Weed Control Trials On Selected Ornamentals at the Ohio State University

Weed control continues to be a major expense for nursery growers, and many species still have few, if any options for chemical weed control.  Two trials to test for phytotoxicity were set up at The Ohio State University, Columbus, Ohio.  In the first trial, red maple (Acer rubrum ‘Sun Valley’), butterfly bush (Buddleia davidii 'Nanho Blue'), Japanese holly (Ilex crenata 'Convexa'), Norway spruce (Picea abies), red oak (Quercus rubra), lilac (Syringa xtribrida 'Lark Song'), yew (Taxus media 'Runyan'), and Japanese tree lilac (Syringa reticulata 'Ivory Silk') were treated with one of the following herbicides and rates: dimethenamid-p + pendimethalin at 3.0, 5.9, and 11.9 kg ai·ha^-1, dimethenamid-p (EC formulation) at 1.1, 2.2, and 4.4 kg ai·ha^-1, imazosulfuron at 0.84, 1.7, and 3.4 kg ai·ha^-1, mesotrione – G at 2.4, 4.7, and 7.1 kg ai·ha^-1, and flumioxazin at 0.42, 0.84, and 1.7 kg ai·ha^-1.  Dimethenamid-p (EC formulation) injured red maple with increasing rates with the highest rate providing higher than commercially acceptable ratings.  The first application was much more injurious than the second application.  The butterfly bush showed injury from both the imazosulfuron (especially at the higher rates) and the mesotrione – G (at all rates).  Mesotrione – G was also very injurious to the Japanese holly, lilac, and yew at all rates.  The highest rate of dimethenamid-p (EC formulation) did injure the lilac, especially after the second application.  In the second trial, arborvitae (Thuja occidentalis ‘Smaragd’), viburnum (Viburnum plicatum tomentosum ‘Mariesii’), hydrangea (Hydrangea macrophylla ‘Forever Pink’), Japanese holly (Ilex crenata 'Convexa'), daylily (Hemerocallis 'Fathers Best White'), dogwood (Cornus sericea 'Cardinal'), serviceberry (Amelanchier canadensis 'Rainbow Pillar'), boxwood (Buxus ‘Antarctica’), Norway spruce (Picea abies), and rhododendron (Rhododendron ‘PJM’) were treated with sulfosulfuron at rates of 0.07 and 0.13 kg ai·ha^-1 and halosulfuron at a rate of 0.07 kg ai·ha^-1.  One treatment was a second application of the 0.07 kg ai·ha^-1 rate of sulfosulfuron, 4 weeks after the first treatment.  Sulfosulfuron was injurious to all species tested except rhododendron; however, the species exhibited varying degrees of injury, and rate and number of applications was also important for the amount of injury.  Dogwood and hydrangea showed the most injury from the sulfosulfuron than any other species.  The second application of sulfosulfuron caused injury symptoms to either reappear or increase in intensity.  Halosulfuron did not injure hydrangea or rhododendron whatsoever.  All other species showed some injury to halosulfuron, but like sulfosulfuron, to varying degrees.