Three experiments were conducted to evaluate the effectiveness of postemergence applied diuron (Direx 4L) for oxalis control and crop safety in container nursery crops. The first experiment, treated on March 15, 2001, evaluated diuron at 0.14, 0.28, 0.56, and 1.12 kg ai/ha (0.125, 0.25, 0.5, and 1.0 lb ai/A) rates without a surfactant and provided a maximum of 74% oxalis control in liriope and 57% in camellia. Diuron caused slight to no injury on ‘Pink Icicle’ camellia, ‘Anthony Waterer’ spirea and ‘Big Blue’ liriope which dissipated completely by 60 DAT. Experiment two, treated on April 6, 2001, used higher rates and included a surfactant. Diuron provided excellent oxalis control at rates ≥ 0.56 kg ai/ha (0.5 lb ai/A) by 21 days after treatment (DAT) and reduced oxalis shoot fresh weight (SFW) and shoot dry weight (SDW) by ≥ 95%. Unlike the first experiment, no injury was observed on either liriope or camellia. Experiment three, treated on April 15, 2001, evaluated diuron rate and oxalis size on oxalis control. At 9 DAT a significant diuron rate × oxalis size interaction occurred, but at 15 and 21 DAT only diuron rate affected oxalis control for the sizes of oxalis tested. At 21 DAT, rates of 0.56 kg ai/ha (0.5 lb ai/A) and higher provided excellent oxalis control. These data indicate that diuron provides excellent oxalis control when applied postemergence in tolerant nursery crops.

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Author notes

This research was funded in part by The Horticultural Research Institute, 1000 Vermont Avenue, NW, Suite 300, Washington, DC 20005.

2Graduate Research Assistant.

3Professor of Horticulture.

4Assistant Professor of Horticulture, North Willamette Research and Extension Center, Oregon State University.

5Professor of Agronomy and Soils.

6Alumni Associate Professor of Horticulture.