Any method of application that would increase efficiency, safety and longevity of preemergent herbicides used in ornamental culture would be of significant interest to nursery and landscape managers. This experiment is composed of two studies with three objectives: 1) determine the efficacy and duration of weed control with three common weed species, Common Chickweed (Stellaria media), Annual Bluegrass (Poa annua), and Prostrate Spurge (Euphorbia maculata) of seven mulches treated with herbicides flumioxazin 1X (0.38 kg ai ha-1) and 0.5X and oryzalin 1X (0.90 kg ai ha-1) and 0.5X, the mulches alone, directed sprays of the herbicides, oryzalin + oxyfluorfen (granular) 1X (0.27 kg ai ha-1), and a Geodisc™ for a total of 43 treatment combinations, 2) assess the phytotoxicity of the different herbicide treated mulches, direct chemical applications, and Geodisc™ on Ligustrum X vicaryi `Golden vicary’, Buxus microphylla `Wintergreen’ and Juniperus horizintalis `P.C.Youngstown’; and, 3) to compare the long-term duration of efficacy of three herbicides acetochlor 1X (2.80 kg ai ha-1), flumioxazin 1X, and oryzalin 1X, applied alone or in combination with two mulches (pine nuggets and Douglas fir) for a total of 12 treatments.
For the first study, evaluations of efficacy and phytotoxicity were conducted via dry weights and visual ratings at 45 and 115 days after treatment (DAT) in 2001 and 2002. For efficacy, ratings were based on a 0-10 scale; a rating of 7 and above represents commercially acceptable control, and for phytotoxicity, ratings were based on a 1-10 scale with 3 and below commercially acceptable. The two years produced significantly different results for efficacy and phytotoxicity visual ratings and dry weights. Seventeen of the 43 treatments gave a rating of 7 or higher for efficacy at 45 DAT in 2001 and 3 in 2002. At 115 DAT, 5 treatments gave a rating of 7 or higher in 2001 and only one gave a rating of 7 or higher in 2002. In 2001, the five treatments that gave commercially acceptable control at 45 and 115 DAT were: rice hulls + flumioxazin at 0.5 and the 1.0X rate, hardwood + flumioxazin at 0.5 X rate, pine nuggets + oryazalin at the 1X rate, and Geodisc™. In 2002, only one treatment gave commercially acceptable control at both evaluation dates, oryzalin + oxyfluorfen. When averaged across both evaluation dates and the three species, the directed spray of oryzalin + flumioxazin and 0.5X flumioxazin were the most phytotoxic treatments in 2001. In 2002, the most phytotoxic treatments were the directed spray of flumioxazin and oryzalin + flumioxazin. The Ligustrum was the most affected by phytotoxicity for both years.
Study 2 started on August 1, 2001. Visual ratings were taken twice at 121 DAT and 303 DAT. Dry weights were also taken at 303 DAT. The data indicates a significant reduction of weed control with the direct sprays compared to the herbicide treated mulches. At 121 DAT, four treatments gave a rating of 7 or higher. One of these treatments (acetochlor) was a direct spray. At 303 DAT, 6 treatments gave a rating of 7 or higher, again only one was a direct spray (acetochlor). The best three treatments at both evaluations that gave a visual rating of 7 or higher were the douglas fir + acetochlor, acetochlor alone, and pine nuggets + oryzalin. The least efficacious treatments were the direct application of oryzalin, untreated pine nuggets and the acetochlor + pine nuggets. The direct applications of oryzalin and flumioxazin gave a rating of below 7 at both evaluation dates.