The presence of herbicides in runoff water after application to container plant nurseries warrants investigation of methods to reduce the amount of runoff. During the summer of 2000, field research was conducted at a commercial nursery to determine the effect of a delay in irrigation after herbicide application on herbicide levels in runoff water. Two studies were conducted in June and August. Isoxaben and oryzalin were sprayed on container plants in production beds at a rate of 1.4 and 2.9 kg ai ha⁻¹, respectively. Treatments were pulse irrigated either immediately or 24 h after herbicide application. Pulse irrigation consisted of three 30-min irrigation cycles, with a 90-min rest between cycles, that supplied a total of 1.8 to 2.0 cm of water. Runoff samples were collected from both treatments after 0, 15, and 30 min of runoff flow from each pulse cycle for 3 consecutive d of pulse irrigation. The maximum isoxaben detected in June for the immediate irrigation treatment was 2.2 μg ml⁻¹. The maximum isoxaben detected in August was 2.0 μg ml⁻¹, also from the immediate irrigation treatment. The total isoxaben detected for the treatments ranged from 5.5 to 9.1% of the amount applied. The maximum oryzalin detected in June was 3.8 μg ml⁻¹ for the immediate irrigation treatment. In August it was 2.8 μg ml⁻¹ (for the immediate irrigation treatment). The total oryzalin detected for the treatments ranged from 4.6 to 8.4% of the amount applied. There were no treatment differences in concentrations and amounts of isoxaben and oryzalin. Efficacy was similar for the treatments in both studies. Delaying irrigation onset after herbicide application did not reduce total levels of isoxaben and oryzalin in runoff water. Both herbicides are stable chemicals with relatively long half-lives, and an irrigation delay of 24 h did not cause degradation that resulted in lower levels of runoff.

Nomenclature: Isoxaben; oryzalin.