica rapa) and on air potato, the invasive plant that originally got me interested in this area of research. I determined that the extract I created did in fact have a statistically significant impact on both plants. This year, I focused on testing Palmer’s amaranth (Amaranthus palmeri) due to the emerging problem of glycine resistance. If invasive plants like Palmer’s amaranth become resistant to Round-Up® (glycine), they are extremely difficult to control. 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’s amaranth. I thought that both invasive weeds on environmentally sensitive land and glycine-resistant plants might be managed with the same product.

Research question

What is the effect of Lantana camara extract on Amaranthus palmeri


If a Lantana camara solution is applied to Amaranthus palmeri plants during their life cycle, then the Amaranthus palmeri plants’ growth and germination rate will be reduced compared to the control. 


  • Amaranthus palmeri seeds acquired from a university researcher
    A tabletop with equipment used in the student's plant experiment, including a blender, a bowl of cut leaves, and trimming shears.
  • 108 9-inch black growers’ pots
  • Mechanical potting machine
  • Commercial mist watering system
  • 3 eight-foot white plastic folding tables
  • 24-foot water mat materials
  • Metric ruler
  • Coin
  • Cheesecloth
  • 120 grams Lantana camara leaves
  • 400 mL distilled water
  • 2 spray bottles
  • Garden hose
  • Trimming shears
  • Triple-beam balance
  • Heavy garden gloves
  • Non-latex gloves
  • Large graduated cylinder
  • Ten clear petri dishes
  • Germination paper
  • Computer
  • Statistix statistical analysis software
  • Microsoft Excel program
  • Spray cleaner with bleach

2012-13 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’s amaranth. Then I visited a local nursery and obtained 108 nine-inch black growers’ pots, potted mechanically, ensuring that they contained equal amounts of identical commercial potting mix.


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.


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 120 grams of Lantana camara leaves that I picked at my grandmother’s house. 


I whirled the Lantana leaves 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 Lantana extract to each of 54 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 54 pots.


After allowing the plants to grow for eight weeks, I harvested and analyzed each Palmer’s 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. 

Laboratory Study

I was also interested in how the Lantana extract might impact the germination of Palmer’s amaranth. I placed ten seeds on germination paper in each of ten petri dishes, spaced equally apart. This work was done at home in my kitchen on a windowsill, where they all received the same amount of light and maintained 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 Lantana 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. Lantana camara extract reduced plant height, mass and germination at statistically significant rates. Undertaking the statistical analysis was the most challenging part of the project for me, and I still have a lot to learn. 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. For this study, a P-value of less than 0.05 indicates statistical significance. To determine significance, I ran an analysis of variance (ANOVA) using the statistical software program Statistix.


Plant Height

Data indicates that Lantana camara extract reduced the height of Amaranthus palmeri. In the field study, the mean plant height in the control group was 118.5 cm and treatment group was 105 cm, a difference of 13.5 cm. The difference between the two groups had a P-value of 0.0104, which 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.



Plant Mass

Data indicates that Lantana camara extract reduced the mass of Amaranthus palmeri. In the field study, the mean mass for the control group was 54.25 grams compared to treatment group mass of 44.25 grams, a difference of 10 grams. The difference between the two groups’ masses had a P-value of 0.0006, 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 Lantana extract.

Plant Germination 

Data indicates that Lantana camara extract reduced the germination rate of Amaranthus palmeri. In the laboratory germination study, the mean germination rate for the control group was 62%, with a germination rate of 8% for the treatment group. This is statistically significant, with a P-value of less than 0.0004 and a confidence ratio higher than 99%.

Learning From the Data

The data paints a picture indicating that Lantana camara 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 harness the natural power of allelopathic plants for our own benefit and for the benefit of sensitive environmental lands? Can other allelopathic plants be discovered? In the future, I hope to investigate these important questions, as well as use mass spectrometry to determine the chemical composition of Lantana camara leaf oil. 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 chemical. 

I am also curious about the possibility that I could be only getting a fraction of the active ingredient. I need to know more about whether or not it is water-soluble. If not, the distilled water in the extract is not the best base. This deserves further study.


Achhireddy, Nagy R., and Megh Singh. “Allelopathic Effects of Lantana (Lantana camara) on Milkweed Vine (Morrenia odorata).” Weed Science 32.6 (1984): 757-61

Charles, Dan. “Farmers Face Tough Choice on Ways to Fight New Strains of Weeds.” National Public Radio, 7 Mar 2012. Retrieved from the World Wide Web on 1 Aug 2012. http://www.npr.org/blogs/thesalt/2012/03/07/147656157/farmers-face-tough-choice-on-ways-to-fight-new-strains-of-weeds

“Designing the Perfect Weed—Palmer Amaranth.” Delta Farm Press. Retrieved from the World Wide Web on 1 Aug 2012. http://deltafarmpress.com/management/designing-perfect-weed-palmer-amaranth

Ferrell, Jason, Sergio Morichetti, and Ramon Leon. “Amaranthus palmeri, Palmer amaranth.” Electronic Data Information Source—University of Florida IFAS Extension. Retrieved from the World Wide Web on 1 Aug 2012. http://edis.ifas.ufl.edu/ag346

“Identifying the Enemy.” The Bulletin: Pest Management and Crop Development Information. University of Illinois Extension. Retrieved from the World Wide Web on 1 Aug 2012. http://bulletin.ipm.illinois.edu/pastpest/articles/200122g.html

“International Survey of Herbicide-Resistant Weeds.” Herbicide Resistance Action Committee. Retrieved from the World Wide Web on 1 Aug 2012. http://www.weedscience.org/Case/Case.asp?ResistID=5256

Webster, Theodore M. “CAES Publications | UGA.” College of Agricultural and Environmental Sciences—University of Georgia. Retrieved from the World Wide Web on 1 Aug 2012. http://www.caes.uga.edu/Publications/pubDetail.cfm?pk_id=7