Controlling Weeds in Nursery and Landscape Plantings

Bulletin 867

Controlling Plant Damage From Herbicide Residues

To determine whether active amounts of an herbicide remain in a soil, a simple test called a bioassay can be conducted. It involves planting the seeds of herbicide-sensitive plants in some containers filled with soil from the test site and in other containers with soil from a site where no herbicides were applied. The following steps are recommended:

1. Fill containers with soil at depths of 0 to 2 inches, 2 to 4 inches, and 4 to 6 inches from several areas within the field to be tested for herbicide residue.

2. Fill other containers with soil from a site where no herbicides were applied.

3. Plant seeds of an herbicide-sensitive broadleaved plant (lettuce or bean) and a sensitive grass (oats or annual ryegrass) in each of the containers.

4. Carefully germinate and grow the plants for 2 to 3 weeks. Be careful not to over- or underwater the plants. Subirrigation is best because it does not disturb the soil surface.

5. The seedlings grown in the soil from the untreated site should grow vigorously. If the seedlings grown in the soil from the test site die or are stunted and twisted, an herbicide residue is in the soil.

If planting in soil where injury from herbicide residues is a possibility, dip the moist roots of the transplants into dry activated carbon or into a slurry of 1 pound activated carbon per gallon of water before setting.

If sensitive plants are to be transplanted into soil containing harmful levels of herbicides, or if an overdose of herbicides is applied, activated carbon may be spread on the field to neutralize the herbicide. From 100 to 200 pounds of activated carbon per acre should detoxify normal herbicide residues. For an overdose, apply the carbon at a rate of 150 pounds times the number of pounds of suspected active ingredient per acre of the applied herbicide. For example, if simazine (Princep) was broadcast at a rate of 6 pounds ai/A, then 6 times 150 or 900 pounds of activated carbon should be applied per acre. Apply the carbon in a band under the plants, rather than broadcast, so that only one-quarter to one-third of the 900 pounds is needed.

If dry powdered charcoal is used, spread it evenly over the affected areas. Some charcoal is treated so it can be added to water to form a slurry and be applied as a spray. Incorporate the charcoal 4 to 6 inches deep, with a rototiller or similar implement. This process ensures that the charcoal is intermixed with the herbicide in the soil. After the charcoal is incorporated, water the area thoroughly every day for 3 to 4 days before planting. If possible, wait a few more days because, under some conditions, deactivation is slowed.