Weeds Be Gone! Experiment with Factors that Affect Roundup® Weed Killer

In order to experiment with glyphosate, you will need to kill some plants. There are several ways to obtain the plants: you can buy them as trays at a nursery, you can grow them from seed, or you can work with weeds in your yard. The procedure below involves working with plants bought at a nursery. Be sure to care for the plants not being used in current trials so they are healthy when you are ready to use them.

Determining Effects of Varying Glyphosate Concentrations

In the first section, you will determine how the concentration of glyphosate affects its activity. This test is called a dose-response test. Caution: Perform this test in a well-ventilated area.

1. Label the plants 1–15, using a permanent marker to write on the Popsicle sticks and then inserting them in the soil of each plant.
2. Label the empty spray bottles 1–5 with the permanent marker and masking tape.
3. Adjust and test the bottle nozzles so they are all set at approximately the same spray setting—not a singe stream, but not a spray that just produces a fine mist either.
4. Put on your disposable gloves and fill the bottles as follows:
   1. Bottle # 1: Add 400 mL of water.
   2. Bottle # 2: Add 100 mL of Roundup and 300 mL of water (25 percent strength).
   3. Bottle # 3: Add 200 mL of Roundup and 200 mL of water (50 percent strength).
   4. Bottle # 4: Add 300 mL of Roundup and 100 mL of water (75 percent strength).
   5. Bottle # 5: Add 400 mL of Roundup (100 percent strength).
      • Use the spray bottle for # 5 rather than the bottle the Roundup came in so that all of the spray nozzles are the same.
5. The plants will be sprayed with the following solutions:
   1. Plants 1, 2, 3: With water only
   2. Plants 4, 5, 6: With the 25 percent solution
   3. Plants 7, 8, 9: with the 50 percent solution
   4. Plants 10, 11, 12: With the 75 percent solution
   5. Plants 13, 14, 15: With the 100 percent solution
6. Following the directions on the Roundup bottle, begin by spraying plants 13, 14, and 15 with the full-strength Roundup (the 100 percent solution).
   1. Be careful not to spray neighboring plants. Use the cardboard to block the untreated plants.
7. Using the same amount of spray on each plant, repeat the same spray application for the other concentrations. Spray enough that the treated leaves are covered with liquid.
8. In your lab notebook, record the effect of the glyphosate on the plants each day for 10 days.
   1. You can observe the plants for a shorter or longer period of time than 10 days. Vary the time, if it seems appropriate.
   2. Provide light and water to the untreated plants so they remain healthy until you are ready for the remainder of your trials.
   3. Use a scale from 1–5, with 1 being "very healthy" and 5 being "dead."
   4. Use your judgment about the definition of the plant's health; just be consistent with all of the plants.
   5. Record the plant's health score (1 to 5) in your lab notebook as a reference for all the tests.
   6. Take photographs or draw pictures of the plants at each stage of health.
9. Graph the concentration (x-axis) vs. the time (y-axis) it took for the plant to die.
   1. Graph each plant separately.
   2. Also graph the average results for each group.
10. Repeat steps 1–9, for a total of three trials. Use clean materials and new plants each time.

Determining Effects of Spraying Varying Surface Areas with Glyphosate

In this section, you will determine the minimum area that needs to be treated in order to kill the plant. This is a test of exposure (surface area) vs. effect.

1. Label the plants 16–30, using a permanent marker to write on the Popsicle sticks and then inserting them in the soil of each plant.
2. The plants will be sprayed as follows:
   1. Plants 16, 17, 18: With water only
   2. Plants 19, 20, 21: One-leaf coverage, full-strength Roundup
   3. Plants 22, 23, 24: 25 percent coverage, full-strength Roundup
   4. Plants 25, 26, 27: 50 percent coverage, full-strength Roundup
   5. Plants 28, 29, 30: 100 percent coverage, full-strength Roundup
3. Count the leaves of each plant to determine the number of leaves that should be sprayed to achieve 25 percent and 50 percent coverage. For example, it the plant has 20 leaves, spray just 5 of them for 25 percent coverage. For 100 percent coverage, spray all of the leaves. Record all data in your lab notebook.
4. Protect untreated leaves from mist. Don't spray if there is a breeze that could spread the liquid.
5. Spray plants 16, 17, and 18 with water. Spray the whole plant. Each leaf you spray should be covered with liquid. This is the control group.
6. Continue spraying the rest of the plants, as indicated in step 2. Spray the same amount on each leaf. Each leaf you spray should be covered with liquid.
7. Observe the plants for 10 days.
8. Provide water and light to all of the plants.
9. For each plant, record the time and health score, as you did in the first section, each day for 10 days. Vary the time if it seems appropriate.
10. Graph the results, with the area covered (25 percent, 50 percent, etc.) on the x-axis and the time on the y-axis.
11. Also graph the time to first signs of toxicity and the time to death for each plant.
12. Repeat steps 1–11, with fresh plants, for a total of three trials.