Science project

Which Plants Can Resist Walnut Tree Allelopathy?

Research Questions:

  • How does the juglone solution react with tomato plants?
  • How does the juglone solution react with radish plants?
  • How many days does it take for the plant that receives 30 ml of juglone solution to die?
  • How long does it take for the plant that receives 60 ml of juglone solution to die?
  • Which plant will die first?
  • Which of the two plants tested seems to be tolerant of juglone?

The type of relationship between plants in which one produces a substance which affects the growth of another is known as “allelopathy."


  • Radish seeds
  • Young tomato plants in pots
  • Distilled water
  • Black walnut tree hulls (or leaves, bark or roots)
  • Metric measuring cup
  • Plastic storage containers
  • Large cooking pot
  • Stopwatch
  • Potting soil

Experimental Procedure:

  1. Gather three young tomato plants and place each in a separate flower pot or container.
  2. Radishes are very well suited to growing in containers. Before planting the seeds soak them in water for not more than ten minutes.
  3. Place three radish seeds into three separate small flower pots or containers. Make sure the soil is loose with no rocks or other obstructions. This allows the roots to grow to their proper size without any barriers.
  4. Radish seeds usually germinate within a week. Begin the next part of the investigation when all of the radish plants are about the same height as the young tomato plants.
  5. Place the black walnut hulls, (leaves, bark or roots) in a pot and fill the pot half full of distilled water.
  6. Bring the hull-water mixture to a boil. Boil the hulls and water for twenty minutes. Use the stopwatch to keep track of time.
  7. After twenty minutes, use a large spoon to remove the hulls from the water.
  8. Once the liquid has cooled, transfer it to one of the plastic storage containers.
  9. Place all of the plants in an area where they will receive equal amounts of sunlight.
  10. Using a metric measuring cup measure 60 ml (about 2 oz) of juglone solution and add this amount to the first radish plant. Again measure 60 ml of juglone solution and add to the tomato container.
  11. Label the two plants' containers “60 ml of Juglone.”
  12. Measure 30 ml (about 1 oz.) of juglone solution and add 30 ml of distilled water. Pour this amount into the second radish container. Repeat the same procedure with the tomato.
  13. Label the plant containers "30ml Juglone + 30ml water."
  14. Thoroughly rinse out the measuring cup. Water the third radish plant with 60 ml of distilled water only. Label the container “Distilled Water Control.” Repeat the procedure with the third tomato plant.
  15. Pour juglone solution on each of the plants at the same time every day. Make sure that the soil is moist but not wet.
  16. If the juglone solution does not moisten the soil, add additional distilled water to the plant so that it is moist.
  17. Keep track of the amount of juglone solution made, and make more, as needed, following steps 5 through 8.
  18. Repeat step 16 every day for at least seven days. Water the plants at the same time every day.
  19. Observe the plants noting any changes. Record all observations in a table similar to the one shown.
  20. Use phrases like “no visible change,” “change in leaf color,” “withering of steam,” “stunting of growth,” wilting of plant,” etc., when describing the plants’ appearances.

Terms/Concepts: allelopathy, juglone, allelopathic effect, autotoxicity, walnut toxicity, Black Walnut Tree, juglone toxicity; Why would fruit (hulls), dead leaves and stems fallen to ground from the Black Walnut Tree affect nearby plants?


  • Allelopathy in Sustainable Agriculture and Forestry, by Ren Sen Zeng, Azim U. Mallik, and Shi Ming Luo, pp. 304-318 (Springer, 2008).
  • Plant Secondary Metabolism, by David S. Seigler, pp. 83-85 (Kluwer Academic, 1998).
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