Duration and spectrum of preemergence (PRE) weed control following isoxaben application at 0.56, 0.84, and 1.12 kg ai/ha in spring, fall, or spring plus fall (double) application were evaluated by field experiments. Residual activity of isoxaben was assessed by monitoring weed counts at two locations for 12 mo after treatment (MAT). Buckhorn plantain and dandelion control from spring-applied isoxaben at 1.12 kg/ha was > 90% at 4 MAT in Blacksburg but dropped to < 51% at 12 MAT. Isoxaben at 1.12 kg/ha applied in fall provided ≥ 90% control of buckhorn plantain for 8–9 MAT, with control ranging from 69 to 91% at approximately 12 MAT. Dandelion control with fall-applied isoxaben at 1.12 kg/ha ranged from 60 to 75%, 8–9 MAT. Fall application of 1.12 kg/ ha isoxaben at Virginia Beach controlled henbit and cornspeedwell for 2–3 MAT. Multiple applications of isoxaben improved weed control at Virginia Beach, but results were inconclusive at Blacksburg. In comparison, oxadiazon controlled white sweet clover and dandelion for 1 MAT following spring application in Blacksburg, but < 65% broadleaf control was observed at 4 MAT. Broadleaf weed control following single application of oxadiazon at 3.36 kg/ha was < 60% at 9 MAT Poor weed control resulted from spring applications of isoxaben and oxadiazon when a 25-cm precipitation occurred within 2 wk after application in one study.

Field studies were conducted to determine the effect of season of the year, sunlight exposure time, and mulch color on paraquat photodegradation rate on polyethylene mulch. Experiments were established in winter, spring, and summer, with white and black polyethylene mulch, and paraquat-applied films were exposed to sunlight for 1, 4, 8, 24, 30, 48, 72, or 96 h after herbicide application. There was significant effect of the season by mulch color by time of exposure interaction on paraquat concentration recovered from mulch eluants. Winter paraquat photodegradation was lower than during the other seasons. At 48 h of sunlight exposure, predicted photodegradation on white mulch was 67, 83, and 88%, during winter, spring, and summer, respectively, whereas these values were 66, 82, and 84% on black mulch. The difference in paraquat photodegradation in winter with respect to the other seasons may be attributed to reduced ultraviolet radiation in winter, when solar radiation has to penetrate a larger atmosphere mass. In practical terms, transplanting on paraquat-applied mulch requires a minimum of 96 h during the spring and summer seasons, when concentrations were 5% or less, whereas a longer waiting period might be necessary in the winter.