Nursery container preemergence herbicides must be applied multiple times, usually every 6 to 8 wk, in order to maintain acceptable weed control. Nursery growers have identified extended duration of container preemergence activity as a research priority for reduction of herbicide usage and costs. The objective of this study was to determine if the combination of slow-release (microencapsulated [ME]) formulations of alachlor and acetochlor with wood-based organic mulches could provide extended efficacy and reduced phytotoxicity vs. over-the-top (OTT) sprays or mulch alone. Efficacy and phytotoxicity studies were conducted over 3 yr with various plants. Both acetochlor formulation OTT sprays reduced spirea shoot dry weights at 45 and 110 days after treatment (DAT) compared with the controls, and emulsifiable concentrate (EC) acetochlor OTT spray also reduced shoot dry weights of rose. No herbicide-treated bark mulch (TBM) combination reduced rose or spirea shoot dry weights. EC acetochlor hardwood (in 2003) was the only treatment to provide 100% weed control at 45 and 110 DAT. The addition of EC or ME acetochlor to mulch reduced phytotoxicity and extended efficacy in 2002 and 2003; alachlor EC or ME TBM did not. Regardless of bark type, 3-yr average EC and ME TBM were 80% more effective than untreated bark mulch (UBM) and 83% and 98% more effective at 45 and 110 DAT, respectively than their comparable OTT sprays. Of the eight treatments that received ratings above commercially acceptable, averaged over dates and years, the three providing the least phytotoxicity and greatest extent, consistency, and duration of efficacy were all TBM combinations: EC acetochlor Douglas fir or hardwood bark, EC acetochlor pine, and ME acetochlor pine. TBM-reduced phytotoxicity compared with OTT sprays.

Nomenclature: Acetochlor; alachlor; Carefree Beauty Rose, Rosa × hybrid ‘Carefree Beauty’; spirea, Spirea japonica L. f. ‘Little Princess’.