Legislatively mandated reductions in open field burning have increased grass seed growers reliance on herbicides to control seedling grasses in established stands of perennial species. In nonburned fields, a dense flush of seedling weeds and volunteer crop emerges with the onset of fall rains, and additional germination of some species occurs throughout the fall, winter, and spring. Because rainfall patterns in western Oregon are erratic during late summer and early fall, the choice of when to apply preemergence herbicides is difficult. Herbicides applied too early may dissipate before the rains arrive, whereas herbicides applied too late may not prevent germination and seedling establishment. Pendimethalin was least affected by a 3- to 4-wk exposure on dry soil waiting for rain, followed in order of increasing sensitivity by oxyfluorfen, metolachlor, and trifluralin. Most treatments controlled California brome less effectively than perennial ryegrass. Duration of 85% or greater control of subsequently oversown perennial ryegrass averaged 69 d for 2.2 kg/ha pendimethalin, 11 d for 0.42 kg/ha oxyfluorfen, and 4 d for 2.2 kg/ha metolachlor, and duration of 50% or greater control averaged 98, 33, and 18 d for the same treatments. Doubling of herbicide rates doubled the duration of control in 15 out of 27 cases but increased the duration only an average of 75% in the other 12 cases. Average time to doubling of weed populations over those occurring when herbicides and weed seeds were applied on the same day was 12, 10, and 11 d after application for pendimethalin, oxyfluorfen, and metolachlor, respectively. The similarity of these periods suggests that the major difference in performance of these herbicides with time was how close to complete control they had initially achieved. Pendimethalin was the superior preemergence herbicide for controlling California brome and perennial ryegrass in these established stands, but oxyfluorfen and metolachlor also were useful if applied near germination.